Failure of 6 volt battery to hold a charge


I just bought a 1939 Cadillac Lasalle Coupe 50. Beautiful car. Total frame off restoration.
I am having trouble with the 6 volt battery. It either will not hold a charge or is not charging when the engine is running. The battery is brand new with less than 100 miles on it. When driving, the “Battery” indicator on the dash never crosses over to “C” for charging. Has anybody else had this problem? Do I have alternator issues?


Are you sure it’s an alternator and not a generator?

If it is a generator, perhaps it needs to be re-polarized.

I’ve never heard of (doesn’t mean they don’t exist) 6 volt alternators…

Good luck, and hope this helps!


You are correct. It is a generator. Talked to restorer of the car and he believes it is a voltmeter problem.


If you’re battery isn’t charging and the gauge isn’t showing a charge then I’m not sure why he’s suggesting it’s the meter. I would consider it’s most likely one of these things…

*The voltage regulator needs to be polarized. I’m not familiar with pre-war GM, but on my 52 Chevrolet the voltage regulator is about 4" x 5" black box mounted to the firewall. It might even be stamp-labeled “Delco” or “Delco-Remy”. IIRC without looking at my car, there should be five terminals with wires connected to them. One of them will be marked or stamped “F”. This is your Field Terminal. Get a jumper wire and connect one end to your battery’s positive terminal after charging it independently. Briefly brush or touch the other end of the jumper to that terminal on the regulator marked “F” once or twice. Remove the jumper wire. Start your car, raise the rpm above idle and watch your gauge to see if it moves past “C” with the elevated rpm. If it does, everything should be kosher.

*As mentioned the generator needs to be polarized. With the engine off, use a jumper wire between the “Gen” and “bat” terminals on the regulator briefly. Repeat the check of the gauge by starting the car.

*Your generator isn’t charging as it should. Could be brushes, which while very slow, are a wear item. Not a huge deal but requires removal and disassembly of the generator.


Thanks. I’ll try that.


Hope that helps. If you want to thank me, post up a pic of that LaSalle. :wink::+1:



Here you go Jim. I have a buddy coming over this afternoon to try your fix.
John P


Good catch on the voltage regulator, I’ve never tried polarizing one of those.

I wasn’t going to mention brushes on the generator, because, to me, it seemed unlikely that they could be that worn, unless they were original brushes, perhaps. Although, brushes to change out would be cheaper than the whole generator. Of course, with that era of vehicle, it might be just what could be found…

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We tried reversing the polarity of the voltage regulator as you suggested and it appeared to initially work.

We connected a wire to the “B” terminal on the voltage regulator and then brushed the other end across

the “F” terminal and got sparks. The car easily started and it appeared the needle on the gauge moved

closer to “C.” But after driving it for an hour the battery once again dragged and did not start the car.

I am wondering if it is a battery problem. The car came from Ontario, Canada and they had temperatures

down to -35 degrees. The battery could have frozen and I may have lost a cell or two. I have an

emergency 6 volt battery I carry in the trunk that is new and fully charged. I plan to switch it out

with the old one and see if that makes a difference.

Also, I’m wondering if when I hook up the trickle charger (which I do every night) does this undo the reversing

of the polarity?

Any thoughts or ideas?

Thanks for your help. Did you get the picture of the car I sent?

John P


It sounds like you polarized the Generator but did you polarize the Voltage Regulator as described above as well?
It is possible you have a bad battery, but if the charging system and gauge are working properly, it still should have showed it charging with the engine running.
And no, a battery tender/charger won’t affect the polarity of the generator or regulator. The only time you should have to re-polarize them is if you disconnect one from the other.


Thanks Jim. We will give it another try.
John P


Jim, we tried to reverse the polarity again and no luck. We attached a wire to the “B” (Battery) terminal of the voltage regulator and then brushed the “F” terminal of the voltage regulator twice. We got sparks. Were we supposed to attach the wire to the battery itself? I’m thinking I may have a dead battery. It was bought new but installed over a year ago. But using a voltmeter the battery indicated 6.6 volts after charging with a trickle charger overnight. Maybe it’s the generator which was checked and seemed fine.

John P


Yes, to polarize the REGULATOR you use a jumper from the positive battery terminal to the ‘F’ for FIELD terminal on the regulator.



Thanks for all the advice. I will run a wire from the positive battery terminal to the “F” terminal

on the voltage regulator. But I have a question. In trying to access the battery on a 1939

Cadillac LaSalle Coupe, it is located under the floor board on the drivers side directly

under the drivers seat. I can peel the carpet back to barely see the metal plate on top

of the battery box but cannot access it without peeling the carpet back all the way across

to the passenger side and also removing at least the accelerator. Otherwise, I have to

put the car on jacks or on a lift to access it. That appears to be a rather cumbersome

way to access a battery that must be checked regularly for fluid levels. I would not

think Cadillac would do something like that. Have you ever heard of this?

John P


Again, not familiar with pre-war GM. I think by post-war they had moved the batteries to the engine bay for the passenger cars. But from what I recall on some early commercial vehicles…yes, the carpet would have to be pulled up (it shouldn’t be glued). Can’t really tell you anything definitive on the accelerator pedal. That would surprise me if you had to remove that.
I know the car is new to you. There has to be re-production owner and repair manuals out there for your LaSalle. And likely a LaSalle-specific forum with downloadable info that would probably really give you some guidance.