Frequently Asked Questions - CSS script
In this Manual:
- 1. What is the CSS test used for?
- 2. How the CSS Test algorithm works
- 3. Which signals need to be recorded
- 4. How to connect to the CKP sensor signal wire
- 5. Types of CKP sensors
- 6. What the signal of an inductive CKP sensor looks like
- 7. What the signal of a two-level CKP sensor looks like
- 8. Where to connect the black "crocodile" clip from the USB Autoscope IV's power cable
- 9. Where to connect the red alligator clip of the USB Autoscope IV power cable
- 10. How to obtain synchronization signal from Cylinder #1 on a gasoline engine
- 11. Shape of synchronization pulses for a gasoline engine
- 12. How to obtain synchronization signal from Cylinder #1 on a diesel engine
- 13. Shape of synchronization pulses for a diesel engine
- 14. Selecting the device operating mode before conducting the test
- 15. How to perform a CSS test on a gasoline car
- 16. Why the sequence of actions in the CSS test is important
- 17. How to perform additional manipulations during the CSS test
- 18. How to perform the CSS test on a gasoline engine with an electronic throttle
- 19. How to perform a CSS test on a vehicle where the CKP signal disappears when the ignition is turned off
- 20. How to perform a CSS test on an older vehicle without a CKP sensor (and on vehicles where connecting to the CKP signal is difficult, as well as on engines equipped with transformer-type CKP sensors)
- 21. How to perform a CSS test on a diesel engine
- 22. How to save recorded signals to a file
- 23. Reliability of CSS test results
1. What is the CSS test used for?
What is the CSS test for?
A common task for a technician is to identify the cause of misfire of the air–fuel mixture in the cylinders of an internal combustion engine. To solve it, a diagnostic scanner is usually used, which allows you to read error codes from the memory of the engine control unit.
Errors P0301…P0312 directly indicate the number of the cylinder operating with misfires. For example, the P0301 code indicates a «Misfire in Cylinder #1» and P0302 indicates a «Misfire in Cylinder #2»... However, the scanner does not provide information about the causes of the malfunction. And it can be in such systems: engine mechanics, ignition system, fuel system. Sometimes the engine control unit cannot even determine the number of the faulty cylinder, and then it saves the error code P0300 – "Multiple misfires". Thus, when using a diagnostic scanner to identify misfires, it often remains unclear how to resolve the problem. For this reason, an alternative tool was developed—the CSS test.
The CSS test is primarily designed to detect misfire in the cylinders of an internal combustion engine and to identify the causes of this. Designed for diagnostics of cars and commercial vehicles with gasoline and diesel engines, engines running on gas and biofuel. It can also be used to diagnose other machinery equipped with internal combustion engines.
This test quickly and easily identifies cylinders experiencing misfires and identifies the area of the malfunction without the need to disassemble the engine or its systems. The CSS test allows you to:
- Compare the dynamic compression in the engine's cylinders
- Test the functionality of the ignition system, not only as a whole but also in relation to specific cylinder numbers
- Check the operation (efficiency) of fuel injectors
- Assess the operation of the ignition timing subsystem
- Identify defects in the crankshaft reluctor wheel
- Evaluate the signal level from the crankshaft position sensor
- Perform an in-depth analysis of the signal from any sensor or actuator related to the engine control system
2. How the CSS Test algorithm works
The input signal used for the CSS test is the signal from the crankshaft position/rotation speed sensor (CKP sensor). This signal is recorded from the internal combustion engine being tested and serves as the primary data source for analyzing cylinder performance and identifying potential issues.
The crankshaft sensor
The crankshaft sensor signal was chosen because it contains very detailed information about the nature of the engine's rotation. This rotation occurs and changes due to pushes generated by the combustion of the air-fuel mixture in the cylinders. The characteristics of these pushes are the most crucial information when diagnosing the causes of misfires. Through the analysis of the crankshaft sensor signal, the CSS test provides to the technician information about the force and timing of pushes from combustion events in the engine cylinders.
Simultaneously with the crankshaft sensor signal, a synchronization signal is recorded, corresponding to the ignition moment of the air-fuel mixture in Cylinder #1. This signal identifies the timing when the piston of Cylinder #1 is near the top dead center (TDC) at the end of the compression stroke and the beginning of the power stroke. It allows you to identify the operation of specific cylinders, that is, to distinguish the pushes produced by one cylinder, from the pushes produced by another cylinder.
The CSS test algorithm is integrated into the software of all USB Autoscope device models as a CSS script. However, the recommendations in this article pertain specifically to the latest version of the device — USB Autoscope IV.
3. Which signals need to be recorded
For both gasoline and diesel engines, the signal from the Crankshaft Position/Rotation Speed Sensor (CKP sensor) must be recorded as the source of information about the engine's instantaneous rotation speed. It is essential to use the CKP sensor signal specifically for this purpose. Neither the camshaft sensor signal nor the signal from the speed/position sensors located in the distributor or on the diesel fuel equipment are suitable for this purpose, since the toothed discs of these sensors have insufficient mechanical rigidity with the engine crankshaft.
As the synchronization signal corresponding to the ignition moment of the air-fuel mixture in Cylinder #1, the ignition pulse of the first cylinder is used for gasoline engines.
Synchronization signal obtained on a gasoline engine
As the synchronization signal for the ignition moment of the air-fuel mixture in Cylinder #1 on a diesel engine, the control signal for the fuel injector of Cylinder #1 is used.
Synchronization signal obtained on a diesel engine
It should be noted that for both gasoline and diesel engines, if access to Cylinder #1 is difficult, a synchronization signal from any other cylinder can be used. However, in this case, when starting the signal analysis, it will be necessary to specify the correct cylinder sequence and indicate the number of the cylinder used for synchronization.
