1. Ignition Control Module (ICM): This is a prime suspect. The ICM controls the firing of the coil pack. A faulty ICM will prevent spark even with power reaching the coil.
* Testing the ICM: This requires a bit more advanced diagnostics. You'll likely need a multimeter or a specialized ignition testing tool to check for proper voltage and signal at the ICM inputs and outputs. A Haynes or Chilton repair manual for your specific year and model will show you the proper testing procedures and wiring diagrams. Replacing the ICM is often the solution in these situations.
2. Crankshaft Position Sensor (CKP): The CKP tells the ICM when to fire the coil pack based on the engine's rotational position. A faulty CKP will send incorrect or no signal, resulting in no spark.
* Testing the CKP: You can test the CKP for continuity and resistance using a multimeter. Again, your repair manual will provide the specifics for your vehicle. A common issue is a damaged sensor wire or connector. Inspect these carefully.
3. Camshaft Position Sensor (CMP): (If equipped - some 1991 models might not have one) Similar to the CKP, the CMP provides the ICM with the camshaft's position. A malfunctioning CMP can disrupt the ignition timing. Test this the same way as the CKP.
4. Power Distribution Issues: While you've checked power *at* the coil pack, ensure the power *to* the ICM and other ignition components is also correct. Check fuses and relays related to the ignition system. A wiring issue (broken wire, bad connection, corrosion) anywhere in the circuit can be the culprit.
5. Ignition Switch: Although less likely, a faulty ignition switch could prevent the correct power from reaching the ignition components.
6. TFI Module (if equipped): Some older Ford engines used a "Thick Film Ignition" module. If your Explorer has one, this is another potential source of the problem. Look up your specific engine's specifications to see if it utilizes a TFI module.
Systematic Troubleshooting Steps:
1. Visual Inspection: Carefully inspect all wiring and connectors related to the ignition system. Look for any signs of damage, corrosion, or loose connections.
2. Check Fuses and Relays: Verify all fuses and relays associated with the ignition system are good.
3. Use a Repair Manual: This is crucial. It will provide wiring diagrams, testing procedures, and component specifications specific to your 1991 Ford Explorer 4.0L. Don't attempt advanced diagnostics without a reliable manual.
4. Multimeter Testing: Use a multimeter to check voltage and continuity at various points in the ignition circuit, following the procedures outlined in your repair manual.
Important Note: Working with the ignition system involves high voltage. Disconnect the negative battery terminal before starting any testing or repairs to prevent injury.
If you're not comfortable performing these tests, it's best to take your Explorer to a qualified mechanic. Improperly diagnosing and repairing an ignition system can lead to further damage.
