Owner's Problem Description
The owner reports that their 1979 Ford E-Series Van starts and runs, but then shuts down unexpectedly. They have replaced various components including the key switch, spark plugs, wires, cap, rotor, coil, and ignition module. When the key is turned on, a spark is observed, and another spark occurs when turning off the key, but there is no spark during cranking. After a few minutes, the spark returns, allowing the van to start again. The issue tends to occur at low RPMs during slowing down or stopping, while it runs fine at high RPMs. The vehicle had been sitting for over a year in a tow lot.
Professional Technical Analysis
For a 1979 Ford E-Series Van with an approximate mileage of 100,000+, intermittent spark loss was observed, primarily during low RPM conditions. The owner reported that after a year of inactivity, the vehicle starts but shuts down under low RPMs, while running normally at higher RPMs. The ignition system has been extensively serviced, including a new key switch, plugs, wires, rotor, coil, and module, yet the issue persists. Initial diagnostics showed sparks upon key engagement and disengagement, but no spark during cranking. This suggests an intermittent fault likely related to the ignition control system or a failure in the signal from the distributor pickup coil, especially during momentary low power. The critical diagnostic step involves testing the ignition components for continuity and proper resistance, particularly focusing on the distributor pickup coil which can fail thermally or mechanically. The vehicle's electrical connections, especially those subjected to corrosion from prolonged inactivity, should also be inspected. It's crucial to monitor for any voltage drops during cranking that could indicate a weak battery or poor electrical ground affecting the ignition system. Diagnosing this issue systematically will require the use of an oscilloscope to check for signal output from the pickup coil and proper voltage levels at the ignition module, ensuring that the entire circuit from the ignition switch to the coil is functioning correctly.
Possible Causes
Most common causes (ordered by frequency):
- Distributor Pickup Coil Failure: The pickup coil in the distributor may be experiencing thermal breakdown or intermittent failure. When the ignition system is under load, such as during cranking, the resistance may exceed acceptable limits, causing a loss of signal. Resistance across the coil should be tested; typical values range from 400 to 600 ohms. If the resistance is outside this range, the pickup coil needs to be replaced (OEM part #DUR-121). - No spark during cranking; resistance values outside expected range indicate possible coil failure.
- Ignition Control Module (ICM) Intermittent Failure: The ignition control module could be failing intermittently. This module controls the timing and delivery of the ignition signal. When it overheats or experiences internal failure, it may cut off the signal to the coil. Testing the module's output during cranking with a multimeter is essential; expected voltage at the output should be 12V. - Intermittent spark during low RPM operation; overheating may affect module performance.
- Electrical Connection Issues: Corrosion or loose connections in the ignition system can lead to voltage drops, especially under load conditions. This can cause intermittent loss of spark. Inspect all connectors and grounds, especially those on the ignition module and coil. Voltage testing at these connections should reveal a stable voltage of at least 12V during cranking. - Loose or corroded connections found during inspection; voltage drop observed during cranking.
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 Spark Testing: Begin by checking for spark at the distributor cap with a spark tester while cranking the engine. If no spark is detected, proceed to check the ignition coil and ICM for proper operation. Ensure the battery is fully charged and connections are clean.
- STEP 2 - Resistance Testing of Pickup Coil: Disconnect the distributor harness and measure the resistance of the pickup coil terminals. The expected resistance should range from 400 to 600 ohms. If the resistance is out of spec, replace the pickup coil.
- STEP 3 - ICM Testing: Test the ignition control module for proper output voltage while cranking. The output should be at least 12V. If the output is low or absent, replace the ICM with a quality OEM part (OEM part #F57Z-12029-A).
- STEP 4 - Electrical Connection Inspection: Inspect all electrical connections in the ignition system for corrosion or looseness. Pay particular attention to grounds and connections to the ignition coil. Clean and secure connections as necessary.
