Alternator whine under load
 

I have a 1986 Targa 911. The car is essentially stock with 70K miles. I recently noticed a whine coming from the alternator when the lights are turned on. Very distinct and repeatable. I replaced the belt and the bearings felt smooth and tight when I checked them. The noise remains. Is this indicative of an alternator going bad? Any help would be appreciated. Jeff

Check the battery first.

No load, no noise.
Under load, noise.

Could be the bearing(s). A relatively easy fix for a motor shop (automotive or otherwise). In addition, they can check it as well, replace brushes, etc. The most time-consuming part is R&Ring the unit (and that's relative).

A very tight drive belt adjustment could cause this (worn bearings) as well.

Sherwood

I had noise when I over tightened my belt.

In your case, probably loading it just puts it over the edge. Try moving one shim over and see what happens.

Please post your results

I had a similar issue and it turned out to be a worn bearing. While having it fixed, I had the brushes and regulator replaced as well and they cleaned it up completely - smooth sailing after that. A lot cheaper than getting a rebuilt one and you only lose it for a day.

Thanks. Battery is solid. I will probably pull it and have it reworked. Nothing else left to try.

Now I am really frustrated! I pulled the alternator and had the bearings replaced (just cause it was cheap) and bought a new battery for good measure. Alternator checked out fine. Everything seemed good until I drove it some. I got home and the battery was dead. The alternator light does not light when the key is turned on. The bulb is good. There is only 10.9V across the socket when the ignition is on (the oil light works and has 11.9V across it. I went from the hot side of the alternator light socket direct to ground and it is 12V. Schematic shows the "ground" side of the light socket going through the Alarm system to the terminal on the alternator. I suspect my original problem of "noise under load" was because the battery was getting drained. Where to next?

Did you check the VOLTAGE REGULATOR Brushes? Located in the back of the alternator.

The alternator shop ran it through it's paces and their tester and it all looked good. Seems unlikely they missed something but you never know. The fact that the alternator light will not come on makes me think it is something else.

So you did connect all 4 wires correct?

You say you drove it home, does that mean the Auto Electrician changed the bearings and refitted the alternator and changed the battery? If so, charge that battery and return the car to shop and go get a coffee while they fix it. Its easy to damage the diodes in an alternator when changing the bearings, its harder to get the wiring wrong but I guess its possible on a bad day. Presumably the alternator light was working properly when you/they refitted the alternator? A new/good battery should run the car an hour or so with a dud alternator depending on what you have turned on, how far is that run from the shop to home and were the lights and aircon etc all on?

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.

Just a suggestion, but when is the last time you cleaned all the grounds? The ground wire from the transmission to rear firewall can get pretty nasty. I don't think that would cause an alternator whine, but as you sort your electrical issues, it's generally a good idea to start with known, clean grounds.

An alternator will have a very noticeable whining sound when there's an internal short,
i.e. a diode or the stator winding. Some alternators will have a very slight whining
sound when under a heavy load, i.e. close to full output current. So a battery with
a bad cell could result in a heavy current load and cause a whining sound.

Hmm, powdercoated the housing and fan? I wonder if there is a good earth connection from the alternator case to the engine now? Was there a seperate earth lead from one of the mounting studs to a engine case bolt? These are a hassle to refit and maybe a previous owner left it out, you still get a ground through the mounting until you powder coat everything. Make sure its fitted and the connection good, its got to take 70 amps. There are also 2 fine wires at the rear of the rotor that connect to the slip rings that can be damaged easily when removing bearings etc., but if the shop tested everything after the bearing swap then its got to be wiring and earth connections right?

Good input. The ground strap looks good. I have been trying to avoid pulling the alternator again but it is clear that no charging is occuring so I will have it re-checked. While it is out I will do conitinuity and resistance checks.

Well, if you have not done so might as well clean the top of the heads, mine were clogged with debris.

Pulled the alternator and found the blue wire on the blade connector had come off. Put it all back together and charges perfectly. But guess what. The orginal "whine under load" is back even after the bearing replacement. I have given up and ordered a rebuilt alternator. Thanks to all for the suggestions.

Interesting, I wonder if its more about the belt and pulley protesting once the load goes on. Can you get the noise to come and go by turning on the lights etc? Mind you some times you just get strange noises from motors and alternators where the stator or rotor has a resonant frequency that we can hear. Same as the annoying buzz that some transformers put out when the laminations work loose.

Replaced the alternator. The whine remains when the lights are turned on. Still very distinct. I went ahead and pulled the belt again and ran it without it. No whine. So I think you are right that is is somehow related to the belt and pulley. I used 600 grit paper and smoothed both sides of the pulley. It is a new belt and the tension is right. Whine remains. I am out of answers. I find it hard to believe I am the only one that has experienced this. Is there a different brand belt I should try?