Safety Warning
DIY auto repair can cause serious injury, fire, or vehicle damage. These guides are for informational purposes only. Always follow OEM torque specs, wear PPE, and consult a certified mechanic if you are unsure. You are solely responsible for your safety.
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HOMESERVICE MANUALSGMC1991FORWARD CONTROL 5.7 K, AUTOMATICREPAIR AND DIAGNOSISENGINE PERFORMANCESYSTEMENGINE CONTROLS - SYSTEM/COMPONENT TESTSRELAYS, SOLENOIDS, MOTORS & MODULESRELAYSFUEL PUMP RELAY
1991 GMC Forward Control 5.7 K, Automatic
Fuel Pump Relay
1991 GMC Forward Control 5.7 K, AutomaticSECTION Fuel Pump Relay
- If a prolonged crank is required to start vehicle, fuel pump relay may be faulty. To verify this, start engine. With engine running, disconnect oil pressure switch (fuel pump back-up circuit). If engine stalls, fuel pump relay is faulty. If vehicle continues to run, relay is okay. Check for other causes of prolonged crank.
- To test fuel pump relay, disconnect fuel pump relay. Refer to COMPONENT LOCATIONS at end of article. Apply battery voltage and ground to fuel pump relay winding terminals. To identify fuel pump relay terminals, see WIRING DIAGRAMS article in this section.
- Using an ohmmeter, check continuity between fuel pump relay power and fuel pump relay drive terminals. Continuity should exist. If continuity does not exist, fuel pump relay is defective.
- To by-pass fuel pump relay on vehicle (fuel pump not operating), turn ignition off. Disconnect fuel pump relay connector. Using a fused jumper wire, connect fuel pump test connector to positive side of battery. Fuel pump should run.
- If fuel pump runs, check for faulty connections to relay or replace defective relay. To locate fuel pump test connector, refer to COMPONENT LOCATIONS at the end of this article.
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When to See a Mechanic
Stop DIY work and contact a certified mechanic immediately if any of the following apply:
- • You smell fuel, burning insulation, or see smoke.
- • Brakes feel soft, pull hard to one side, or make grinding noises.
- • The engine overheats, stalls repeatedly, or misfires under load.
- • You are missing required tools, torque specs, or safe lifting equipment.
- • You are not confident in the next step or safety outcome.