4. How to connect to the CKP sensor signal wire
To record the CKP sensor signal, connect a signal cable with a needle probe to input #5 of the USB Autoscope IV. Insert the needle of the signal cable under the seal of the CKP sensor connector so that it is securely pressed against the contact terminal of the signal wire without damaging its insulation.
A needle probe is used for connection
In some cases, it is necessary to use an insulation piercing clip.
Use of an insulation piercing test clip
If access to the electrical connector of the crankshaft position sensor is very difficult, the same signal can be recorded from the opposite end of the wire by connecting to it not from the crankshaft position sensor side, but from the side of the engine control unit's electrical connector.
Connection to the CKP sensor from the side of the ECU's electrical connector
5. Types of CKP sensors
There are several types of crankshaft position/rotation speed sensors (CKP) for internal combustion engines. They differ in the type of sensing element and the type of output signal.
1) The so-called inductive type CKP sensor (actually an induction sensor). The voltage waveform of its output signal resembles a sine wave. The output signal of such sensors is sometimes referred to as an analog signal.
Inductive type CKP sensor output signal
The amplitude of the inductive crankshaft position sensor signal depends on the specific sensor, its installation depth, and the crankshaft rotation speed. All of these parameters are automatically controlled by the CSS test algorithm.
The inductive type CKP signal is suitable for use with the CSS test.
2) Hall effect CKP sensors (used in older vehicles) and magnetoresistive sensors (used in modern vehicles, known as MRE sensors) generate a rectangular-shaped signal. The output signal of these sensors is sometimes referred to as a two-level signal.
Hall effect CKP sensor output signal
The two-level CKP signal is suitable for use with the CSS test.
3) The transformer-type CKP sensor (some CKP sensors produced by Siemens) is schematically part of an auto-generator, the active component of which is located inside the engine control unit and is made in the form of a specialized integrated circuit (usually produced by Philips). Two of the three pins of such a sensor carry sinusoidal signals.
Transformer-type CKP sensor output signal
The amplitude of these signals is not the same. The frequency of these signals is around 100 kHz, but it depends on the specific sensor. That is, when replacing one working sensor with another, the frequency of the signal will change slightly. However, on one winding of the sensor, the frequency will always be exactly the same as on the second winding. In other words, the frequency of the signals on the two windings of the same sensor is always absolutely identical.
The distance between the working surfaces of the transformer-type crankshaft position sensor and the tooth of the triggering disc affects the phase shift between the signals on the two windings of the sensor. That is, the phase difference between these two signals changes depending on whether the tooth of the toothed disc is near the working surface of the sensor or not.
The transformer-type CKP signal is not suitable for use with the CSS test.
If it is absolutely necessary to perform the CSS test based on the standard transformer-type CKP sensor, in this case, the signal of the sensor itself is not recorded, but rather the output signal of the integrated circuit servicing it. However, since this integrated circuit is located inside the engine control unit, accessing it would require opening the ECU housing.
The output signal of the integrated circuit that converts the signals of the transformer-type CKP sensor is suitable for use with the CSS test.
There is another method for performing the CSS test on vehicles equipped with a transformer-type crankshaft position sensor, which is described in section 20 of this document.
Note:
To check the functionality of transformer-type crankshaft position sensors, there is a specialized test called "SiemensRPM." It is not intended for detecting misfires in the cylinders of the tested engine, but rather allows measuring the phase shift between the signals in the two windings of the sensor and displays the dynamics of these changes.
6. What the signal of an inductive CKP sensor looks like
Typically, an inductive crankshaft position sensor consists of a coil of copper wire wound around an iron core, into which a permanent magnet is embedded. Because the permanent magnet generates a magnetic field, there is always a small magnetic flux present in the sensor's core. The operation principle of an inductive CKP sensor is based on the effect of changing magnetic flux in its core when the teeth of the reluctor wheel approach or move away from the sensor: When a tooth is close to the sensor’s working surface, the magnetic flux is relatively high. When a tooth is far from the sensor’s working surface, the magnetic flux is relatively low.
The output signal of the inductive sensor does not respond to the magnitude of the magnetic flux in its core. It only responds to the rate of change of this flux. This physical phenomenon is called "electromotive force (EMF) of self-induction." The magnetic flux can change in either direction—whether increasing or decreasing. In other words, an inductive crankshaft position sensor reacts only to the variable component of the magnetic flux in its core. This means that voltage will be generated at the sensor's output both when the tooth of the reluctor wheel approaches the sensor and when it moves away. Only the signal's polarity will change. Additionally, this means that the value of the output voltage depends on the crankshaft's rotational speed, as this directly affects the rate at which the tooth enters and exits the sensor’s magnetic field.
The output signal of the inductive sensor
With the engine off, the voltage difference between the terminals of the inductive sensor coil will be zero, regardless of whether a tooth of the reluctor wheel is near the sensor's working surface or not. This is because, in both cases, the magnetic flux in the sensor's core remains unchanged. The shape of the output signal of an inductive crankshaft position sensor primarily depends on the arrangement of the teeth on the reluctor wheel, as well as their size and shape.
Shape of the output signal depends on the arrangement of the teeth on the reluctor wheel
Shape of the output signal depends on the arrangement of the teeth on the reluctor wheel, as well as their size and shape
Damaged teeth of the reluctor wheel
The signal amplitude also depends on the distance between the working surface of the inductive sensor and the teeth of the reluctor wheel: the closer the sensor is to the wheel, the more the teeth alter the magnetic flux in the sensor core, resulting in a higher output signal voltage. Thanks to this effect, the signal from an inductive CKP sensor can be used to detect runout of the toothed disc. This occurs due to improper attachment of the disc to the crankshaft (when the geometric center of the disc does not align with the geometric center of the crankshaft). As a result, the teeth on one side of the disc pass closer to the sensor's working surface than those on the other side, causing greater or sharper changes in the magnetic flux in the core. Consequently, one group of teeth generates pulses of higher amplitude, while the other generates pulses of lower amplitude.
