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I have a 51 CJ3a that I have an small charging issue with. My Ammeter goes negative when I start or turn on lights, but never goes plus. If I disco the battery while it is running then it dies. Does this sound like a generator or voltage regulator problem?
It looks like your question has been out for a while and no answer, so here goes.
Yep – you have a dead charging system. From your description, there is no way to figure out which component is bad. There is always the possibility of a bad connection or wire harness. Look at terminals closely for loose or corroded crimped barrels on wire ends. If you got the Jeep from somewhere else, is it wired right?
But – if everything else checks out – how to figure out what is the problem? There is no easy way to check out a voltage regulator, so if the generator is functional, replace the regulator. How to check the generator? Not too hard.
You will need a 6-volt lantern battery (one of the square ones - it helps if it has screw terminals vs. the springs, but either will work) and a 12-volt test light (a turn signal bulb will work).
Look at your generator. I have attached a picture of mine. There are places for three wires. The screw that goes into the generator case is the ground. The other two terminals are the generator armature connection (the bigger of the two terminals – the upper terminal on my generator) and the field terminal (the smaller of the two – the lower terminal).
Disconnect the armature and field terminals from the vehicle wiring harness. Connect your test light from the armature terminal to ground – make sure it’s really ground. Connect the negative terminal of the lantern battery to ground. Have a test lead on the positive terminal of the lantern battery that you can clip on the field terminal.
Start the engine and let it run at idle. Unless your generator has a lot of residual magnetism in the field laminations, the light should be dark. If it glows a bit – good sign! Now, clip the positive terminal of the lantern battery to the field terminal of the generator. The 6-volt lantern battery is providing the current the generator needs to excite the field coils. I don’t advise using the vehicle battery – it can provide ‘way more current than we need here!
If you have a good generator, the test light should be glowing by now. Accelerate the engine a bit and the test light should get brighter as the engine revs up. A generator running this way should be able to make the lamp burn very brightly, even though it is a 12-volt lamp. If you get no light or a very dim light – you probably have a bad generator.
But, if the generator makes the test lamp light up – probably a regulator problem. We had one new regulator die on us as we brought the '48 truck to life and this is exactly the way we isolated the problem. As a further word or two of wisdom; it doesn’t hurt to have a good auto electrical shop look at an old generator. Brushes do wear down and it doesn’t hurt to have the commutators turned or cleaned every 50-years or so. If the brushes wear down to the point that the brush holders hit the commutator - bad news!
The other choice is to find a good auto electrical shop that can still test generators and have them look at it for you. The modern trade stores really do a good job of checking alternators, but bring in a 65-year old, 6-volt Jeep generator and you get a “what’s that” look.
Good luck, let us know what you find.