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 4.3 W, STANDARDREPAIR AND DIAGNOSIS (SINGLE PAGE)ENGINE PERFORMANCESYSTEMENGINE CONTROL SYSTEM & COMPONENT TESTSIGNITION SYSTEMTIMING CONTROL SYSTEMSKNOCK SENSOR CIRCUIT (MODELS USING EXTERNAL SPARK CONTROLLER MODULE)
1999 GMC Forward Control 4.3 W, Standard
Knock Sensor Circuit (Models Using External Spark Controller Module)
1999 GMC Forward Control 4.3 W, StandardSECTION Knock Sensor Circuit (Models Using External Spark Controller Module)
- An open or short circuit on IC module control wire to PCM/VCM will cause a loss of 12-volt IC module signal. This will cause PCM/VCM to fully retard ignition timing.
- Install scan tool. Using a metal object, tap on engine next to knock sensor and note knock parameter. Knock should be indicated on scan tool.
- If a scan tool is not available, backprobe PCM/VCM knock sensor signal terminal with a DVOM. With engine idling, 8-12 volts should be present at this terminal. Using a metal object, tap on engine close to knock sensor. Voltage signal at PCM/VCM terminal should drop to zero volts, and then return to original voltage when knock signal ceases.
- If voltage signal does not respond as described, check knock sensor-to-module signal. On vehicles equipped with automatic transmission, it may be necessary to place transmission in Drive for timing change to occur.
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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.