So my friend's 1990 EF seemed to lose the alternator last week as the battery light came on and his lights started to get dim to the point they couldn't be seen, and the car no longer started. I replaced the alternator, zero change. I just replaced the wiring harness, zero change. There is continuity between the large white wire off the alternator to the main fuse box, and then to the battery... Fuses all check out OK in the fuse box. The grounds seem OK but I didn't see one hooked up to the alternator when I replaced it (no extra wires in the area at all), could that be the problem? (EDIT: actually that appears to be on the green plug to the regulator I think)

 

I should also mention that the tach stopped working when this problem came up.

 

Well I'm actually fairly certain the replacement alternator is a dud according to voltage readings. I'll have to replace it again.

 

Disconnect the ground from the body and the block
Clean both surfaces
Connect ground

 

The replacement alternator I bought turned out to be dead. I grabbed one from the junkyard and threw it in.. works great.