1. Ignition Switch: A faulty ignition switch can prevent power from reaching the ignition system. Test the switch's power output and continuity at various positions (accessory, run, start).
2. Crankshaft Position Sensor (CKP Sensor): This sensor tells the ECM where the engine is in its cycle to time the spark. A faulty CKP sensor will prevent spark entirely. It's a common failure point. Check its wiring harness for damage, corrosion or loose connections. Test the sensor's resistance (check your vehicle's specifications for the correct resistance value). A multimeter is essential for this step.
3. Cam Position Sensor (CMP Sensor): (Depending on the specific 1992 F-150 engine) Similar to the CKP sensor, a faulty CMP sensor prevents proper spark timing. Test it in the same way as the CKP sensor.
4. Ignition Control Module (ICM): Some Ford F-150s have a separate ICM, sometimes integrated into the distributor. If you haven't already replaced this, it's a suspect. This is often overlooked and functions as an interface between the ECM and the ignition coil.
5. Wiring Harness: Look for damaged, corroded, or broken wires in the harness that runs to and from the ignition system components (especially those near the distributor or coil). Pay close attention to the connections, looking for loose, burnt or corroded pins. A visual inspection isn't enough; you need to carefully check each connection.
6. Power and Ground Circuits: Ensure the ignition system receives adequate power and has a good ground connection. Trace the power and ground wires to the components; poor connections in these circuits can cause intermittent or complete failure.
7. Distributor (If equipped): If your engine has a distributor, check the cap, rotor, and pick-up coil inside for wear, cracks, or damage. A worn or damaged distributor can prevent spark.
8. Fuel Pump Relay/Circuit: While you say there's no spark, this is a precaution. Verify the fuel pump is working, as an easy-to-check first step, in case there's a related issue causing the problem or was misdiagnosed in the past. A bad fuel pump relay can sometimes be confused with no-spark scenarios.
9. ECM Programming/Calibration: While you say you replaced the ECM, there's an outside chance (less likely) it isn't fully compatible or properly programmed for your specific vehicle. Did you get a remanufactured ECM specifically matched to your truck's VIN or a generic one?
Diagnostic Tools: A multimeter and a diagnostic scanner (OBD-I for this year) are extremely helpful. A scanner might display diagnostic trouble codes (DTCs) that will pinpoint the fault more accurately. Many auto parts stores offer free code scanning.
Important Note: Working on the ignition system involves high voltage. Disconnect the negative battery cable before starting any work to avoid injury. If you're not comfortable performing these tests or repairs, take the truck to a qualified mechanic. These are just potential problem areas, and it may require systematic diagnosis by a professional to pinpoint the root cause.
