* Distributor Cap and Rotor: These are the most common causes. Cracks, carbon tracking, or simply wear can prevent proper spark distribution. Inspect them carefully for any damage or wear. Replace them as a set; it's cheap insurance.
* Ignition Module (or Pickup Coil inside the Distributor): The ignition module (sometimes integrated into the distributor itself on older vehicles) is responsible for triggering the coil. If it's failing, you'll get power to the coil but no spark from the distributor. A failing pickup coil (inside the distributor) will have similar symptoms. Testing the module usually requires a multimeter and a specific testing procedure found in a repair manual. It's often easier and cheaper to replace the distributor as a whole unit if this is suspected.
* Distributor Shaft: Worn or damaged distributor shaft bearings can cause the rotor to be misaligned or not rotate smoothly. This can interrupt the spark. Inspection requires removing the distributor cap and carefully observing the rotor's movement.
* Worn or Broken Distributor Gear: This gear meshes with the camshaft gear and drives the rotor. If it's worn or broken, the rotor won't rotate correctly. This is less common but possible.
* Wiring Issues within the Distributor: Although you say you have power *to* the distributor, there could be a broken wire *inside* the distributor itself. This is hard to diagnose without careful inspection or replacement.
* TFI Module (if equipped): Some 4.9L engines used a Throttle Position Sensor (TPS) based ignition system, or similar, requiring a TFI module (Thick Film Ignition). A failing TFI module can cause a no-spark condition even if the coil is receiving power. This is another component to check.
Troubleshooting Steps:
1. Visual Inspection: Start with a thorough visual inspection of the distributor cap, rotor, and wires. Look for cracks, burns, corrosion, or anything out of the ordinary.
2. Replace Cap and Rotor: This is the cheapest and easiest first step. Replace them with new parts.
3. Check for Spark at the Coil: While you say you have power at the coil, double-check for actual spark. Use a spark tester (available at auto parts stores) to confirm. A weak spark might also indicate a problem.
4. Test the Ignition Module (or Distributor): This is more involved and may require a multimeter and a wiring diagram. A repair manual specific to your 1991 F-150 4.9L will be crucial for this.
5. Check for Codes (if possible): Does your truck have a diagnostic port? Check for any stored trouble codes that might point to the problem.
Recommendation: If you are not comfortable with electrical diagnostics, it's best to take your truck to a qualified mechanic. Replacing the distributor as a complete unit might be the most cost-effective solution if the problem isn't immediately obvious. A new or rebuilt distributor often solves many of these issues at once. Remember to check your spark plug wires and spark plugs as well while you are at it.
