Your 1991 Ford pickup is equipped with an external voltage regulator. The regulator handles voltage from the alternator to the pickup's electrical system. The regulator does its job efficiently and works for a very long time although it can fail during normal operation. To check the voltage regulator's operation is a fairly simple procedure requiring a minimum of expertise.
Locate the voltage regulator. The regulator is a black, square box located on the left side of the engine compartment bolted to the fenderwell.
Expose a small amount of bare wire on the red wire going in and going out of the voltage regulator using a utility knife.
Attach each end of the jumper wire to the exposed portions of the voltage regulator wire.
Attempt to start the vehicle and determine if it continues to operate. If the vehicle continues to run, the voltage regulator is faulty and will need to be removed and replaced.
Remove the jumper wire and use the black electrical tape to cover the exposed bare wire.
Follow the steps outlined in your service manual for removing and replacing the voltage regulator.
