Hey-
So I have a new battery, and just put on a new alternator that's been bench tested to work, so why isn't it charging my battery? When the vehicle is running, and I plugged in my battery overnight to charge it, the voltage does nothing but diminish on the battery and I need to recharge it in a few days. This is very frustrating! The plug that plugs into the alternator has 3 prongs, and on my multimeter, all 3 prongs have some voltage on them. 2 of the prongs read the exact same charge that my battery reads, and the third reads a much smaller charge. So I would assume that the wires are OK, so what's wrong? Does anyone know what these 3 different wires that plug into the alternator are for, assuming that the alternator grounds on the engine block where it is attached? The car is a 1975 300D. Thanks for any insight.

 

with the car off there should be 11.8 or better volts at the battery terminals. running voltage should be 13.5 on up to 14 volts anything more is bad. what are the readings you have? i know my 26 year old 70,000 mile alternator regulator is lazy and i hit a spot in engine rpm that the volts fluctuate enough to dim then brighten the lights. but at idle i have 13.8 volts at the battery terminals. are you sure it is hooked up right with the wires in the same position?

 

My gut feeling is its not a charging issue but a battery draw issue. Sounds like something is staying on, diminishing the battery overnight. Verify if you have a voltage draw and if you do, pull each fuse one at a time to locate the circuit affected.

 

Thanks for the replies-

I still can't figure it out though. All of the wires are hooked up correctly- the alternator just accepts a plug that only goes in one way. It's tough to explain, but I know for sure that there's just no voltage going from the alternator to the battery terminal. I need to charge the battery on a plug-in-the-wall charger, and then the battery reaches around 13 volts when it's fully charged. Every time I start the car, it pulls some electricity off of the battery, and the ending voltage is always less than before the last time I had started it.

There is no electricity drain. I have tested all fuse terminals, and on top of that, the voltage doesn't diminish by any measurable degree when the car sits overnight etc. It's been a couple weeks now that I can't figure this out and it's getting pretty frustrating, I really don't want to admit defeat and bring it to the shop. Any other ideas folks?

 

The 12 volts comes from the battery to energize the alternator field. Not from the alternator to the battery. Any decent shop can hook up a machine to measure voltage and most importantly amperage being put out from the alternator. With the alternator load tested, it should generate around 40 to 50 amps under load. Years ago, I bought a hand held amp gauge that you would put close to the wire you wanted to measure and it would read amperage produced. Worked off induction. I'm sure I bought it at a PEP BOYS or GRAND AUTO type of store. Holding the gauge at the wires at the alternator, then a friend holding the engine RPM to 2000 and then turning on all accesories should load the alternator enough to see the output. Also does the red battery light function in the gauge cluster? Stay on in the run position and go out once you start the car? If not a burnt out battery bulb can interrupt the charging circuit.

 

Hey- I would just like to thank everyone that's been offering advice. It looks like my belt was just too loose. I changed the belt that drives it as a last ditch effort, after trying everything else I could think of.

You see, the belt that was previously on there was really very loose, but I figured that since it was still turning the alternator faster than I could see, even if it was slipping there should have been at least SOME voltage coming out of it. So I guess I still don't know exactly how the alternator works, but tightening the belt did it.