Application:
- Fuel pump
- Fuel priming pump for a diesel engine
Voltage and current waveforms of a properly functioning fuel pump. Engine start and running at idle.
- 1 – supply voltage
- 2 – current of the fuel pump
Voltage and current waveforms of a malfunctioning fuel pump. Engine start and running at idle.
- 1 – supply voltage
- 2 – current of the fuel pump
Examples of current waveforms of malfunctioning fuel pumps with poor contact between brushes and commutator. Engine is running at idle.
Voltage and current waveforms of a malfunctioning fuel pump, the armature is seizing. The current going in to the pump increases and results in overload, burned relays and connectors as well as fuses blowing.
- 1 – supply voltage
- 2 – current of the fuel pump
- A – engine start
- B – point in time where the fuse blew
Using a fuse adapter usually makes it easy to measure current from the fuel pump. Is there a purpose in measuring fuel pump current? The fuel pressure and/or a restricted fuel filter cannot be accurately determined based on current consumption because different fuel pumps have different power requirements. Even if the current consumption data is inaccurate, it can still be useful for giving an initial direction in the troubleshooting process. Here we are going to go one step further and compare the different values obtained when measuring "direct" vs. "alternating" currents.
Most all fuel pumps have a commutator type motor, which has brushes that will intermittently lose contact with the armature as the brushes move from one commutator bar to the next.
We cannot readily look inside the motor to determine the degree of wear of those brushes. Worn out brushes are often diagnosed as a failure of the pump mechanical parts, such as the impeller. The actual failure is technically unimportant since the fuel pump will need to be replaced in either case.
Since there is intermittent brush contact with the armature, current consumed by the fuel pump motor will be in the form of an uneven or pulsating DC current. As the brushes and/or the commutator wears out, the contact will gradually get worse, with more and more uneven current. As the amount of ripple current increases, the steady or DC current part of the total current will decrease. A fuel pump consumes a certain amount of power to pressurize the fuel. Where the brushes do not make contact, the motor will slow down and where contact is re-established, the motor will speed up.
This increases wear and tear on the commutator segments that do make contact, causing ever larger areas of poor contact. Eventually the flow and/or pressure of the fuel pump will be affected. Poor contact zone will increase, reducing the flow rate and pressure of the fuel pump.
If the fuel pump stops on a commutator segment that does not make electrical contact, the pump will not restart. This occasionally happens when a vehicle with a worn fuel pump is shut off. The vehicle will not restart. A common “test” is to knock or bang on the fuel tank while attempting to start the vehicle. The vibration may move the brushes or the commutator just enough to re-establish electrical contact. If the vehicle restarts, it is assumed that the problem is the fuel pump, which it often, but not always, is.
A very common situation that many have experienced is this one: A no-start vehicle is towed in. Upon initial diagnosis, the vehicle starts and runs with no problem detected. This scenario may or may not be connected to the fuel pump since the vibration from being towed in may have affected any number of electrical connectors, modules, or relays. One simple method that can be used to verify is as follows: Use a fuse adapter and insert in the fuse box instead of the fuel pump fuse. Connect your DMM set to the 10A scale or your current probe to the adapter and proceed to start the engine. Measure both the AC and the DC component.
If the AC current value is approximately 10% of the DC value or less, then the brushes and/or the commutator in the fuel pump is in good condition. If the AC current is approximately 50% of the DC current or more the fuel pump needs to be replaced.
An oscilloscope image showing the current waveform from a pump with worn brushes
An oscilloscope waveform taken from a fuel pump with severely worn brushes. Notice that the DC and AC component as shown on a current clamp is almost equal at 2.3 and 2.5 A respectively.
Start up current waveform from a good fuel pump
A good fuel pump will have a fairly substantial «inrush» current on initial start up. This is because a stalled armature basically represents an electrical short. As the armature starts to move, electrical current is induced in to the windings. This is because the windings move in a magnetic field. The induced current is in the opposite direction of the applied current and will counteract or «fight» the incoming current. The counteraction causes the current consumption to go down to a value that depends on the load applied to the pump motor. The load is heat, friction, and the actual work the m otor is doing. The «humps» are caused by the commutation where the brushes move from one bar to the next.
Start up current from a fuel pump with too high current consumption
If the fuel pump is experiencing high friction from, for example, worn out bearings, the steady state (where the motor has settled down) will be higher than normal. The actual specification will vary some, depending on the fuel pump type and the normal system pressure. Higher system pressures will cause higher current draw. It is advisable to test several good fuel pumps to get a feel fro what normal values are.
Contact problems due to worn commutator and/or brushes
Severely worn brushes and/or commutators can show as intermittent glitches in the waveform. Please note that fuel pumps can be equipped with an axial or a radial commutator. The radial commutator design tends to show a “dirtier” waveform.
The armature is not firmly connected to the impeller
This waveform shows loss of mechanical connection between the electrical motor and the impeller.
The fuel pump is experiencing vapor lock
If the intake strainer (sometimes called a sock) is plugged, the fuel pump impeller may start to cavitate where the fuel is literally evaporating at the impeller. The low pressure caused by the plugged intake is adding to the low pressure behind the impeller vanes, causing the fuel to «boil».
A stalled or seized fuel pump. Waveform is taken as the starter is operated
The fuel pump may stall or have stalled. This could be caused by debris in the fuel tank such as rust particles, or a failure of the pump itself, such as the impeller breaking apart. A current waveform will clearly show this issue. The steady state current consumption of a stalled pump will be very similar to that seen as inrush current when an operational fuem plump starts up. The current will be 2 – 3 times that of normal running current.
The current ramping technique can be used to check just about all electric motors that utilize commutators and brushes.
