Diagnostic help please
 

Stock 79 SC had a problem today. Installed a new AC compressor (along with a lot of other new AC parts) and have driven it 2 or 3 times since but I had a problem this morning. I started the car and let it warm up for a few minutes and came back into the garage to head out and I heard an unusual sound under the hood and thought it sounded like it might be coming from the alternator.

I looked at the dash and the alternator light was on. Thinking that perhaps I overtightened the AC compressor belt, I loosened it a little and checked for the alternator light and even though it was still on, my cigarette lighter volt meter was reading steady at 12.4 V and not dropping at all. I took it out thinking that perhaps the light would go off and it did not. Car was running fine until a terrible whine and really loud buzzing sound started coming from the engine. I pulled over immediately and looked under the lid but no clue there. Started it up and turned back for the house and it happened again. Loud squeal and really loud buzzing, took it out of gear and the car stalled as I coasted to a stop.

I did start it up just to get it over a few feet to where a tow truck could hook it up and both the alternator light and the green oil pressure light were on. Could I have done damage to the alternator by overtightening the AC belt? What else might have been damaged if that were the case? I'm a novice wrencher in case that's not obvious at this point and I'm really heart broken that I may have caused some real damage. Car is on its way to an indy shop in Bonita Beach. Any thought or hypothesis is welcome,
Thanks

how's the tension on your alternator belt? If engine's running, you should see high 13/low 14v if alternator's working properly? here's a link to an article on how to check/adjust alternator belt tension https://www.pelicanparts.com/techarticles/101_Projects_Porsche_911/03-Fan_Belt/03-Fan_Belt.htm

The tension felt fine and looser than the AC belt. I am most concerned that I caused some damage to the alternator and thus damage to something more critical. That buzzing and squealing was so loud, it sounded damaging! I won't know for a few days but it's got me very concerned.

Based on your report that the alternator light came on and you only had 12.4 volts I would guess that your issue is with the alternator. You could have removed the AC belt to eliminate it as a source of the noise. Is the alternator original or has it been rebuilt previously.

A terrible whine sound could be an alternator bearing failure that would coincide with the light and low voltage output.

To the best of my knowledge the alternator is original. I'm also wondering if over tensioning the AC belt could cause crank shaft related damage?

No, it’s pretty much impossible to damage the crank from an over tightened belt. What could get damaged, however, is the bearing inside of the compressor clutch. You might want to get one of those universal belt tension tools. It could help you figure out what is “just right” from too tight. Also, remember that a new belt will settle into the grooves and stretch a wee bit in the first few minutes of operation, so it should be rechecked after the first use.

How many miles on the motor? I had to rebuild my original alternator at 90K due to bad exciter diodes. Still sounds like a failure of the alternator. A loss of alternator/loose belt would cause the alternator idiot light and overheating due to loss of engine cooling fan.

If the engine was running 12.4 volts is too low should be about 13.8 to 14.4. I was getting whining sounds which turned out to be the alternator bearings. I had it rebuilt at a local shop for $110 along with a new regulator.

Targa80 - It's got a little over 130k on the clock. If it is just the alternator maybe it's unrelated to anything I did and that would make me feel a little bit better. Maybe it's just coincidental and it was time for a new alternator. I'm just not sure while in the midst of that noise the engine would stall and then fire right up again to allow me to put it in a safe spot if it was the alternator. Man, that was a loud and scary sound so again, if it's only the alternator I will consider myself lucky.

alternator/fan belt should be pretty tight, wondering whether noises you were hearing were caused by alternator/fan belt slippage? Should be easy to confirm/rule out