1. Ignition Coil Problems:
* Coil Failure: The coil itself might be bad. This is a common culprit. Testing the coil's primary and secondary resistance (using a multimeter) is crucial. A visual inspection for cracks or damage is also important.
* Coil Wiring: Check the wiring to and from the coil for breaks, corrosion, loose connections, or chafing against the engine. A poor ground connection for the coil is particularly problematic.
2. Distributor Issues:
* Distributor Cap and Rotor: These are prone to wear and carbon buildup, leading to poor spark distribution. Cracks or corrosion in the cap are common. Replace these as a preventative measure if they look even slightly questionable.
* Distributor Pick-up (or Ignition Module): The pickup coil (or the integrated ignition module in some setups) senses the crankshaft position and triggers the coil. A faulty pickup won't signal the coil to fire. Testing requires a multimeter or oscilloscope.
* Distributor Shaft Wear: Excessive wear on the distributor shaft can lead to misalignment and poor spark delivery.
3. Ignition Control Module (ICM):
* ICM Failure: The ICM (if present; some 1988 models might have a simpler system) controls the ignition timing and coil firing. A bad ICM prevents the coil from receiving the signal to fire.
4. Wiring and Connections:
* Broken Wires: Trace all wires from the coil, distributor, and ICM (if applicable) to ensure no breaks or shorts. Pay close attention to areas that experience vibration or heat.
* Poor Ground Connections: A bad ground connection anywhere in the ignition system can significantly impair its function. Clean all ground connections thoroughly.
* Connectors: Inspect all connectors for corrosion, loose pins, or damage.
5. Other Potential Problems (Less Likely but Possible):
* Ignition Switch: A faulty ignition switch might not be sending power to the coil circuit.
* Ballast Resistor (if equipped): Older systems sometimes have a ballast resistor in the coil's primary circuit. A failed resistor could lead to low voltage and weak or no spark.
* Fuse/Circuit Breaker: Check the relevant fuses and circuit breakers.
Troubleshooting Steps:
1. Visual Inspection: Carefully examine all wiring, connectors, the coil, distributor cap and rotor.
2. Check for Power: Use a multimeter to check for power at the coil's positive terminal when the ignition is on.
3. Test the Coil: Test the coil's primary and secondary resistance. Refer to a wiring diagram or repair manual for the correct values.
4. Test the Pickup (or ICM): This may require a multimeter or oscilloscope and some specialized knowledge.
5. Replace Wear Items: Replace the distributor cap and rotor as a preventative measure.
Without testing, it's impossible to pinpoint the exact cause. A wiring diagram specific to your 1988 Ford F-150 6-cylinder is essential for accurate troubleshooting. A repair manual will also be invaluable. Consider taking it to a mechanic experienced with older vehicles if you're not comfortable with electrical diagnostics.
