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 DIAGNOSISEXTERNAL PAGESDIFFERENT CARSECTION 235 (ENGINE CONTROLS - THEORY & OPERATION - 3.4L)IGNITION SYSTEMSELECTRONIC IGNITION SYSTEMPCM TO ICM CIRCUITS
1999 GMC Forward Control 5.7 R, Automatic
PCM To ICM Circuits
1999 GMC Forward Control 5.7 R, AutomaticSECTION PCM To ICM Circuits
WARNING: This page is about a different car, the 2003 Pontiac Grand Am, 2003 Oldsmobile Alero, 2003 Chevrolet Monte Carlo, and 2003 Chevrolet Impala. However, it is still accessible from the selected car via links, so may be relevant.
- Low Resolution Engine Speed, 3X Reference (PCM input) - From the ICM, the PCM uses this signal to calculate engine RPM and CKP. The PCM also uses the pulses on this circuit to initiate injector operation.
- Low Reference (PCM Input) - This is a ground circuit for the digital RPM counter inside the PCM, but the wire is connected to engine ground only through the Ignition Control Module (ICM). This circuit creates a common ground plane and assures there is no ground drop between the PCM and ICM.
- IC Timing Signal (PCM Output) - ICM controls spark timing while the engine is cranking, this is called bypass mode. Once the PCM receives 3X reference signals from the ICM, the PCM applies 5 volts to the IC timing signal circuit allowing the ICM to switch spark advance to PCM control.
- IC Timing Control (PCM Output) - The IC output circuitry of the PCM sends out timing signals to the ICM on this circuit. When in the Bypass Mode, the ICM grounds these signals. When in the IC Mode, the signals are sent to the ICM to control spark timing.
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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.