Improper attachment of the disc to the crankshaft
Additionally, the signal amplitude also depends on the specific sensor, as factors such as the strength of the built-in permanent magnet's magnetic field, the core configuration, the winding parameters (number of turns, wire diameter, geometric dimensions of the coil), and others play a role.
It should be noted that some non-original sensors may generate a signal with opposite polarity (inverted) compared to the original ones.
The polarity of the signal from an inductive CKP sensor depends on the direction of the winding and the polarity of the built-in permanent magnet. Typically, incorrect polarity of the CKP signal prevents the engine control system from functioning properly.
7. What the signal of a two-level CKP sensor looks like
Most crankshaft position sensors with a two-level output signal are based on either a Hall effect element or a magnetoresistive sensing element. These sensors are called "two-level" because the voltage of their output signal can take only one of two states: low level (when the sensor's output voltage is close to 0 V), or high level (on different cars it can take values of 3.3 V, 5 V, 8 V, 12 V… and others; it depends on the control unit with which the sensor works). The transition between these states occurs very quickly. As a result, the voltage waveform of a two-level CKP sensor has a rectangular shape with a fixed amplitude.
Voltage waveform of a two-level CKP sensor
Both the Hall effect and magnetoresistive elements respond not to the rate of change of the magnetic flux but to its absolute value. This means a CKP sensor based on one of these elements can function even when the engine’s crankshaft is not rotating. For example: in one model of the sensor, the signal goes to a low level when a tooth on the reluctor wheel is near the sensor's working surface, and to a high level when the tooth is far away. In another model, the behavior could be the opposite.
Pulse frequency depends on the engine's rotational speed; signal amplitude remains constant.
The duty cycle of the signal depends almost entirely on the shape of the teeth on the timing wheel.
Output waveform of two-level CKP sensor
Output waveform of two-level CKP sensor
Damaged teeth of the reluctor wheel
A common fault with magnetoresistive sensors is instability of the falling edge of the signal, which is most noticeable when the last tooth passes the sensor before skipping teeth on the disk.
Damaged teeth of the reluctor wheel
Some cars use sensors whose circuitry contains a so-called "monostable multivibrator". The output signal pulses of these sensors always have the same duration; only the duration of the interval between pulses changes.
8. Where to connect the black "crocodile" clip from the USB Autoscope IV's power cable
All measurements made using the analog inputs of the USB Autoscope IV are referenced to the black alligator clip from the device's power cable. Therefore, this cable, specifically its black alligator clip, must always be connected during any measurement. The power cable connects to the "+12 V" connector on the back panel of the USB Autoscope IV.
Since measurements are relative to the black alligator clip from the power cable, the quality of the obtained signal depends on the chosen grounding point on the vehicle. There are three main grounding points in a car: vehicle body, "–" terminal of the battery, engine housing. Ideally, the black alligator clip should be connected directly to the point where the engine control unit's ground wires converge. In modern vehicles, this point is typically located on the vehicle body. If access to this point is difficult, the black alligator clip from the USB Autoscope IV power cable can be connected to any grounding point on the car body that does not carry high currents (e.g., starter current or alternator current). In this case, the quality of the signals will be nearly unaffected.
Black crocodile connection to the vehicle body
Connecting the black alligator clip to the "–" terminal of the battery is not recommended due to the high level of electrical interference at this point. The only exception is the ElPower test, during which the black alligator clip should specifically be connected to the battery's "–" terminal.
Connecting the black alligator clip to the engine ground is also not recommended due to significant interference levels at this point.
9. Where to connect the red alligator clip of the USB Autoscope IV power cable
The universal cables of the USB Autoscope IV can connect not only to measuring adapters but also to transducers included in its standard kit (or optional sensors, amplifiers, or adapters). Almost all of these are active devices, meaning they require a power supply to operate. Power is supplied to the transducers via the same universal cable through which the signal from the transducer is transmitted to the analog input of the USB Autoscope IV. The universal cable receives power from the analog input connector of the device. Voltage is present on the analog input connectors only if the device is connected to a power cable, the crocodile clips of which are connected to a voltage source. To supply voltage to the device, the red crocodile clip of the power cable should be connected to the "+12 V" terminal of the battery of the vehicle being tested, but only after the black crocodile clip of the power cable has already been connected to the vehicle ground.
Red crocodile connection to the "+12V" terminal of the battery
This will provide power to the transducers connected to the USB Autoscope IV universal cables. It will also provide power to the preamplifier of the high-voltage probes used to test the ignition system, which are connected to the "In+", "In–" and "In Synchro" connectors located on the front panel of the device.
Connecting to the "+12 V" terminal of the vehicle's battery is typically convenient only for cars where the battery is located in the engine bay or under the front seat. For vehicles with batteries positioned under the rear seat or in the trunk, a special "+12 V" power terminal is often provided in the engine bay. In such cases, the red clip of the USB Autoscope IV power cable should be connected to this terminal.
Red crocodile connection to special "+12 V" power terminal
For vehicles where connecting to the "+12 V" battery terminal or a special "+12 V" power terminal is difficult or impossible, the red clip from the USB Autoscope IV power cable can be connected to one of the vehicle's fuses. Naturally, you should make sure that there is supply voltage at the terminals of this fuse. To check for power, the USB Autoscope IV is equipped with a "12 V" LED indicator located on the front panel, which lights up only when the device is powered.
Not all vehicles have a nominal on-board voltage of 12 V. Trucks and other large vehicles often operate at a 24 V system voltage. However, even in 24 V systems, power terminals with a +12 V voltage are available. This is because such vehicles are equipped with two 12 V batteries connected in series. The nominal voltage of both batteries is 12 V, so the voltage at the terminals of the power cable connecting one battery to the other is 12 V. It is recommended to connect the red crocodile clip from the USB Autoscope IV power cable to one of the terminals of this power cable.
