Rob Edwards

Mkay, I have Hebert's 91 S4 alternator on the bench, deciding what (if anything) to do to it. I popped off the pulley (24mm impact socket) and realized that there's an 8mm hex hole broached in the pulley shaft. So I figured I could drive the alternator up to speed with the impact wrench, and listen to the bearing:



Sounds good to me, I think:



Next, I pulled the old voltage regulator. The slip rind surfaces are a bit dirty but the surfaces are not worn appreciably. My gut says leave them alone, maybe clean up with a pencil eraser while spinning?



Next, the voltage regulator- looks like the brushes are half worn down, so a definite replacement candidate:



But the old VR has a 75 ohm, 3W resistor soldered across the terminals, while the new one does not. Does this need to be swapped? Or the two VR's ohm-metered to see if they're the same? Or just install the new one and take it somewhere to be tested? Or learn to stop worrying?

BC

I have several of these, and when I get them rebuilt, sometimes I am not sure what I am actually paying for. So it would be interesting with the pictures as assistance to learn more about these old units.

Maybe the resistor is for creating something different in the regulator's.... regulation?

Leon Speed

Very timely.

Bill Ball

Not sure about that resistor - could be for noise. Hopefully Alan will pop in.

I would leave the slip rings alone too. You could disassemble the alternator and mount the rotor on a lathe and turn the rings as I have had done, but only if they have a wear groove that prevents the brushes from clearing the groove's lip on assembly.

+/- on the bearing(s). If they make no noise then you are fixing something that isn't broken. Still, it might be worth opening the case and looking at the rear bearing and its spacer/holder. Mine was shot - but then again, mine was making a racket too, so I was motivated to get in there.

This is one of those philosophical things. The WYAIT fanatics would have you replace the bearings, even though you aren't IT yet.

Rob Edwards

Continued confusion. Just for grins I measured resistance 1) across the VR terminals themselves, and 2) across the slip ring brushes, comparing old vs. new.

There is a huge disparity in resistance between old and new across the terminals, I suspect 'cause on the old one I'm just measuring that 75 ohm resistor. But the slip ring appears to see the 'same' resistance at the brushes. Must be the reason Anderson can sell these as replacements....

Old: 78 ohms



New: 917 ohms



But measured on the slip ring brushes directly, readings are the same:

Old: 863 ohms



New: 873 ohms

IcemanG17

odd...very odd

my short story....I noticed the 928 Estate always had "shaky" voltage....which eventually turned into spikes....spikes where it would PEG 16V.....lights would get bright, fans were turning TURBO...scary stuff....

In an effort to save a few $$$ I change ONLY the voltage regulator (techinically Bill did) literally the next track day the Alt itself died...... So I installed a rebuilt unit from 928intl and ZERO issues since..but it still is a bit shaky on the gauge....which I figure is the wonderful 27 year old wiring...

ZEUS+

The voltage regulator is transistorized and cannot be tested with an ohmmeter. The resistor is for an additional pathway to ground if the rotor goes open to turn the warning light on. Without the resistor the light may not come on if the rotor goes open. There should be continuity from slip ring to slip ring. Slip rings to shaft should show no continuity, if they do rings are grounding. The rings should be cleaned with a fine emery paper, dont use an eraser.

JHowell37

FWIW, you don't need the impact gun to spin it to check for bad bearings. Spinning by hand works fine as long as you can get most of the dirt, sand, and leaves out of the alternator. That one looks quite clean as it is. All you need to do is give it a spin by hand and listen for it hitting rough spots, or see if it quickly stops. Based on the appearance of it, and the fact that the slip rings look practically brand new, I'd bet the bearings are fine.

I've removed voltage regulators from a few 928 alternators and don't ever recall seeing that resistor. I too am curious about its purpose.

Is that alternator rebuilt, or original? What's wrong with it?

mark kibort

when you rebuld these guys, its usually for broken armature wires. replace the armature for $20 and then you are good to go. you get a new commutator that way, the brushes will last near forever on our cars and I dont know why, and while your in there, you should buy the slightly larger pulley to slow the altnernator down a little unless its a pure street car and you sit in traffic a lot.

ZEUS+

Alternators do not have an armature, starters do.

Rob Edwards

Its (sic) for a street car that will sit in traffic a lot.

17prospective buyer

Just a nomenclature mixup, starters and alternators work on the same fundamental principle, just that the starter field windings and armature windings draw current, on an alternator AC is induced via a rotating magnetic field through the stator.

I'd test the diode trio and rectifier bridge as well, it's an easy test and doesn't cost anything. Diode test, go from each side of the bridge heat sink to the diode terminals (negative on heat sink, positive on diode), should be around 0.550 voltage drop on each one indicating they are "doing their job" correctly, and no voltage drop for the other way around. (positive on heat sink, negative on diode terminal).

mark kibort

really? why dont you tell me the difference then.
an alternator IS a motor that is being spun, not spinning something else.

JHowell37

What is the problem with this alternator? Is it original or rebuilt?

Rob Edwards

No problem with the alternator, it's a rebuilt unit (not sure when but there's a rebuilder decal on it) that seems to be in very good shape.

The only reason I opened it up and started asking dumb questions is because it is going to go back into Dan Hebert's Turquoise '91 S4, that will go back into service in Phoenix, and will thus be subjected to headlights/A/C/carputer/stereo on while idling in traffic in 100o+ heat, so I just want to make sure it's functioning as well as possible. Dunno how well it works, as the car's been apart for 2+ years now.

Looking back through the records, Kie bought the Turquoise car in January of 2007, at which point it had front end damage and 91,000 miles. During his ownership he replaced the alternator, then sold the car to Dan in October 2009 with 99,000 miles on it.

So the 'rebuilt' alternator from Kie's ownership has no more than 8000 miles on it, which is consistent with its appearance and good-sounding and -feeling bearings. But no way did they replace the voltage regulator given how badly its brushes are worn. And that shouldn't be surprising given that a new VR is $60-70...

So I think that this alternator is fine, but the question is whether it would benefit from a new VR- I doubt it makes any performance difference (at least 'til the brushes wear away and destroy the slip rings) but hell, maybe as the brushes wear the spring pressure pushing them on the slip rings decreases and the contact isn't as solid? I have no idea, hence putting it out there in front of more-talented-than-mine eyes. I have no idea what the critical determinants of alternator current output vs. rpm are, and how the 'age' of various components in an alternator affect this. Just trying to learn.