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.
1989 GMC Forward Control 5.7 K, Automatic, 3L80/M40FACTORY OEM SERVICE PROCEDURE
Test Description
NOTE:
Test numbers refer to test numbers on diagnostic chart.
- Continue with test even if engine will not idle. If idle is too low, "SCAN" will display 80 or more counts (steps). Occasionally an erratic or unstable idle may occur. Engine speed may vary 200 RPM or more up and down. Disconnect IAC. If condition is unchanged, IAC is not at fault. There is a system problem. Proceed to step 3).
- When engine was stopped, IAC valve retracted (more air) to fixed park position to provide increased airflow during next engine start. "SCAN" will display 100 or more counts.
- IAC MUST be disconnected before performing this step. Test light will confirm ECM signals by a steady or flashing light on all circuits.
- There is a remote possibility that one of the circuits is shorted to voltage that would have been indicated by a steady light. Disconnect ECM and turn ignition on. Probe terminals to check for this condition.
Factory Service Tools
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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.
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