For vehicles with a 24 V on-board voltage where connecting to a "+12 V" terminal is difficult or impossible, it is permissible to connect the red clip from the USB Autoscope IV power cable to a "+24 V" terminal or to a vehicle fuse that has a +24 V voltage present at its terminals.
10. How to obtain synchronization signal from Cylinder #1 on a gasoline engine
The ignition spark is used to obtain synchronization pulses from Cylinder #1 on a gasoline engine. A black high-voltage synchronization probe is used for this purpose. This probe is attached to the high-voltage wire of Cylinder #1. This wire is typically present in classical and DIS ignition systems.
Sync probe attached to the high-voltage wire of Cylinder #1
The connector of the black high-voltage synchronization probe is connected to the "In Synchro" input, located on the front panel of the USB Autoscope IV.
Modern engines are equipped with individual ignition coils, which are almost always located directly above the spark plugs. In this case, the high-voltage wire needed for synchronization is missing. Therefore, an additional high-voltage extension is used to connect the synchronization probe. This extension cord consists of a special high-voltage adapter to which the vehicle's high-voltage cable is connected.
Adapter for high voltage extension cable and ignition wire
One end of the extension is connected to the high-voltage terminal of the ignition coil for Cylinder #1, and the other end is connected to the spark plug it serves. After that, the synchronization probe should be attached to this additional high-voltage wire.
Adapter for high voltage extension cable and ignition wire
On vehicles equipped with ignition modules, the process is similar. However, several high-voltage extensions will be needed in this case. The synchronization probe is attached to the high-voltage wire of Cylinder #1, just as always.
Four adapters for high voltage extension cables and ignition wire kit
If the synchronization signal is taken from a cylinder other than the first, the "Synchro on cylinder" parameter must be correctly specified when starting the signal analysis.
11. Shape of synchronization pulses for a gasoline engine
When performing the CSS test, it is necessary for the synchronization pulses to indicate the top dead center at the end of the compression stroke of Cylinder #1. Near this point, the ignition system of the gasoline engine generates what is called a "working spark." This is why, on spark-ignition engines, the synchronization pulses are obtained from the ignition spark, which is recorded using a high-voltage synchronization black probe. The output signal of this probe is fed to the specialized "In Synchro" input of the device. It differs from other inputs by the presence of a peak detector, which reliably registers short-term peak values of the signal. As a result, the shape of the received synchronization pulses takes a specific shape.
Shape of synchronization pulses on a gasoline engine
Along with the "working spark", the synchronization probe also records the so-called "waste spark", which in DIS ignition systems always alternates with the "working spark". The "waste spark" can also be generated in individual ignition systems in cases where the engine control unit has not recognized the phase sensor signal (CMP). In ignition systems that generate the "waste spark", spark discharges between the spark plug electrodes occur twice as frequently as in normal conditions. Like the "working spark", the "waste spark" is generated by the ignition system near the top dead center. However, unlike the "working spark", the "waste spark" occurs at the end of the exhaust stroke, not the compression stroke.
The amplitude of the synchronization pulses with the moment of occurrence of the "working ignition spark" is almost always higher than the amplitude of the synchronization pulses with the moment of occurrence of the "waste spark". This happens because the amplitude of the synchronization pulse directly depends on the breakdown voltage of the ignition pulse that formed it. The ignition pulse that forms the "working spark" is generated by the ignition system at the moment when the pressure in the cylinder is high (near the top dead center at the end of the compression stroke). And the higher the pressure between the spark plug electrodes, the higher the breakdown voltage of the ignition spark. Therefore, the peak amplitude of the synchronization pulse with the "working spark" of ignition is large.
The ignition pulse that forms the "waste spark" occurs at a time when the pressure in the cylinder is low (near the top dead center at the end of the exhaust stroke). Therefore, its peak amplitude is small. Thus, the useful synchronization pulses have a larger amplitude compared to the amplitude of the synchronization pulses from the "waste spark".
Synchronization probe also records so-called "waste spark"
As mentioned above, for the CSS test algorithm to work correctly, the synchronization pulses must point to the top dead center exactly at the end of the compression stroke. Therefore, it is very important that the synchronization pulses from the "working spark" of the ignition are clearly distinguishable on the recorded waveform against the synchronization pulses from the "waste spark". If their amplitudes are very similar and the useful synchronization pulses are poorly distinguishable, then in this case the synchronization sensor is moved from the high-voltage wire of Cylinder #1 to the high-voltage wire of any other cylinder. It is important not to forget to correctly indicate the synchronization cylinder number at the moment of starting the signal analysis.
In addition to the ignition pulses of the “working” and “waste” sparks, the synchronization probe can also detect interference from the ignition spark in other engine cylinders.
Interference from the ignition spark in other engine cylinders
Most often, these interferences have a small amplitude. If their amplitude is too large and the useful synchronization pulse is poorly distinguishable against their background, then in this case the synchronization probe is moved from the high-voltage wire of Cylinder #1 to the high-voltage wire of any other cylinder. It is important not to forget to specify correctly the synchronization cylinder number when starting the signal analysis.
In many multi-spark ignition systems, a series of impulses is generated at certain engine operating modes instead of a single high-voltage pulse. The first pulse in the series is the primary one and often the most powerful, while the remaining pulses are supplementary. The main purpose of these additional pulses is to prevent misfires in cases where the first high-voltage discharge fails to ignite the air-fuel mixture.
Series of impulses is generated In many multi-spark ignition systems
The first ignition pulse, which actually ignites the mixture in the cylinder, has the highest amplitude among the series of pulses. This happens because the subsequent discharges occur when the cylinder already contains electrically conductive plasma from the flame rather than a fresh air-fuel mixture. Consequently, the breakdown voltage of the additional ignition pulses is automatically reduced.
