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 MANUALSGMC1999FORWARD CONTROL 5.7 R, AUTOMATICREPAIR AND DIAGNOSIS (SINGLE PAGE)ENGINE PERFORMANCESYSTEMENGINE CONTROLS TESTS W/CODES - DIESELCIRCUIT TESTSNO MALFUNCTION INDICATOR LIGHT (MIL)
1999 GMC Forward Control 5.7 R, Automatic
No Malfunction Indicator Light (Mil)
1999 GMC Forward Control 5.7 R, AutomaticSECTION No Malfunction Indicator Light (Mil)
- Perform ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK . After performing OBD system check, go to next step.
- Attempt to start engine. If engine starts, go to next step. If engine does not start, go to step 6.
- Check fuse No. 4 (Gauges). If fuse if faulty, go to step 14. If fuse is okay, go to next step.
- Turn ignition on. Using test light connected to ground, probe ignition feed terminal at instrument cluster connector. See WIRING DIAGRAMS article. If test light illuminates, go to next step. If test light does not illuminate, go to step 11.
- Turn ignition off. Disconnect Blue PCM harness connector. Turn ignition on. Connect a fused jumper wire between ground and MIL control circuit of PCM harness connector. See WIRING DIAGRAMS article. If MIL illuminates, go to step 9. If MIL does not illuminate, go to step 10.
- Check fuses to PCM ignition and battery feed circuit. If fuses are okay, go to next step. If fuse(s) are faulty, go to step 13.
- Turn ignition off. Disconnect PCM harness connectors. Turn ignition on. Using test light connected to ground, probe PCM harness connector ignition and battery feed circuits. See WIRING DIAGRAMS article. If test light illuminates on both circuits, go to next step. If test light does not illuminate on both circuits, go to step 12.
- Check for faulty or poor PCM ground connections. Repair as necessary. After repairs, go to step 20. If ground connections are okay, go to next step.
- Check for poor connection at PCM harness connector. Repair as necessary. After repairs, go to step 20. If connector is okay, go to step 15.
- Check for open in MIL control circuit. Repair as necessary. After repairs, go to step 20. If circuit is okay, go to step 16.
- Repair open in ignition feed circuit to instrument cluster. After repairs, go to step 20.
- Locate and repair open in PCM battery feed or ignition circuit. Repair as necessary. After repairs, go to step 20.
- Locate and repair short to ground in PCM ignition feed circuit or PCM battery feed circuit. Repair as necessary. After repairs, go to step 20.
- Locate and repair short to ground in instrument cluster ignition feed circuit. After repairs, go to step 20.
- Replace PCM. Program PCM using required equipment. After repairs, go to step 20.
- Check for poor connection at MIL control circuit to instrument cluster connector. Repair as necessary. After repairs, go to step 20. If circuit is okay, go to next step.
- Remove instrument cluster assembly. Inspect MIL bulb. If bulb is okay, go to step 19. If bulb is faulty, go to next step.
- Replace MIL bulb. After replacing bulb, go to step 20.
- Replace instrument cluster. After repairs, go to next step.
- Start engine and operate vehicle under conditions which original symptom was noted. If system does not operate properly, go to step 2. If system operates properly, go to ON-BOARD DIAGNOSTIC (OBD) SYSTEM CHECK .
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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.