RussB

Remember what I said about the stock 90 Amp alternators being inadequate for turbo cars . . .

If it were me, I'd take this opportunity to swap out the 90 Amp for a 110 Amp alternator and its mount (from most 1990 and 91 200s and many 100s). I'm sure that used 110 Amp ones will be the same price as the 90 Amp alternators.

mcorrow

I need to spend as little as possible to make it "reliable" again, then I'll look at swapping for the larger one.... (else I make the gods unhappy)....

Will check the brushes....

RussB

Confuscius say, "Man who fight with wife by day, get no peace at night".

mcorrow

Confuscius was extremely wise man.....

Will Ng

That's why Confuscius stayed a single man! hahah!

RussB

He probably never drove an Audi with a marginal alternator.

mcorrow

Picked up the pulled alternator from the junkyard. Turns out it came from a Jetta, and has a "remanufactured" sticker on it, so it was not the original(the one in the 87 also has a remanufactured sticker on it).... Pulled the Voltage Regulator off it, and the brushes are about 3/4" long..... sounds like it may have been relatively new...
Got home and pulled the VR off the one on the car, and low and behold! the brushes were just "nubs" (thanks RussB)... put the VR from the junkyard alternator on the old alternator, and found out that the lower screw hole would not hold the screw(hole was too large)..... not sure how it was being held in there before (maybe locktite?).... anyway, I started the car, and tested the output (voltage) from the old alternator - 14.0 volts.... I decided that I was not happy with the fact that the lower screw was not going to work properly, so I pulled the alternator off...
Warning: before removing the alternator, do not run the car... there are pipes or hoses right above that get really hot (burned my knuckles pretty badly, while trying to get the upper bolt out)... Anyway, got the alternator out, put the newer alternator in, dropped in the newer Voltage regulator... (turns out this alternator had the smaller pulley on it!... more power at lower engine rpms!....Thanks for the suggestion!).. tested the output, 14.0 volts (13.99-14.01)... let the car idle for 2 hours with the high beams on, blower on high, radio on, with a voltmeter accross the battery... It did not seem to be draining the battery...)

Question: Does anyone know how the alternator warning light works? Never once did I get the alternator warning light to come on (except when starting).... I'm guessing that the old alternator setup was producing nearly the right voltage, but just not enough power (or voltage) to charge the battery... is that how it fails when the brushes wear out?

I think I'm going to add a voltage gauge (and maybe a boost gauge while I'm there)....
Thanks all!

RussB

If I recall properly, the alternator warning light is in series with the alternator's field winding, which is fed with +12 Volts from the battery. It will glow only when the winding draws current from the battery -- when the alternator's stationary. When a healthy alternator's turning it creates its own field current, so it won't suck current from the battery, so the bulb won't light.

I suppose that if the brushes wore to the point that field current couldn't flow through them, the alternator light would glow.


From Wickipedia:

"In many older designs of car, the field windings are initially supplied via the ignition switch and charge warning light, which is why the light glows when the ignition is on but the engine is not running. Once the engine runs and the alternator is generating, a diode feeds the field current from the alternator main output, thus equalizing the voltage across the warning light which goes out. The wire supplying the field current is often refered to as the "exciter" wire.

This system is simple and avoids the need for a heavy duty switch in the main alternator output circuit, which can carry very high currents--up to 100 amperes (though typical cars have 40-60 ampere alternators). One drawback of this arrangement is that if the warning light fails or the "exciter" wire is disconnected, no priming current reaches the alternator field windings and so the alternator will not generate any power. However, some alternators will self-excite when the engine is revved to a certain speed. The driver may check for a faulty exciter-circuit by ensuring that the warning light is glowing with the engine stopped. Modern systems have more complex electronic monitoring and should alert the driver when such problems occur."<ul><li><a href="http://en.wikipedia.org/wiki/Alternator">http://en.wikipedia.org/wiki/Alternator</a</li></ul>

RussB

<ul><li><a href="http://mapage.noos.fr/matushansky/confucius.html">http://mapage.noos.fr/matushansky/confucius.html</a</li></ul>