Asiandude

Seeking some advice from the 928 experts on this forum:
My car is a 1987 S4 auto, 70,000 miles, well cared-for daily driver.
Recently while driving I'm seeing the voltage meter shoot up to 16 volts, also a high-pitched noise from the front of the car (probably the alternator) the central warning light flashes and tacho needle swings. If I pull over and restart the car the problem goes away for a while..then returns again. Normal charging at idle and driving is a little over 14 volts. I have replaced the voltage regulator and the condenser but the problem remains, the battery is new.
So, am I looking at replacing the alternator or is there another issue to be considered? The alternator is original afaik so probably 37+ years old. I live in Brunei in S.E Asia and all parts have to be shipped-in so a heavy alternator is an expensive purchase.
Thanks for any advice given. Joe.

Mrmerlin

Did you check the slip rings before installing the new regulator they should be smooth you might need to get them replaced a rebuilder can do that

What brand of regulator did you install it should only be Bosch

Petza914

A battery can't take 16v for very long. You run the risk of damaging it or it exploding. I think it's new alternator time.

Asiandude

Slip rings looked ok, the regulator was a Bosch item from Pelican. I guess it may be an internal fault in the alternator...

soontobered84

Why not try replacing the regulator built onto the alternator? It's not that difficult to do, would be less expensive, and could solve your shipping issue into Brunei.

Asiandude

The Rectifier assembly? Thought that was a job for the rebuild guys, will look into it, thanks.

Alan

I'd suspect the battery connections are loose and its dropping out of the circuit when you see the high voltage.

Alan

Asiandude

Checked, they seem tight enough. If the high voltage occurs and I pull over and turn off then restart, the voltage goes back to normal (for a few days).

Alan

Then based on the same reasoning I'd tighten the the battery ground grounding bolt and/or replace the ground strap - its easy enough to do, and the strap is a well known intermittent failure point.
You can try wiggling the ground strap while the car is exhibiting the issue - move the end near the ground point and see if it changes anything, if needed get better access & wiggle the whole thing.

I'd just replace it anyway - I replaced the strap on my GTS many years ago - and it really needed it.

Alan

depami

When the problem occurs, measure voltage at jump post and at battery. If jump post voltage is high and battery is not, you know you have a connection problem.

Alan

If you are losing the battery ground that won't be the case at all (e.g. battery +ve and Jump post will still be the same relative to chassis ground).

Alan

depami

You are correct. I guess I should have stated the obvious, measure battery post to post, not from chassis. To me, that's a no brainer.

Alan

Actually I think that is a far from obvious distinction for most people...

Alan

Asiandude

Thanks for the suggestions. The ground strap was replaced a couple of years ago and all seems good in that area. I'll do the voltage checks when it next occurs. One additional issue I've just noticed; under hard acceleration the volt meter needle fluctuates like crazy (usually quite steady).

Alan

So maybe not at the battery connections - but still sounds like possibly an intermittent connection somewhere else. Of course it could be intermittent within the battery itself (plate connections etc) - has it been changed since these issues started? can you try another?

Alan