Thread: Bad alternator?

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Ok, so new wire harness, 2 fuses installed, one going to the coil and the other is for the ignition. Engine is grounded to the frame with a big fat grounding strap. Now i have been told that if your alternator has a bad diode inside that it can drain your battery. I have a slow drain and was wondering if this is true and if anyone knows how to test for that? Thanks in advance!

We had that happen on a car years ago. During the week, when we were using it every day, the battery would stay up, but on the weekends it would discharge because we didn't use it Saturday or Sunday. Drove me nuts, but I finally replaced the alternator and that was it. The diode had gone bad letting current bleed off.

If you choose to, your local Autozone or Advance Discount will put a tester on it for you free of charge and let you know whats what.

Don

I got the same thing going on right now in my 72 Vette.
I know it is the alt. but I want to rebuild it since it a chrome plated job.
Kurt

I had a similar problem after putting in my new harness last year. it turned out(after buying new alternator) that one of my wires had frayed the coating in just one spot and was grounginf out. if you get the car running, you can find out if the alternator is throwing a charge with most home charger/tester units. good luch and check those pinch points

An alternator produces alternating current (AC) by the principles of electromagnetic induction. Accessories connected to your car’s charging system require a constant flow of direct current (DC) at a fairy steady voltage. You can't charge a battery with alternating current, so it must be rectified to direct current.

Ripple voltage or (AC voltage) can be measured using a digital multimeter. Make sure your meter is on an AC scale. Connect the black lead to a good ground (bolt on the engine or bare metal) and the red lead to the “battery" terminal (sometimes labeled “BAT” or simply “B” – usually a red wire) on the back of the alternator, (not at the battery). A good alternator should measure less than .5 Volts AC (that’s point five i.e., one-half a volt) with the engine running. A higher reading indicates damaged alternator diodes – typically open. (First diagram)

To check alternator diode leakage, connect the multimeter in series with the alternator output terminal when the car is not running. This is accomplished by disconnecting the battery lead wire and connecting the digital muitimeter’s red lead to the battery terminal on the alternator and the black lead to battery lead. Leakage current should be a couple of milliamps at most; more often, it will be on the order of 0.5 milliamps on a current scale of your multimeter – more that a few milliamps indicates a shorted diode and this will definitely zap your battery in a day or so. (Second diagram)

Hope this helps..
Glenn

a quick easy way to test alt. start car take off battery cable if it stays runing alt should be good.

Awesome support guys! I came home tonight and hooked the battery up and left it for about fifteen min, came back and the alt was warm to the touch. Well, I guess ill take care of that this weekend since were suppose to have rain/snow out here in wonderful Utah! Thanks again guys!

The only thing disconnecting the battery cable when running tells you is that your alternator is producing enough current to power the ignition. An MSD 6a requires 1 amp per 1000 RPM (on a V8). So at 6000 RPM you'd need 6 amps.

Headlights alone can draw 40 amps. Add the wipers, defroster and hit a turn signal and your draw is 70+ amps. That's why the heater motor slows down on those icy mornings when we have lights, heater, heated seats etc on and then turn on the rear window defogger. The battery compensates - but only slightly as it is stored energy.

A lot of new vehicles come with 100 amp plus to accommodate accessories and the killer stereo systems.

Your "warm" alternator indicates a shorted diode..