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.
Chart A-3, Engine Cranks But Will Not Run: Notes
Electronic Spark Timing (EST) is controlled by ECM. Ignition module sends a reference signal on Yellow/Red wire to ECM when engine is cranking. With engine speed at less than 400 RPM, ignition module will control ignition timing. When engine speed is greater than 400 RPM, ECM applies 5 volts on by-pass line (Yellow/Green wire) to switch timing control to ECM (EST mode). In EST mode, PCM controls ignition timing.
- If CHECK ENGINE light is on with ignition on, ECM is receiving voltage. ECM is located under left side of instrument panel, left of steering column, and contains 24-pin and 32-pin connectors. If TPS voltage exceeds 2.5 volts, engine may be in clear flood mode, resulting in starting problems. Engine will not start without reference pulses; scan tester should indicate engine speed during cranking.
- If engine speed is indicated during cranking, ignition module is receiving a crank signal; however, if spark is not present during cranking, ignition module is not triggering ignition coil or secondary ignition problem exists.
- Test light should flash, indicating ECM is controlling fuel injectors.
- Determines whether ignition module is not generating reference pulse or wiring or ECM is defective. When connecting and removing test light with battery voltage on Yellow/Red wire, a reference pulse should be generated. If engine speed is indicated, ECM and wiring are okay.
- Checks for ignition voltage at fuel injector wiring harness. Disconnect wiring harness before probing Red/Green wire. Reinstall wiring harness connector after checking for voltage.
- Checks for open in Blue/White or Blue/Yellow wire from connector to ECM. Ensure fuel injector wiring harness is installed.
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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.