* Coil itself: The most common cause. The coil's internal windings can break down with age and heat, increasing resistance and reducing its ability to generate the high voltage needed for spark. The insulation can also degrade, leading to shorts. A hot coil is more susceptible to these issues.
* Ignition Control Module (ICM): The ICM is the "brain" that controls the ignition system. It sends the signal to the coil to fire. If the ICM is overheating or has internal component failure exacerbated by heat, it may stop sending the signal correctly or at all once it reaches operating temperature. This is especially likely given the age of the vehicle.
* Worn or damaged wiring: Worn insulation on the wiring harness leading to the coil, or a poor connection at the coil, can increase resistance. When the engine heats up, this increased resistance can lead to a voltage drop, preventing the coil from firing properly. Look closely for cracks, fraying, or corrosion.
* Insufficient grounding: A poor ground connection anywhere in the ignition system can cause problems, particularly when things heat up. A bad ground will restrict the flow of current, impacting the coil's performance.
* Pickup Coil (in the distributor): While less likely, a failing pickup coil (or its wiring) can cause intermittent or complete ignition failure. This is less likely to be *heat* dependent, but if it fails, it might seem worse when hot because the increased engine speed might trigger a weak signal.
* Poor Cooling: Sometimes, the coil may simply not have adequate airflow or cooling. Excessive heat buildup in the engine compartment can stress it beyond its limit.
Troubleshooting Steps:
1. Visual Inspection: Carefully inspect the coil and its wiring harness for any signs of damage, burning, or corrosion. Check the connections for tightness and cleanliness.
2. Check the ICM: If possible, test the ICM using a multimeter or an ignition system tester. This often requires some specific testing procedures and may need a workshop manual. They are often prone to failure in older vehicles.
3. Resistance Test: Use a multimeter to test the resistance of the coil's primary and secondary windings. Compare the readings to the specifications in your vehicle's repair manual. High resistance indicates a problem.
4. Ground Check: Ensure the engine block and the coil have good ground connections. Clean and tighten any connections found to be loose or corroded.
5. Heat Check: If you suspect heat is the primary cause, you could try carefully monitoring the coil temperature with an infrared thermometer while the engine runs to see how hot it's getting.
Recommendation:
Given the age of the vehicle, it's quite possible you'll need to replace the ignition coil and possibly the ICM. Replacing the entire ignition system wiring harness if it looks suspect is also not a bad idea, although labor-intensive. Starting with visual inspection and resistance testing of the coil is the most efficient first step. A repair manual specific to your 1988 Dodge D100 3.9L V6 will be invaluable for this process. It will provide specific testing procedures and component specifications.
