'88 S4 Alternator Issue

TannerWhite · 03-11-2009, 10:42 AM

Hey guys, I've got a slight problem that you all might be able to shed some light on.

I have a 1988 928 S4 with about 140,000 miles on it... and I love it to death. Unfortunately, the air hose connecting to the alternator tore off without me knowing about a month ago, and now my battery won't charge at all. I suspect that the alternator has reached the end of it's life, but a quick trip to Checker had me confused when they told me it was functioning normally. 50% of the time, my voltmeter shows 12 volts, and the other 50% of the time shows much less. I know there is a 200 amp upgrade available, but $700 is unacceptable. Are there other alternators that can be swapped in? Do I have to go with the 115 amp replacement and hope my wiring isn't too shotty to just drain the battery again?

Any and all feedback would be greatly appreciated! I've got the beauty up on a lift right now getting ready to replace my motor mounts and my steering rack, so I thought I'd tackle the alternator at the same time. Thanks again!

Xlot · 03-11-2009, 11:13 AM

If the alternator tests good at a shop its prety safe to assume it works ok.

Other things to investigate are:

Are your grounds clean? The alternator puts out 13-14V when charging the battery, and the needle will be high, then once charged, it'll show around 12V, so the needle will drop. Your low readings could just be due to dirty ground points (there's a bunch of them on the car). Cleaning the ground points and battery contacts is good annual maintenance on 928's anyway.

Is there 12V at the exciter wire with the ignition in the "on" position with the engine not running? The exciter wire is the small post on the back of the alternator - when put to ground, it should register 12V, as the car uses battery power to initiate alternator charging at low rpm's.

If there's no voltage on the exciter wire, the car will only start charging once you rev the engine high enough to excite the alternator. No voltage can be due to a failed charging bulb in the cluster (its in series with the exciter circuit)