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 TESTSFUEL SYSTEM (DIESEL GLOW PLUG SYSTEM DIAGNOSIS)GLOW PLUG CONTROLLER
1991 GMC Forward Control 5.7 K, Automatic
Glow Plug Controller
1991 GMC Forward Control 5.7 K, AutomaticSECTION Glow Plug Controller
- Ensure coolant temperature is less than 80°F (27°C) before beginning test. Place ignition switch in RUN position and allow glow plugs to cycle. After 2 minutes, crank engine for one second. It is NOT important that the engine starts. Return ignition switch to RUN position. Glow plugs should cycle on at least once.
- If glow plugs do not cycle, disconnect controller connector (controller is located on top, rear of valve cover). Connect a 12-volt test light between Purple/White wire terminal of harness connector (terminal "B") and ground. With ignition switch in RUN position, test light should be off. Test light should glow when engine is cranked. See Figure.
- If test light does not glow as specified, repair short or open in engine harness Purple/White wire. If test light operates properly but glow plugs did not cycle, replace controller.
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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.