The control systems of some modern engines, in addition to the main spark discharge, also generate a Post spark, which is supplied to the spark plug at the moment, when the piston has traveled roughly halfway between the top dead center and the bottom dead center during the power stroke. The breakdown voltage of this Post spark is relatively low since the pressure in the cylinder has already decreased by that point.
Post spark generated by control systems of some modern engines
12. How to obtain synchronization signal from Cylinder #1 on a diesel engine
To obtain synchronization pulses from Cylinder #1 on modern diesel engines, electrical pulses sent by the engine control unit (ECU) to the fuel injector of Cylinder #1 are used. This applies to engines equipped with electromagnetic or piezo injectors in Common Rail systems, unit injectors, or PLD sections. It is recommended to capture these pulses using small-sized current clamps, such as the CTi-M or CTi-50. The signal cable connector of the clamps should be connected to Input 4 located on the front panel of the USB Autoscope IV. Only one of the two control wires for the injector should be placed within the jaws of the current clamps.
Small-sized CTi-M current clamps used to obtain a synchronization signal on modern diesel
For diesel engines with mechanical fuel injectors (not equipped with electronic control), synchronization pulses can be captured using a PD-4 or PD-6 piezoelectric transducer (depending on the diameter of the fuel line). These transducers detect the momentary microscopic expansion of the fuel line that occurs during the fuel injection event. The signal from the piezoelectric transducer is pre-processed by a piezo amplifier before being sent to Input 4 on the USB Autoscope IV.
Piezo transducer used to obtain a synchronization signal on diesel engine with mechanical fuel injectors
Some diesel engines with mechanical injectors may feature an important distinction: one injector, typically for a specific cylinder, differs from the others because it is equipped with a needle lift sensor. For such engines, there are two ways to obtain a synchronization signal: Using a piezoelectric transducer (as described earlier) or using the needle lift sensor signal. The signal from the needle lift sensor is preferable because it is significantly less noisy compared to the signal from the piezoelectric fuel line expansion transducer.
To record the signal from the needle lift sensor you should connect a signal cable with a needle probe to Input 4 of the USB Autoscope IV. Insert the needle probe under the seal of the sensor’s connector so that it firmly contacts the terminal of the signal wire without damaging its insulation.
Recording synchronization signal from the needle lift sensor
Not all fuel injection systems of this type have an injector with a needle lift sensor installed in the first cylinder of a diesel engine. In this case, it is important not to forget to specify correctly the synchronization cylinder number when starting the signal analysis.
Be aware that in some engines, the needle lift sensor on the diesel injector does not generate a signal. In such cases, synchronization for the CSS test cannot be performed using this injector.
13. Shape of synchronization pulses for a diesel engine
For the CSS test, synchronization signals must indicate the top dead center (TDC) at the end of the compression stroke for Cylinder #1. In compression-ignition engines, fuel injection occurs near this point. This is why synchronization for diesel engines uses synchronization signals with the fuel injection moment. The method of obtaining this signal depends on the type of fuel injection system. It can be the electrical control signal for the injector of Cylinder #1, a signal from the fuel line expansion transducer for a mechanical injector, or a signal from the needle lift sensor in a mechanical injector. The shape of the synchronization pulses varies depending on the method used to obtain them and the specific design features of the fuel injection system.
Voltage pulses occur at the terminals of the electrical connector of an electronically controlled diesel injector more frequently than this injector produces fuel injections. And for synchronization of the CSS script, pulses are required that indicate only real fuel injections into the cylinder of diesel serviced by this injector. The challenge is that due to the characteristics of the electrical system, voltage pulses appear not only during injections by the this injector but also during fuel injections by injectors for other cylinders in the engine. It should be noted that power is not supplied to the injector constantly, so voltage pulses occur not only at the control terminal, but also at the power terminal of the injector electrical connector.
Voltage pulses at the terminals of the electrical connector of an electronically controlled diesel injector
Voltage waveform on the control terminal of the diesel injector not suitable for use as a sync signal
Obviously, the voltage waveform on the control terminal of the diesel injector is not suitable for use as a CSS script synchronization signal. The voltage waveform on the power terminal is also not suitable.
But still, it is possible to extract a useful control signal from these signals. To do this, it is necessary to record voltage waveform simultaneously from both injector terminals – both the control and the supply terminals, with subsequent use of the software function "Calculate signal difference".
Software function "Calculate signal difference"
In this case, the recording of the voltage difference between the terminals of the injector's electrical connector is simulated. And this difference occurs only when fuel injection is performed by this injector. Therefore, the signal of the voltage difference between the terminals of the diesel injector's electrical connector is suitable for synchronizing the CSS script. However, this method is not recommended as the main one for obtaining a synchronization signal on a diesel engine.
There is a much simpler and more reliable way to obtain a synchronization signal on diesel engines with electronically controlled injectors - this is the use of small current clamps to obtain current pulses in the electrical circuit of the injector. Unlike voltage pulses, a current pulse in the circuit of an electronically controlled diesel injector occurs only when the control unit gives a command to open the control valve of this particular injector. Therefore, the injector control current is a reliable source of synchronization signal when testing diesel engines equipped with electromagnetic or piezo injectors of the Common Rail system, or electronically controlled pump injectors or PLD sections.
The shape of the injector control current pulses is determined by the type of control valve, which can have either an electromagnetic or piezoelectric drive, as well as other features of the engine management system.
Shape of the injector control current pulses is determined by the type of control valve
Both types of current pulses are suitable for synchronizing the CSS script.
On modern diesel engines, the required dose of fuel is injected into the cylinder not at once, but in several portions one after another. The last portion is the largest, and is called the "main injection". All the portions preceding it are called "pre-injection". On the injector control current waveform, this looks like a series of pulses, the last of which is longer than all the previous ones.
Injection of required dose of fuel on modern diesel engines
In some diesel engines, "Post Injection" is used—after the main injection during the power stroke, one or more small amounts of fuel are injected into the cylinder. The presence of such pulses does not disrupt the synchronization of the CSS script.
