Alternator whine under load

[Lorenfb]

"So I think you are right that is is somehow related to the belt and pulley."

Most likely not!

[Porboynz]

So to recap you replaced the bearings in the original alternator and the same whine remained. You then replaced the alternator with a completely different unit and still the same whine under load? Then you took a close look at the pulley and belt, cleaned it up with emery paper and replaced the belt and still got the whine under load.
Do the top and bottom pulleys run true? Right number of spacer washers? I would try a different brand belt, and make sure its the right spec. Maybe there is a mismatch there somewhere with later and earlier model parts, how about a photo of the fan pulley and belt? Our host sells the top pulley outer, might be worth a try with a new belt. Driving your Porsche should be a whine free experience in all senses of the word, so keep at it, you will solve it.

[Lorenfb]

"got the whine under load"

Now what can it be? Load, a strange term!

[Porboynz]

Load is probably a quaint English term that made its way to NZ and maybe not so much used in the US. I would use it as follows:
"load up the engine to check for clutch slip by applying the brakes while driving along in gear" or in this case "apply a load to the alternator by switching on the lights etc" Load in this case is the added work the alternator needs to do to counteract the extra current being used by the lights etc. If you have 55 amp alternator and its busy replacing the battery charge that was used to start the engine then you add in the additional current of the lights and other accessories like blower fan and heated rear window is going to give it some serious exercise, or load. We call a trunk a boot here (throw your golf clubs in the boot mate) and a hood is a bonnet. (thats a flat 6 under the bonnet mate). Oh, we drive on the left side of the road as well, in right hand drive cars, and so it goes on.

[Lorenfb]

"got the whine under load"

Maybe one might measure the load current of the alternator with an ammeter
on the lead coming directly from the alternator to determine whether the
alternator is at near capacity, right? Or maybe one might continue replacing
parts.

[wwest]

Originally Posted by 1986vero911

Thanks for the responses so far. I did all the work myself (including powercoating the fan and housing while they were out...sweet) so there is no coffee break to be had. I was very careful about the rewiring while re-installing. I really believe it was not charging before I started the project so incorrect wiring is even less likley. Driving time after repair was about an hour so that fits with no charging. I just charged the battery and checked output. It is 12.5V off and 11.7V running so clearly there is a problem.

"..powdercoating..."

Changes the airflow path through the fan housing enough to create the whine...?

You lowered the distance between the fan blade tip and the inside of the fan housing ever so slightly....

That would make, does make, a BIG difference in the sound of the compressor blade section in a jet engine.

[rolls 912]

Source where the whine is coming from. If you suspect it's the belt or the pulley, grab some hose, put it next to your ear and the pulley, shroud etc. this old school method works very well for hearing and diagnosing strange noises.

[1986vero911]

Thanks for the ideas. I ran it without the belt and the noise does not exist with lights on or off. It made the same noise before powdercoating and only makes it with the lights on so that is not likely to be the problem. Shims and belt tension is right.
I do not like being a "change out stuff until it stops" guy but my troubleshooting took me there unless I wanted to rebuild alternator brushes. I will drive it some more and see if the new belt gets broken in. After that, the last option is trying a different outer pulley half. I can probably borrow one just to troubleshoot. Annoying!

[anc]

Did u ever find out the cause of the noise under load? I have a similar issue w/ my mb 300e and can't track it down!

[takair]

I know the thread is old, but is it possible the whine is coming from the radio/stereo speakers? If so, it could be a radio ground problem.

[Demetri]

I have a whine coming from my smog pump. Not sure how I’m going to fix it. But this thread helped me pinpoint it. Thanks!

Screw driver to the ear trick can help locate virbration, which is the source of the sound. Metal end on culprit and ear on handle. Helps to have a long screwdriver.