Owner's Problem Description
The owner reports that while driving their 1991 Ford Explorer, the vehicle suddenly shut off and would not restart. Now at school for auto class, they are troubleshooting the no-spark condition. They have tested the ignition control module (ICM), coil pack, and crankshaft position sensor, but the vehicle still does not produce spark. The vehicle does not have a camshaft position sensor as it is a VIN X model.
Professional Technical Analysis
The 1991 Ford Explorer equipped with a 4.0L V6 engine (VIN X, approximately 150,000 miles) is experiencing a complete lack of spark, which is critical for engine operation. The symptoms indicate that the vehicle abruptly shut down while driving, suggesting a possible failure in the ignition system. Initial tests included the ICM, coil pack, and crankshaft position sensor. The absence of a camshaft position sensor is noted, which is typical for this engine configuration. The ICM should normally receive a stable power supply (12V) and provide a spark to the coil pack. If the ICM is functioning correctly, it should produce a signal to the coil, resulting in a spark at the plugs. The crankshaft position sensor (CPS) is essential as it informs the PCM about the crankshaft's position, allowing for appropriate ignition timing. Testing the CPS should reveal a voltage output of approximately 0.5V to 5V, depending on engine rotation. A malfunctioning CPS can lead to a no-spark condition as the PCM would not receive feedback on the crankshaft's position. The vehicle's previous repair history, if any, may provide insights into recurring electrical issues. Current assessments indicate that the vehicle is non-operational and poses safety risks if driven in this condition. It would be prudent to check for power at the coil pack and review wiring integrity leading to the ICM and PCM, as intermittent wiring failures could contribute to the no-spark situation.
Possible Causes
Most common causes (ordered by frequency):
- Ignition Control Module (ICM) Failure: The ICM may be malfunctioning due to thermal breakdown or internal component failure. This is a known issue in older Ford vehicles. Testing for power input and output signals at the ICM can confirm if it's operational. If the ICM fails to provide the necessary signals, the coil will not produce spark. A new ICM should be used (OEM P/N F57Z-12A297-A) which typically costs around $120-$200. - No spark at the ignition coil, lack of power output from the ICM, possible stored fault codes related to ignition system.
- Crankshaft Position Sensor (CPS) Malfunction: A faulty CPS will fail to send the correct signals to the PCM, which can result in a no-spark condition as the PCM relies on this input for ignition timing. The CPS should display a resistance of 200-500 ohms and a voltage signal of 0.5V to 5V during operation. If faulty, replace with an OEM CPS (P/N F57Z-12K073-A), approximately $50-$80. - No signal detected from the CPS during testing, potential stored codes indicating CPS failure.
- Wiring Issues: A common cause of no-spark conditions can be damaged or corroded wiring leading to the ICM or coil pack. Inspect the wiring harness for continuity and integrity. Resistance values should be below 5 ohms for good connections. Repair or replace any damaged wiring before further diagnostics. - Visual inspection reveals frayed or corroded wiring, continuity testing shows intermittent connections.
Diagnostic Steps
Professional Diagnosis Process
Follow these systematic steps to accurately diagnose the issue. Each step builds on the previous one to ensure accurate diagnosis.
- STEP 1 - Initial Scan and Data Collection: Use the GeekOBD APP to perform a complete OBD-II scan to retrieve any stored fault codes related to the ignition system. Check for any freeze frame data that could provide insight into the conditions at the time of the failure. Pay attention to any codes that may indicate issues with the ICM or CPS.
- STEP 2 - Power and Signal Testing: Verify that the ICM is receiving the required power (12V) at its power terminals. Additionally, use a multimeter to check the output signal from the ICM to the coil pack during cranking. Proper signal should be detected; if absent, ICM may be faulty.
- STEP 3 - Crankshaft Position Sensor Testing: Disconnect the CPS connector and check its resistance using a multimeter. Resistance should be between 200-500 ohms. While cranking the engine, measure the voltage signal output from the CPS; it should vary from 0.5V to 5V. If these values are not met, replace the CPS.
- STEP 4 - Wiring Inspection: Conduct a thorough inspection of the wiring harness leading to the ICM and coil pack. Look for any signs of wear, corrosion, or breaks in the wires. Use a multimeter to test for continuity and ensure resistance is below 5 ohms. Repair any issues found before re-testing for spark.