In the normal operation of a diesel engine, fuel is injected into the cylinder at the moment when compressed and highly heated air is present there. This allows the fuel to self-ignite, producing combustion that powers the engine. However, when the engine control unit switches to the mode of cleaning the diesel particulate filter (DPF), additional portions of fuel are injected into the cylinder, but they do not ignite or burn. This happens because the fuel is injected during the exhaust stroke, when only exhaust gases at low pressure are present in the cylinder. As a result, self-ignition does not occur, and the fuel is expelled along with the exhaust gases into the exhaust system. The fuel self-ignites only after it reaches the heated DPF filter, burning off soot and thereby cleaning the filter. The presence of DPF cleaning pulses does not interfere with the synchronization of the CSS script.
In some types of diesel engine control systems, the control unit may send very short pulses or series of pulses to the control valves of all injectors, which are not synchronized with the positions of the engine's shafts. However, this does not result in fuel injections into the diesel engine's cylinders. This occurs because the pulses are so short that during their action, only the injector's control valve moves, while the needle of the nozzle does not rise. In this way, when necessary, the control unit reduces fuel pressure, allowing part of the fuel to flow from the high-pressure fuel rail through the injectors to the return line. This method of regulating fuel pressure is used in systems equipped with only a metering valve and not equipped with an electronically controlled relief valve to release fuel from the high-pressure rail into the return line.
ECU may send very short pulses or series of pulses to the control valves of all injectors
In all the cases described above, the CSS test automatically recognizes the pulses of the main diesel fuel injection and synchronizes correctly.
In diesel engines with mechanical fuel injectors, synchronization is performed using a piezoelectric transducer that detects the expansion of the high-pressure fuel line supplying fuel to the injector of Cylinder #1. The output signal of this transducer is not voltage or current but charge. Therefore, the signal from such a transducer firstly goes to a charge amplifier, where it is converted into voltage, and only then is it fed into the oscilloscope.
Synchronization using a piezoelectric transducer in diesel engines with mechanical fuel injectors
It is worth noting that this synchronization method is not sufficiently reliable in two ways. First, piezoelectric transducers are mechanically fragile and can easily break. Second, the signal from the piezoelectric expansion transducer, even after passing through the charge amplifier, often remains so noisy that it cannot serve as a reliable synchronization signal. This is because the sensor not only detects the expansion of the fuel line to which it is attached but also picks up all mechanical vibrations and sounds acting on that line. To reduce noise levels, the high-pressure fuel line for Cylinder #1 can be temporarily freed from its standard clamps that mechanically link it to the fuel lines of other cylinders during the test. This reduces the transfer of pulsations from the fuel lines of other cylinders but does not always achieve the desired result. Other noise reduction measures are not possible here.
It should be noted that the signal from the piezoelectric expansion sensor of a diesel's high-pressure fuel line has a rather specific waveform and contains a lot of additional information that can, to some extent, indicate the condition of the diesel's mechanical fuel system components. This diagnostic method was used when such diesel engines were more common.
In diesel engines equipped with electronically controlled fuel systems and mechanical injectors, it is recommended to use a needle lift sensor, built into one of the injectors, as the source of the synchronization signal if such a sensor is available and functioning properly. Otherwise, a piezoelectric transducer that detects the expansion of the high-pressure fuel line can be used for this purpose.
The needle lift sensor generates two electrical voltage pulses of opposite polarity: the first at the start of fuel injection and the second at the end of fuel injection.
Synchronization using a needle lift sensor signal
Please note that the injector with the needle lift sensor is not always installed on the first cylinder of the diesel engine in all injection systems of this type. Therefore, it is important to correctly specify the synchronization cylinder number when initiating signal analysis.
14. Selecting the device operating mode before conducting the test
3 preconfigured modes for the CSS test
15. How to perform a CSS test on a gasoline car
After completing all necessary connections and turning on the device in CSS mode, follow these steps for a gasoline engine:
- start the engine and wait for the idle speed to stabilize
- in the USB Oscilloscope program window, start signal recording
- after a few seconds, gradually increase the engine speed to 3000 RPM
- release the accelerator pedal abruptly and wait for the idle speed to stabilize
- press the accelerator pedal sharply, and upon reaching 3000…4000 RPM, turn off the engine while keeping the accelerator pedal completely pressed until the engine comes to a complete stop. It is crucial to ensure that the throttle remains wide open during this process
- stop the signal recording
16. Why the sequence of actions in the CSS test is important
The recommended sequence of actions during a CSS test ensures that the maximum amount of information about the efficiency of the engine's cylinders is captured in the shortest possible time. Typically, the test takes no more than 1 minute, and each step in the sequence has its own specific purpose.
The first deceleration (last for diesel engines)
Although it may not be obvious, the most critical step in the CSS test is the deceleration of the engine after the first smooth throttle (or the last for diesel engines). This step is crucial because the analysis algorithm functions correctly only if, during this deceleration, the minimum possible amount of air enters the engine's cylinders. This effect is achieved because, after the throttle is released, the engine speed remains high and the engine continues to rotate by inertia while the throttle valve is closed. As a result, little to no air enters the cylinders, and combustion does not occur.
In this section, the CSS test algorithm examines the engine rotation pattern under conditions where there is almost no air in the cylinders and no ignition. It serves as a model of crankshaft rotation with no jolts in the cylinders. This is where the so-called "calibration" of the CSS test is performed.
"Calibration" of the CSS test is very important, as it affects the reliability of the cylinder efficiency display at all other stages of the test. The quality of calibration is affected by the speed and completeness of the throttle valve closing at the end of the snap throttle.
