Thread: Did I kill my alternator?

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I stupidly shorted out my electrical system while attempting to jump start my car (don't ask). The reason I had to jump start at all was because the battery is 10+ years old (says a lot for Diehard batteries) and was showing its age. I have a cutoff switch that I believe has had made a big difference even though I start the car every three weeks during the off season.

After replacing the battery with a new Diehard this morning, I find that the charging system shows 13 volts only above 700 engine rpm, below that it drops to 8 volts (as low as the gauge goes). While running, the gauges flickered, especially the voltage gage, and stepping on the brakes brought the voltage down to 8 (or less). After running until the engine was warm, I shut it off and found that the battery was completely dead - the turn signals wouldn't even work.

It sounds to me like the alternator has a problem, assuming the brand new battery is not the issue. I'm confused though, at how the alternator can keep the engine running, but not charge the battery. I'm going to run the multimeter tests described in another post just to see if I can get any more insights. I'll also take the alternator to a local shop for testing, but it's now Saturday afternoon and it's difficult to wait until Monday AM. This is a nice weekend, too.

Thanks everyone.
Dorsey

I should have mentioned that this a 60a three-wire GM alternator with internal voltage regulator.

if you can, try full fielding the alternator, there should be a small slot where you can test the alternator with a screwdriver. itll scare the crap outta you but at least you know it alive.

Better check your charge fuse it will usually be a 7.5 amp fuse in the exciter circuit. If you shorted the battery out that is the first fuse I would check.

Thanks to all of you, I was able to test the alternator and verify that it's working properly. In order to make tests at the battery I had to drop it out of the frame, which is difficult because it's mounted underneath.

I found that the voltage at the battery posts was correct at 12.6, but at the starter was only ~7.1v. To make a long (and embarrassing) story short, when I installed the battery earlier, I must have knocked the positive terminal loose. As soon as I reset and tightened it, all was well again, 14v+ at the terminals while charging, no flickering, and very little voltage drop when I turn on the lights.

So, I guess my mechanical skills are on full display here. My only defense is that mounting the battery is difficult because of the close space and awkward access. Had it not been for the thorough diagnosis all of you provided, I might have just bought a new alternator.

Again, many thanks for your quick replies and support.

Originally Posted by DennyW

You mean, does the dash light that says alt light up red when you turn the key on ?

Yeah I mean