The alternator on a 1990 Ford F-150 charges the battery while the engine is running. The alternator is driven by the serpentine belt. When the engine is running and the alternator is working properly, the battery's voltage should be about 14.5 volts. Most auto parts stores have specialized equipment to test your alternator and confirm it needs to be replaced.
Shut off the engine and open the hood. Use a socket wrench to disconnect the negative battery lead. Inspect the connector and post; use a wire brush to remove any corrosion.
Disconnect the wires from the back of the alternator. Label each wire with tape to ensure that it is reattached to the correct post. Some wires are held on with a nut and washer; others simply snap on to the post.
Remove the tension from the serpentine belt. The tensioner is next to the alternator. Turn the large bolt counterclockwise to release tension. Slide the belt off the alternator pulley.
Remove the two bolts that secure the alternator to the engine block. Lift the alternator straight up to remove it from the engine.
Install the new alternator by attaching the wires to their respective posts. Next, bolt the new alternator in place. Put the serpentine belt over the alternator pulley and bolt the tensioner back into place. Reattach the negative battery cable to complete the installation. Close the hood.