Last RPM reset
Let's consider the differences between the last RPM reset and the first one (valid only for spark ignition engines). During the last RPM reset there is also no ignition of the air-fuel mixture in the cylinders, because the engine is forcibly stopped. But there is a very significant difference from the first reset: during this measurement phase, the throttle is maximally open and does not prevent air from entering the cylinders. Thus, compression and decompression of the freely entering air takes place in the cylinders, but there is no ignition of the air-fuel mixture. At the same time, whenever the air compressed in the cylinder will work like a spring: as soon as the piston passes the top dead center, this pre-compressed air starts pushing the piston toward the bottom dead center, accelerating the engine crankshaft. This section is suitable for evaluating the performance of the mechanical part of the engine only, since here neither the fuel supply system nor the ignition system has any influence on the engine rotation, or the ignition system. Here the so-called “dynamic compression” is checked.
The accuracy of the dynamic compression display is influenced by the correct calibration of the CSS test.
Snap throttle
Snap opening of the throttle valve during the last snap throttle will cause ignition system malfunctions to manifest themselves (only applies to spark ignition engines). As soon as the throttle valve is fully open, it no longer restricts the airflow into the engine cylinders. The cylinders' air intake reaches its maximum, causing the peak pressure in the engine cylinders to rise sharply. This means that when the ignition impulse is applied, the air-fuel mixture between the spark plug electrodes will also be under high pressure. This circumstance is aggravated by the fact that, unlike in other engine operating modes, the ignition advance subsystem generates spark discharge during snap throttle not before the peak pressure in the cylinder (when the pressure has not yet reached its maximum), but practically at the moment when the maximum pressure is reached. This in turn increases the load on the ignition system.
The point is that the spark discharge breakdown under conditions of increased pressure requires an ignition pulse with increased breakdown (peak) voltage. And the increased voltage in turn increases the requirements to the quality of electrical insulation of all components of the secondary circuit of the ignition system. As we know, electric current always “seeks” the path with the lowest electrical resistance. And, if the ignition system is defective, the current in the secondary circuit of the ignition system “finds” an alternative path: in the place where the electrical strength of the high-voltage insulation is broken, the spark discharges right there instead of between the spark plug electrodes. In this case, the spark discharge no longer occurs between the spark plug electrodes, and the air-fuel mixture is not ignited. In other words, a misfire occurs due to an ignition failure.
It should be noted that under such conditions, the cylinder's performance is reduced not partially but completely, as the ignition spark cannot partially ignite the air-fuel mixture—the mixture either ignites or it doesn't.
Thus, ignition system malfunctions become evident during snap throttle.
Smooth throttle
The section at the beginning of smooth throttle best reflects the accuracy of the fuel supply system, because it is here that the dirt / wear of fuel injectors is most clearly manifested, resulting in the fact that in some cylinders of the engine a large dose of fuel is injected, and in others - a smaller one. This occurs because the fuel injection doses are minimal here, and when the fuel injector is dirty or worn, the accuracy of small fuel doses is most affected.
Idle speed
The uniformity of cylinder operation at idle speed primarily depends on dynamic compression and the cylinder-to-cylinder uniformity of fuel injection. It can also be influenced by air leaks into the intake manifold and other factors. Of course, if no spark or fuel is supplied to any of the cylinders, that cylinder will not function at idle speed either.
17. How to perform additional manipulations during the CSS test
Sometimes situations arise where it is necessary to extend the standard methodology of the CSS test. For example, it may be required to check how the cylinders respond to sudden enrichment of the air-fuel mixture. Typically, to perform such a check, a small dose of any gaseous or liquid easily flammable fuel is injected into the intake path of an engine running at idle speed, and the engine's reaction is observed. If such (or any other) check needs to be performed as part of the CSS test, the following recommendation should be followed: any additional manipulations must be carried out strictly between the first and last throttle! That is, for the example under consideration, injection of an additional fuel dose can be carried out after the first throttle while the engine is idling, after which a second throttle is carried out following the recommendations of the standard methodology.
If, during additional manipulations on a spark ignition engine, the engine stalls, it is not critical. The important thing is that information will be recorded about the response of the engine cylinders to the additional manipulation. But it will not record information about the condition of the ignition system and dynamic compression in the engine cylinders. In this case, to evaluate these parameters, it will be necessary to repeat the CSS test, but this time strictly according to the standard methodology.
18. How to perform the CSS test on a gasoline engine with an electronic throttle
On modern cars with spark ignition engines, instead of a mechanical connection between the accelerator pedal and the engine's throttle valve, an electronic pedal and electronic throttle control are used. When performing the CSS test on such vehicles, during the final stage of measurement, the throttle may close even though the accelerator pedal is held down (this depends on the model of the car and engine). In this case, the CSS test will not be fully completed, specifically; dynamic compression in the engine cylinders will not be measured. If the task is to check compression using the CSS test, then in such a case, to shut down the engine, instead of turning off the ignition, it will be necessary to interrupt the power supply to the fuel injectors or fuel pump.
To interrupt the power supply, it is recommended to use a remote fuse switch. The location of the required fuse in the car should be identified, and a remote switch should be pre-connected in place of the fuse. In this case, at the end of the test, the engine must be shut down not in the usual way, but by pressing the button on the remote fuse switch.
19. How to perform a CSS test on a vehicle where the CKP signal disappears when the ignition is turned off
On some vehicles equipped with a crankshaft position sensor (CKP) with a dual-level output signal, during the final measurement stage, immediately after shutting down the engine at high RPMs, the CKP signal may disappear even before the engine comes to a complete stop. In such cases, the CSS test will not be fully completed: the dynamic compression in the engine cylinders will not be measured on a spark-ignition engine. If the technician needs to specifically check compression using the CSS test, it is recommended to use a remote fuse disconnect switch to shut down the engine. For more details, see section 18.
