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Testing Electric Fuel Pumps

What Technicians need to know when testing fuel pumps and related circuits when suspecting a fuel system fault.

There are of course many electrical circuit or fuel pump causes of “low or nil” fuel pressure or flow. Understanding the simple electrical control and load circuit types will assist the technician in quickly and accurately pinpointing the test procedure required for that specific circuit.

Typical PCM positive relay control. (Power side switched) e.g. VE Commodore – Pre SIDI

On this system, the PCM will activate the relay with a power supply to the relay control circuit.

Testing this Circuit:

  • Removing the pump relay and utilising a voltmeter or test light to test for 12V power supply whilst cranking or by using a suitable scanner to command PCM to operate the relay. (Pin 85 relay socket position shown on this image)
    • No signal – would require further investigation of wiring from the PCM and if necessary, the PCM itself.
    • Correct signal – confirms that this section of the control circuit is not the cause of any inoperative fuel pump.
  • The Earth condition must be tested to ensure relay activation. (Pin 86 relay socket position shown on this image)

Testing Load Circuit:

  • A fused power supply is present at the load circuit terminal of the relay whilst the ignition is switched on. (Pin 30 relay socket position shown on this image)
    • This power supply to the fuel pump is interrupted of course by the relay when not activated.
    • This power supply can be tested by a multimeter, generally during cranking to verify the Integrity of the power supply as there are multiple possible areas for voltage drop creation.
    • The load circuit from the relay to the fuel pump may be tested with a “known working” relay refitted to the pump relay socket and testing for correct power supply to the fuel pump.
    • This needs to be tested at both the relay load circuit position as well as the fuel pump. (Pin 87 relay socket position wiring shown on this image)

Note:  there are areas where interruption to the circuit may be created and the circuit integrity must be validated.

Typically: Hitting a large pothole or being involved in a vehicle accident may trigger the Inertia switch which Isolates the fuel pump from the relay if fitted. (for safety reasons)

This inertia switch is generally able to be simply reset. Incorrectly fitting this unit or a loosely mounted unit may intermittently trigger and interrupt the load circuit.

  • If a suitable power supply is evident after the inertia switch and at the fuel pump connection, then further Investigation is required for fuel pump electrical and mechanical integrity and finally – ground circuit suitability at chassis or in PCM.

Typical PCM ground relay control (Ground side switched) e.g. – Ford FG Falcon with inertia switch.

On this system, the PCM will activate the relay by grounding the control circuit. The power supply to the control circuit, generally originates from a suitable relay.

Testing this Control Circuit.

  • Removing the relay and utilising a multi meter to check the supply from the control relay during cranking ensures power supply integrity. (Pin 1 position at relay shown on this image)
  • Activating the relay with a suitable scanner or cranking the engine should result in a ground circuit within the PCM for control wire from Pin 4 relay location shown on this image.
    • No ground signal – would require further investigation of PCM or related connector.
    • Correct ground signal – Confirms that this section of the control circuit is not the cause of any inoperative fuel pump.

Testing Load Circuit:

  • Ensure a fused power supply is present at the load circuit terminal of the relay whilst the ignition is switched on. (Pin 3 position at relay shown on this image)
  • Similar to the previous system, the power supply to the fuel pump is interrupted by the relay when not activated. This image also includes the inertia switch which again must be validated by testing the power supply from Pin 2 shown from this image and ensuring a similar power supply is available at the fuel pump.

Note: the test procedure for this load circuit is similar to the previous system.

Testing Fuel Pump Integrity with an Oscilloscope and Amp meter.

It is important to identify if the vehicle being tested has a standard constant voltage fuel pump circuit or is fitted with a fuel pump driver module prior to carrying any current flow testing. A fuel pump driver module with pulsed width modulated output will of course vary the indicated circuit current flow as the pulsed output varies. These fuel test expectations will vary compared to the standard fuel pump constant power supply system.

Why test the pump circuit current flow?

Carrying out a fuel pressure test on a vehicle will indicate if the fuel pump can supply the fuel pressure required for basic system operation during the test, but not the condition of the fuel pump. A fuel pump that is problematic may still supply the required fuel pressure but may be operating with a higher current draw due to internal issues that could indicate possible reduced fuel flow and possible future fuel pump failure. Any armature and/or commutator issues may also be visible on the oscilloscope.

Note: Be aware that fuel pressure is directly related to current flow and fuel flow. That is – If the fuel pressure has been increased, the current flow also will increase, and the fuel flow will “decrease.” This should be taken into consideration when carrying out relevant tests.

Note the increase in current flow (as the result of fuel pressure increase) and the fuel flow decreases to levels that could potentially cause major engine damage.

The Premier Auto Trade Fuel range includes almost 800 Fuel Pumps from the world’s leading manufacturers, covering over 12 million vehicle applications in Australia and NZ.

When you supply and fit products from Premier Auto Trade you can expect a product designed and tested to the vehicle manufacturer’s specifications offering OE form, fit and function. Premier Auto Trade distributes products throughout Australia via a network of specialised resellers and leading automotive groups.



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