Flakey Alternator -- ??

[curmudge1]

Trying to get my money's worth by tapping into the expertise here.

My new (to me) 99 gas 2-door Tahoe (40K miles) has some odd alternator behavior, seems to me. Maybe someone can help me understand?

I haven't been driving it much since I got it, & a couple of weeks ago, it wouldn't start, weak battery. Charged it up & was OK. But, now I'm sensitized to the electrical system & I notice that the voltmeter would go up to 14V shortly after starting, but while driving with AC on & radio, the voltage would drop, to 12V or below.

Just checked with my handy digital multi-meter, and the battery before starting registered 12.35V (under-hood light was on). Started up & saw the dash voltmeter go up to 14V, checked with the mm & saw about that. So, I turned the AC on, fan up to high & radio on kind loud, & saw the dash voltmeter drop to 12V, the digital mm said 12.35V or so.

Hmmm ... belt looks OK. Do alternators sometimes get weak? This is supposed to be a 105 amp, according to AutoZone online. Should be able to maintain voltaage, I would think.

I'm going to AutoZone & get it tested, but I'd be interested in knowledgeable opinions from this group. Thanks in advance.

[rjschoolcraft]

The voltage regulator could be on the way south. I've seen this behavior before.

[moondoggie]

Good Day!

I had an alternator on a GM product once that lost one diode (or diode pair, I don't know, I think there's six diodes in these things). At light load, it worked fine, producing a little more than 14V. With enough load, however, it couldn't keep up. If I'd kept using this alternator as it was, it would have solved itself - the regulator will still ask for 105A (or whatever the alternator is rated), but is only able to get output from two of the three diode pairs. The remaining functioning diodes probably get pretty hot passing current above their rating.

Most of the GM products I've owned could handle the fan on high & the rear window defroster on, with the engine idling, & still produce ~ 14V.

Hope this helps... ;)

Blessings!

[GMC Hauler]

I had a problem once with intermittent charging. Turns out that the battery cable from the alternator was loose on the battery. I wiggled the connection, and the parts store guy's tool started reading the alternator there.