20. How to perform a CSS test on an older vehicle without a CKP sensor (and on vehicles where connecting to the CKP signal is difficult, as well as on engines equipped with transformer-type CKP sensors)
On vehicles where connecting to the CKP signal wire is difficult or impossible (e.g., due to its absence in older engines), the signal from an external crankshaft position sensor can be recorded instead of the standard CKP sensor signal. For this purpose, it is recommended to use an external optical crankshaft sensor, such as the Laser sensor. During the test, the beam of this sensor should be directed at optical marks that have been pre-applied with white paint on the crankshaft pulley, simulating the teeth of a trigger wheel.
Use an external optical Laser crankshaft sensor
21. How to perform a CSS test on a diesel engine
The procedure for performing a CSS test on diesel engines differs from that for gasoline engines. After making all necessary connections and turning on the device in CSS Diesel mode, the recommended steps are as follows:
- start the engine and wait for idle speed to stabilize
- in the USB Oscilloscope program window, start recording signals
- after a few seconds, smoothly increase engine speed to 3000 RPM
- release the accelerator pedal sharply and wait for idle speed to stabilize
- press the accelerator pedal sharply again, and upon reaching 3000–4000 RPM, shut off the engine by turning off the ignition. At this point, the throttle valve will automatically block air intake to the manifold
- stop recording signals
For diesel engines where the throttle valve does not close during the final measurement stage, or for engines without a throttle valve in the intake manifold, the air supply to the intake manifold must be manually blocked to shut off the engine during the final measurement stage. It is crucial to keep the air supply blocked until the engine comes to a complete stop.
22. How to save recorded signals to a file
As soon as signal recording is stopped, the USB Oscilloscope program automatically saves the data in a temporary file and immediately displays the signals on the screen. If you believe these signals may be needed later (e.g., for your personal archive or for publication on the Internet), it’s best to save them to a file right away. To do this: In the USB Oscilloscope program window, open the menu "File => Save As". Specify the location and filename for saving.
However, analysis of the signals can execute without first saving them to a file, as the tests can also work directly with the temporary waveform file. This means you can select "Analysis => Execute script" immediately after stopping the recording.
When analysis starts, a "Configuration Window" will appear. In this window, you need to specify the parameters of the vehicle being tested. The primary parameter is the "Firing Order". If the synchronization signal was not taken from the first cylinder, you must correctly specify the parameter "Syncro on cylinder".
All parameters entered in the configuration window can be embedded into the waveform file. This ensures that when reopening the file and launching its analysis later, the configuration window will already be preconfigured. To save these parameters, either: Open the menu "File => Save As" and confirm with "Yes," or Simply choose "Yes" in the dialog box "Do you want to save the current file or changes made?" when closing the file.
Note: If the test cannot analyze the recorded signals for any reason, always save the signals to a file and share them on the forum thread discussing this test. This practice helps developers collect materials necessary for improving test algorithms. A link to the CSS test discussion thread is provided below.
CSS Script forum discussion23. Reliability of CSS test results
The reliability of results obtained using the CSS test can vary. It depends on several factors, primarily the quality of the crankshaft position sensor (CKP) signal. This signal contains the crucial information about the nature of changes in the engine's instantaneous rotational speed. Not only do noise levels and interference affecting the signal play a role, but other factors also influence the accuracy of CSS test results, as listed below:
Number of teeth on the CKP sensor toothed disc
The detail of the data regarding the engine's instantaneous rotational speed depends on the number of teeth on the trigger wheel and their even distribution across its working surface. The configuration of the disk teeth determines the detail of the information obtained by the CSS test, as well as the uniformity of the flow of this information. In any case, to achieve acceptable accuracy in CSS test results, the number of teeth on the CKP sensor's toothed disc should not be less than the number of cylinders in the engine being tested.
Rigidity of the crankshaft position sensor toothed disc mounting
Another less obvious criterion for the quality of the CKP sensor signal is the rigidity of the mechanical connection between toothed disc and the crankshaft (or, more precisely, the flywheel). For the CSS test, this rigidity may be insufficient even in engines where the trigger wheel is directly attached to the crankshaft. Insufficient rigidity increases the error in the test results. This error is most noticeable in engines where toothed disc is mounted on the front end of the crankshaft (opposite the flywheel) and becomes more pronounced as the physical length of the crankshaft increases. This is especially evident in inline six-cylinder engines.
This error occurs because the worse the stiffness of the disk mount, the stronger the micro vibrations of the disk in relation to the crankshaft. These vibrations are mistakenly interpreted by the CKP sensor as changes in the crankshaft's instantaneous rotational speed, leading to incorrect interpretation by the CSS test. The issue is particularly severe in vehicles where the toothed disc is equipped with a rubber damper, which significantly amplifies these micro-vibrations.
It is important to note that the issue of toothed disc mounting rigidity only manifests at high RPMs and is almost negligible at low RPMs. Therefore, if difficulties arise due to this issue, performing throttle at lower RPMs is usually sufficient. The RPM threshold above which rigidity issues may occur varies for each engine.
Number of engine cylinders
In addition to the quality of the CKP signal, the number of engine cylinders also affects the reliability of CSS test results. This is due to the mutual influence of cylinder operations on each other. For example: in 4-cylinder engines, the power stroke of one cylinder does not overlap with the power stroke of another. In 5-cylinder engines, such overlap is also absent, as the effective phase of a cylinder’s power stroke ends well before the piston completes its full stroke. In 6-cylinder engines, the influence of one cylinder on another is practically negligible. However, in engines with 8 or more cylinders, mutual influence between cylinders becomes apparent. This makes it difficult to clearly isolate the contribution of one cylinder to the engine’s rotational speed from that of adjacent cylinders in the firing order. This occurs because the next cylinder in the firing order begins its power stroke before the previous one has fully completed its stroke. For this reason, multi-cylinder engines are sometimes tested using the CSS method in two stages: First, one half of the cylinders is tested while the other half is disabled. Then, the second half of the cylinders tested while the first half is disabled. For example, a 12-cylinder engine is tested as if it were two 6-cylinder engines.
