Alternator wont charge battery

[Coalkracker]

Will a test light in line with the plug tell me for sure?

[Stocker]

Yes, unless it's a very small current drain (not enough current to light the test lamp).

[Steeveedee]

If it is a diode, you MIGHT see a spark when you disconnect the wire. I could have been a little more clear about it. If you pull that wire off and the battery isn't discharged in a day or so, that is the problem. The rebuilt alternator I first got was noisy, and I suspected a diode- they sound sort of like a front bearing noise when there's a bad one, but I didn't think much about it until I went to start the truck a few days later and the battery was cold as a mackerel- 0 Volts. I wonder how much life was taken out of that battery by going flat like that. Guess I'll have to keep an eye on it.

[Coalkracker]

I havent gotten around to checking it yet but i do know there wasnt any abnormal noise coming from the alternator but i'll probably have time tomorrow so i'll go ahead and check it out. The truck hasent been on the road in decades so im sure im going to have my fair share of problems as i go through it.

[RichardJ]

On post #43 you reported these voltages with the Key Off.

F 11.3v
2 0v
3 11.9v
4 5.9v

With the Key Off, terminal #4 should not be receiving voltage from the ignition Switch.
Recheck those readings with the Key OFF.

[Coalkracker]

The power draw is indeed coming from the alternator. Parts store ordered me another one that will be here tomorrow morning.

[Steeveedee]

Good to see some resolution. Unless the parts store has a video screen for their alternator tester, a bad diode probably won't show up when it is tested. If they do, there will be a missing lump or two in the output pattern.