[Ecky]

My father's F150 is having an electrical problem, and I've offered to help him.

I'll start by saying that I don't have a service manual, and should probably get one. A wiring diagram for the electrical systems near the alternator might be pretty valuable.

The truck:

1995 F150 I6 5MT, appears to be OBD I with a proprietary (?) port. Truck lived in Florida for most of its life and has zero rust, and about 160k on the clock. The engine bay is spotless and bone stock, with no fluid leaks, and it runs great, except when it doesn't.

The symptoms: Very occasionally while driving it will seemingly quit charging, and after a few miles the spark plugs stop firing and the truck dies. Every time we've had it in the driveway and tested (including letting it warm up and revving it), the alternator puts out proper voltage, and the system sits at ~14.2v. It's tough to see coming while driving because the battery gauge on the cluster hasn't worked for years (the first sign is usually the ABS light coming on), so he has some anxiety about driving it right now.

The battery is healthy, so the first thing we did was order a replacement alternator from RockAuto, which didn't fix the problem - with the new alternator, it shows the same symptoms.

My next guess was that he had a bad ground strap. I was only able to find one strap on the engine, and it looks like new, with no signs of wear. While idling in the driveway, it appears to have a strong ground everywhere when tested with a multimeter - but it would, because it was charging properly.

A visual inspection of the wires going into the alternator doesn't show any obvious problems. There are 5 small wires, and a large red one going into it. The positive cable to the battery looks fine, and I was able to measure 0.20v in one of the small wires in the connector going into the alternator (with the truck off), and a solid ground in another. There's a small green wire that splits off from the rest.

My questions: What could cause it to not charge? How is the alternator controlled in these older trucks? What are some diagnostics I might try?