Help me on this GEN 2 problem

[jvrotter]

I got a 86 Blazer last year, it had a bad alt #2 and poor connections in relay, etc. Had the alt rebuilt and cleaned up the connections. After that got good charge on both batteries, alt2 light came on when starting then went out, everything ok.
10 months later my alt2 light stays on. So I check batteries while engine is running, 14.5v on batt 1 and 12.5 on bat 2. Volt meter on dash is right between yellow and green. Before it was high side of green. I pull off the 2-wire plug and check it, ignition off. voltage on red, no voltage on brown. Ignition on, voltage on red and voltage on brown. According to my cheat sheet that means problem in alt. Drive to my alt shop, they check it and say alt is good, its my wiring.
Am I missing something? Which lead from the alt actually is charging the second battery? I also changed battery so thats not it. Are both alternators the same?

 

[Barrman]

The red wire on the plug you pulled off is what carries the voltage back to the battery. The brown wire is the exciter wire. Put a volt meter probe on that red wire at the plug with the engine running and everything plugged in. If it is 14.4 there, making sure you ground the volt meter to the rear battery when doing this, then the problem is probably a loose or corroded connection between the alt and the battery.

Yes, both alternators are the same in a stock truck.

 

[jvrotter]

OK, I probed the red wire, engine running. Only getting 12.4v. Guess that means the alt is bad. Right? Anything else I should check?

 

[Barrman]

Yes, It sure seems like an alternator issue.