* Ignition Control Module (ICM): This is the most likely suspect. The ICM receives signals from the crankshaft position sensor (CKP) and camshaft position sensor (CMP) to determine the precise timing for the ignition spark. A faulty ICM won't send the necessary pulse to the coil, even if the coil itself is good and has power. This is common in older vehicles.
* Crankshaft Position Sensor (CKP) or Camshaft Position Sensor (CMP): These sensors tell the ICM where the engine is in its cycle. If one is faulty, the ICM won't know when to trigger the spark. A bad sensor will prevent a pulse from reaching the coil.
* Wiring and Connectors: Check all wiring between the ICM, the CKP/CMP, the coil, and the ignition switch for breaks, shorts, corrosion, or loose connections. A simple break in the wire can prevent the signal from reaching the coil. Pay close attention to the ground connections.
* Ignition Switch: While less likely, a malfunctioning ignition switch could prevent power from reaching the ICM or interrupt the signal path.
* Engine Control Module (ECM): The ECM often plays a role in ignition timing and can also contribute to problems if faulty. However, it's less likely than the ICM or sensors.
Troubleshooting Steps:
1. Check the ICM: This is the most logical next step. You'll likely need a service manual or a wiring diagram to locate it and test its output signal. Testing requires an oscilloscope (best) or at minimum a multimeter capable of catching the quick pulse, but even then it's challenging without the proper test points.
2. Check the CKP and CMP sensors: These usually require a multimeter and a specific testing procedure (consult your service manual) to confirm they're providing the correct signals.
3. Inspect wiring and connectors: Visually inspect all wiring related to the ignition system for any damage. Pay special attention to connections for corrosion and loose wires. Clean connectors with electrical contact cleaner.
4. Check for power at the ICM: Verify that the ICM is receiving power from the ignition switch.
5. Test the ignition switch: While less probable, ensure the ignition switch is functioning correctly.
Important Considerations:
* Service Manual: A service manual for your specific vehicle is crucial. It provides wiring diagrams, component locations, and testing procedures. Without it, troubleshooting becomes significantly more difficult.
* Safety: Disconnect the negative battery terminal before performing any electrical testing to prevent injury.
* Professional Help: If you're not comfortable working with automotive electrical systems, it's best to take your vehicle to a qualified mechanic. Improper testing can damage components.
In short, the absence of a pulse to the coil strongly indicates a problem upstream in the ignition control system. The ICM is the most probable culprit, followed by the crankshaft/camshaft position sensors and wiring issues. Systematic testing with a service manual as your guide is essential to find the exact problem.
