Hi all,

My '98 Jimmy just isn't behaving. Two weeks ago I was having trouble with the alternator. The bearings were starting to make noise so I took it down to NAPA and got a replacement. I noticed the first day that I replace the alternator that the voltmeter would drop down to about 12v and all the lights would dim for a while. The voltage would come back up and the lights would go back to normal brightness. The voltage output never dropped enough for a long enough time to discharge the battery to where it wouldn't start.

Today I did some checking of the electrical system. Without the engine running the battery voltage was 12.3v. With the engine running the voltage varied between 13.7v and 11.9v. I watched the voltage across the battery terminals for about fifteen minutes. Most of the time it was around 13.65. I then hooked the multi-tester up to the output of the alternator and the battery negative terminal. I watched the voltage output for about fifteen minutes and got the same results, variation between 14.1v and 11.9v. When the voltage dropped all the lights dimmed correspondingly. I also checked for continuity between the alternator case and the battery negative terminal and that checked out OK. There was 0.2v between the alternator case and the negative battery terminal which is within spec (>0.5v).

The diagnostic tree in the factory manual requires a bunch of other test equipment that I don't have. How do I troubleshoot the problem and keep the lights from dimming while I'm driving?

 

I took the alternator back to NAPA today and they tested it for me. It failed. It was a bad alternator right out of the box. If I had wanted a bad alternator I would have gone to AutoZone from the beginning. It's less than a mile from my house. At least I have what I think is a functional alternator. I'll see over the next couple days if it actually works.