GM/Detroit 6.5L ENGINE CONTROLS – Fuel Pump Relay Circuit Diagnosis

Refer to Fuel Pump Controls

Circuit Description
When the key is turned ON, the PCM energizes the fuel pump relay which sends voltage to the lift pump. The lift pump remains ON during the glow plug cycle. If the engine is not started immediately after the glow plug cycle, the PCM shuts the fuel lift pump OFF and waits until the engine starts. As a backup system to the fuel pump relay, the fuel lift pump can also be turned ON by the fuel pump/oil pressure switch. When the engine oil pressure reaches 28 kPa (4 psi), and the fuel pump relay does not complete the circuit, the fuel pump/oil pressure switch will close and complete the circuit to run the fuel lift pump.

Diagnostic Aids
Possible long crank times or a poor performance problem will exist.

Test Description
Number(s) below refer to the step number(s) on the Diagnostic Table.
2. Checks for proper operation of the fuel pump.
3. Checks the operation of the fuel pump by supplying 12 volts to the fuel pump control circuit.
5. Check the battery feed circuit to the fuel pump relay.
10. Checks for proper operation of the fuel pump relay.


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