Electronic fuel pump

You will require a PicoScope to perform this test. A list of suitable accessories can be found at the bottom of this page.

How to perform the test

  1. Connect the low current amps clamp to Channel A on the PicoScope.
  2. Ensure the clamp is switched on and the 20 A range is selected if applicable. Press the 'zero' button before connecting the clamp to the circuit.
  3. Due to the low current in the fuel pump circuit, it is important that no other electrical loads are connected that might affect the waveform. Place the current clamp on the fuel pump's supply wire (as illustrated in Figure 1) if possible, or bridge the fuel pump relay with a fused 'jump wire' and monitor the current on this wire, or remove the fuel pump fuse and monitor the current in a wire bridging the two terminals.

Example waveform

Waveform notes

The waveform shows a current pulse for each sector of the commutator. The majority of fuel pumps have 6 to 8 sectors—our example has 8. A repeated feature on the waveform can indicate wear and an impending failure. Our waveform shows a lower current draw on one sector and this is repeated when the pump has rotated through 360°.

The current drawn by the fuel pump depends on the fuel pressure but should be no more than 8 amps, as found on the Bosch K-Jetronic mechanically regulated fuel injection system which has a pressure of 75 psi.

Technical information

This type of high-pressure fuel pump is called a roller cell pump. The fuel enters the pump and is compressed by rotating cells that force it through at high pressure. The pump can produce a pressure of 8 bar (120 psi) with a delivery rating of approximately 4 to 5 litres per minute. Inside the pump is a pressure relief valve that lifts off its seat at 8 bar if the fuel flow becomes blocked by an obstruction in the filter, fuel lines or elsewhere. The other (output) end of the pump is home to a non-return valve. When the voltage to the pump is removed, this value closes the return to the tank and maintains pressure in the system.

The normal operating pressure in this system is approximately 2 bar (30 psi), at which the pump draws 3 to 5 amps. Fuel passing across the pump's armature is subject to sparks and arcing, but this is not as dangerous as it sounds because the absence of oxygen means that there will not be an explosion!

The majority of fuel pumps on today's vehicles are fitted inside the petrol (gasoline) tank and are referred to as submerged fuel pumps. The pump is typically next to the fuel sender unit and both units can sometimes be accessed through an inspection hole either in the boot floor or under the rear seat.

Mounted vertically, the pump comprises of an inner and an outer gear assembly called the 'gerotor'. The combined assembly is secured in the tank with screws and sealed with a rubber gasket, or with a bayonet-type locking ring. On some models Volvo fit two fuel pumps, the submerged pump acting as a 'lift' pump to the external roller cell pump.

Modern Fuel Pump System

Figure 4 - Shows a modern system in which the Electronic Control Module (ECM) controls the earth path of the fuel pump relay

AT032-3

Disclaimer
This help topic is subject to changes without notification. The information within is carefully checked and considered to be correct. This information is an example of our investigations and findings and is not a definitive procedure. Pico Technology accepts no responsibility for inaccuracies. Each vehicle may be different and require unique test settings.

Suitable accessories

  • 30 A (low amps) current clamp

    £259.00

  • 20 A / 60 A DC (low amps) current clamp

    £159.00

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Guided test: Fuel Pump