Fuel Pump: Testing and Inspection

Testing fuel pump

Test Data:

Special Tools:

Commercially Available Testers:

Shop Made Tools:

Note: The fuel pump must run audibly for a short time (approx. 1 second) when the ignition is switched on.


A. Delivery




1 Detach fuel pump relay module. Multimeter set to amperes. Test current consumption at contact 7 (terminal 87) and contact 8 (terminal 30).

Current consumption 6 - 10 amperes.

If the readout is greater than 10 amperes, proceed as follows:

a) Vehicle with Catalytic Converter (up to 01/90)



Detach heated oxygen sensor heating coil plug connection (G3/2x1) and repeat measurement. Located in right footwell (arrow), as of 09/89 next to battery.

Test current consumption of heated oxygen sensor, 0.5 - 1.3 A; test heated oxygen sensor if necessary.




2 Detach wiring from multimeter. The delivery of the fuel pump should be tested in the fuel return. Release fuel pressure in fuel tank by removing filler cap. Unscrew fuel return line (51) at the diaphragm pressure regulator and plug.




3 Screw shop-made fuel hose onto diaphragm pressure regulator and hold in measuring glass or beaker.




4 Bridge contacts 7 and at fuel pump relay module. This supplies voltage to the fuel pump.

Arrangement of fuel pump relay module in right of component compartment. Engine 102.961: in front of electrical center (see ill.).

Detach bridge after 40 seconds or before measuring glass/beaker runs over.

Specification: 1 liter in maximum 40 seconds at a minimum voltage of 11.5 volts at the fuel pump.

5 If the delivery is less than 1 liter in 40 seconds, test the following points:
a) Test voltage at the fuel pump. Specification at least 11.5 volts.
b) Check that strainer in supply line fitting of fuel distributor is not blocked.
c) Check fuel lines for restriction points (pinched lines).
d) Clamp off leak line between fuel accumulator and fuel suction hose. Re-check fuel delivery. If the specified delivery quantity is reached, replace fuel accumulator.
e) Replace fuel filter.

6 If the delivery is still insufficient, replace fuel pump.

7 Connect fuel return hose, install fuel pump relay module.


B. Fuel pressure

1 Take off protective box.




2 Unscrew cap nut (arrow) at fuel pump (M3/1). Connect fitting, Part No. 103 589 02 63 00 and pressure gauge, Part No. 103 589 O0 21 00.

3 Detach fuel pump relay module and bridge contacts 7 and 8 (terminals 30 and 87).








4 Take reading on pressure gauge. The pressure must be between 2 - 4 bar. If it is below 2 bar, the fuel pump (M3/1) is defective. If it is above 4 bar, the fuel pump (M3/2) is defective.

5 Disconnect pressure measuring device, connect cap nut, install fuel pump relay module and check for leaks.

6 Install protective box.