The Stovebolt Site Forums The Shop Area The Electrical Bay Battery goes dead on 38 with alternator.

For some reason my battery on the 38 truck I just bought went dead. It is still 6 volt. I disconnected the battery and did some trouble shooting and found as soon as the wire that goes from the starter towards the switch in the cab, it draws the battery completely. When I remove the wire the battery will go back up to 3.5 volts. Something isn't wired correctly under dash. Truck was running and would start when I got it. Has set for 1 week since I last started it. I'm not an electrician but I guess I will learn with some help.

Do you have a one wire alternator? You'll need a wiring diagram either way. I have 6v one wire alternators on both my '38 and '53.
http://chevy.oldcarmanualproject.com/electrical/wiring/index.htm

Try disconnecting the big "charge" lead at the back of the alternator and leaving the rest of the system intact. It's possible there's a leaky diode in the alternator that's putting a constant drain on the battery. Be sure to fully charge the battery first- - - -it should have a specific gravity of 1.265 and/or an open circuit terminal voltage of 6.3 volts or more. Anything less and you'll need to do more charging or replace the battery.
Jerry

Yes it is a one wire alternator.. Wire goes from alternator to starter switch, then a wire goes from there to the discharge side of ammeter. Wire also from battery to starter. Battery is low so I am going to go get a 6 volt charger as I only have a 12 volt charger. Only 3.8 volts .Thank you al so much for the help. Grady.

Battery on charger. New question. Alternator part number says it is 12 volt. I took it off as it doesn't line up right with the other pulleys so the belt is chewed up. Only about 1/2 the size it should be. Don't want to sound dumb, but should I have a negative ground or positive ground alternator? Think I'll take a swim now and cool my brain!!! It's 100 degrees here! Grady

A 12 volt alternator simply cannot work with a 6 volt battery. It will either overcharge the battery and/or blow out all the bulb filaments, burn the ignition points, and possibly destroy the gas gauge. Someone would have had to rewire the stator and rotor of the alternator for a 6V output and then fashion some sort of regulator to limit the voltage to a maximum of 7.5 volts against a fully charged 6V battery.
Jerry

Somebody should be whipped with a wet noodle for putting a 12 volt alternator on a 6 volt system unless as Jerry said it was modified. That’s why your battery won’t hold a charge it’s been overloaded. Good luck, and replace the alternator with a 6 volt one or complete the entire 12 volt conversion.🛠

I have never heard of a "one wire alternator" which has more than one wire.
One wire alternators only have the 10 gauge wire held on by a nut on the back of it. This 10 gauge wire goes to the ammeter. This alternator is energized internally once a certain RPM is achieved.

This company makes a 6 volt alternator. It looks like they convert a 10SI to 6 volt.

https://brillman.com/product/6-volt-1-wire-alternator-40-amp-negative-ground/

My guess is that the original GM part number may still be there which would be confusing.

Well after I got the battery charged and moved the alternator back so belt was straight I got it started again! Only problem was that the alternator wasn't charging. Was looking at a You tube video and had to "excite" the alternator. That is taking a jumper wire and holding it on the stud, while just bumping the one terminal under the black rubber cover. It worked! Was charging a little over 7 volts on a fast idle. Shut the truck off and started it later and same thing happened. Had to excite the alternator again. Hope I don't have to do that each time I start the truck. Thanks for all the info. Ammeter doesn't show that it is charging though? Will have to look into this a little further.

All you usually need to do is rev the engine once. Most alternators will "self-excite" once the speed goes a little above idle. You can attach a wire to the terminal you're using to "excite" things from the ignition switch, or if you're not comfortable keeping the voltage on it all the time, use a push button starter switch. I'd recommend keeping the reference voltage on it all the time the key is turned on.
Jerry

I read the specs which you provided for your one wire alternator. It self energizes. All you need to do is step on the gas to raise the RPM high enough to trigger the energizer and it will continue to charge even at idle speed after that.

Yes it does work and charge when I start it now. But the battery doesn't stay charged. I have ordered a new optima. Should be here early next week then I will ma ke a holder for it in the original box. Thanks for all the help. You guys are awesome!.

Before you swap out the old battery, remove the negative cable from the battery and slowly move into contact with the battery post and look for a spark to make sure that you don't have a short from another source.

One of my vehicles has a persistent discharge. I have been overcoming it with a trickle charger though. I'm pretty sure that the voltage gage is the culprit, since it shows voltage 24/7 (unless i disconnect the battery). I just haven't had the time or the gumption to tear the dash apart to wire it (and probably a couple other gages) properly.

Good time to look for sparks is after dark. Removing fuses one at a time helps ID a circuit that is grounded when all switches are off.

Ed