Owner's Problem Description
The owner reports that the engine of their 1980 Jeep CJ5 continues to run even after turning the ignition key off. This issue started recently, with no prior repairs or parts replacements noted. The vehicle operates normally otherwise, and the owner is seeking a solution to the unusual running condition.
Professional Technical Analysis
The 1980 Jeep CJ5, equipped with a 151 cubic inch 4-cylinder GM Iron Duke engine and likely around 100,000 miles on the odometer, is exhibiting an unusual condition where the engine continues to run after the ignition key has been turned off. This issue can arise from several potential causes. One common explanation is the presence of backfeed, potentially from a faulty diode in the alternator circuit, which may cause voltage to continue feeding back into the ignition system. The ignition switch mechanism itself must also be examined, as any misalignment or wear in the linkage could prevent the ignition circuit from fully disengaging. Additionally, the engine's electrical connections should be inspected for corrosion or shorts that could contribute to this problem. A thorough diagnostic approach should include checking voltage at the ignition switch when off, ensuring it is at 0V, and monitoring alternator output for any anomalies. The continued operation of the engine in this state can be a safety hazard, and immediate attention is required to prevent potential overheating or engine damage due to unintended running conditions.
Possible Causes
Most common causes (ordered by frequency):
- Faulty Diode in Alternator: A defective diode could allow voltage feedback from the alternator to the ignition system, causing the engine to continue running after the key is turned off. This is common in older vehicles and could lead to issues with electrical components if not addressed. - Potential symptoms include a glowing check engine light and abnormal voltage readings at the ignition switch. Tests should confirm voltage levels when the key is off.
- Ignition Switch Mechanism Failure: If the ignition switch linkage is misaligned or worn, it may not fully disconnect the ignition circuit, causing the engine to continue running. This needs to be visually inspected and tested for proper operation. - Check for any physical play in the ignition key position and measure voltage at the switch to ensure it drops to zero when turned off.
- Electrical Shorts or Corrosion: Corroded connections in the ignition system could create unintended paths for current, allowing the engine to continue running after the key is off. Inspecting and cleaning all connections is crucial. - Symptoms may include intermittent electrical issues, and visual inspections may reveal tarnished or corroded connectors.
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 Visual Inspection: Begin by visually inspecting the ignition switch linkage and wiring for any signs of wear or misalignment. Check for any corrosion at connection points, particularly at the ignition switch and alternator.
- STEP 2 - Voltage Testing: Use a multimeter to measure the voltage at the ignition switch when in the 'off' position. The voltage should be 0V. If there is voltage present, trace the wiring back to identify any shorts or faulty components.
- STEP 3 - Diode and Alternator Testing: Test the alternator's diodes for functionality. This can be done by measuring the output voltage and inspecting the diode's resistance. A shorted diode can cause feedback issues.
- STEP 4 - Road Test: After completing the inspections and tests, perform a controlled road test to observe if the issue persists. Monitor the engine's behavior during the test and check for any warning lights.
