The Stovebolt Site Forums The Shop Area The Electrical Bay Alternator Problem?

I rewired my '51 GMC 1/2 ton with a harness from Chevy Duty. I went to NAPA and got a new Delco alternator. Put the harness in along wit a turn signal,also from CD,and everything worked.
The next morning the battery was dead. Put the battery on the charger and put a continuity tester between the + & - cables. Went through the truck and when I disconnect the 10ga wire on the alternator the circuit is broken. This is the wire that goes up to the ammeter.
A buddie, a Toyota mechanic, siad it sounds like the regulator is bad. So I take it back to NAPA and they put it on their tester and say it's charging so it's OK. I tell them about the problem I am having and they just look at me. No answer given. So I go to another NAPA and the guy does the same test with the same results. But he says, i hope isay this right, the alternator has been "energized" and after the engine is shut down it needs some type of load on one of the wires to discharge it. He says to do this with a lightbulb set-up.

Hey I'm just a woodworker and my truck is ready to go except for this. I took it out for a few little trips this weekend and then dissconnect the battery.

OK guys help me out so I can be "On the Road Again"!!

You can cause the alternator to draw current if you hook up the continuity tester backward, so a better way to check is with an ammeter in the circuit or by connecting the negative up on the battery and shunting a bulb between the + terminal and the + battery cable. If you are still drawing current, then yep, the alternator is bad. I just experienced that very thing on my sister's Mazda. The alternator charged ok, but also discharged. A rebuilt one fixed the problem. It is more likely a leaky diode than the regulator, but it is definately bad. I have run delco alternators with nothing but a sense wire (one wire congfiguration) on 2 vehicles now with no trouble. The bulb just lets the alternator come on sooner, by leaking some current to the regulator. It is supposed to be switched with the ignition anyway, so it is out of the circuit when the engine is off. Mine starts charging at 900 rpm, and then keeps charging when idling, so I don't worry about it.

You could move the 10 ga wire to the other side of the key switch. Thats the way I have mine it cuts the battery off when the key is out.

That's not a very good idea, Kedso, alternators don't like to be open-circuited while they are being rotated. You may find that yours has a short life.

Not to mention that an alternator is capable of trying to force 30 - 60 amps (depending on the rating) through your switch . . . which will shorten the life of the switch as well!

Yes, that too. You could find your switch contacts welded together!