* Black wire with red stripe: This is usually the battery sense wire. It goes directly to the positive (+) battery terminal (or a very close, positive point on the battery harness). This wire tells the alternator what the battery voltage is so it can regulate its output. *This wire is crucial for proper alternator function.*
* White wire: This is likely the alternator output wire. It carries the current generated by the alternator back to the battery (and the electrical system). This wire is usually much heavier gauge than the battery sense wire. It connects to the main alternator output terminal and then to a heavy-gauge wire leading to a battery positive terminal or the distribution block.
Important Considerations:
* Third Wire: You mention three wires. The third wire is likely the field wire which often will be a smaller gauge wire (possibly light green). The field wire is the one that provides the alternator with excitation current to start generating electricity. This wire usually goes to the alternator's regulator, which is part of the alternator or may be a separate component. If there's a separate regulator, then the field wire may be routed to a circuit breaker or fuse to protect it. It sometimes appears fused at the alternator end, so careful inspection is vital.
Without a wiring diagram specific to your 1979 F-150's alternator and engine type, this is the best general guidance. A wiring diagram is strongly recommended to ensure accurate connection. Incorrect wiring can damage the alternator or other electrical components. You can often find these diagrams online (through forums dedicated to classic Ford trucks), in a Ford repair manual specific to your truck's year and model, or at a local auto parts store. They may be able to look up the diagram based on your VIN.
Caution: Working with automotive electricity can be dangerous. Disconnect the negative battery cable before working on the alternator wiring to prevent electrical shock or short circuits.
