89 928 turbo

My 89 S4 is acting up. Upon start my voltage guage reads a little over 10, of course, lights are dimmed and draw is on....

What I discovered was that with a couple of sharp rev's, it climbs over 12 and then operates normally at about 13 or so, until I shut the car off and then it mostly does this again.

I have checked the belt, as it seems that it is symptomatic of slippage and will be replaced in any event, but it is tight and in good shape. But I have heard that it is possible that it could be the voltage regulator or related to the "green wire" syndrom. The threads that I checked had discussions more toward timing and such.

Can I get some feedback or a link....something else about an "exciter"....??

thanks,

JT
89 - Turbo Guards Red - Auto

Lizard928

The "exciter" is 12V supplied to the smaller post on the back of the altenator. without that voltage it will not start to charge until revved up to 2500-3000 RPM.

or it could be the regulator.

the first test I would do is to take the cover off the back of the altenator and with the key in the ON position measure voltage between the small terminal and the housing of the altenator.

docmirror

As above. Also sometimes caused by carbon buildup on the brushes of the field. with a quick blip of the throttle, the field gets charged, and the alternator begins working correctly. In most alternators the brushes can be removed and either cleaned or replaced.

Investigate the wiring to the field (small post) on the alternator.

Lizard928

Doc, on our bosch altenators, the brushes are part of the voltage regulator. So replacing the regulator would solve that issue.

docmirror

Excellent. sometimes bushes just need a touching with 400 grit paper, and the ring inside could use a little cleaning with an eraser. Try that first, the replace as needed. It's funny that the brushes are not field replaceable without exchanging the VR? That seems unusual to me.

928ntslow

When I had this issue with my 82, it turned out to be the low voltage bulb. I am not sure if the same components are used in the 89's, but here is what to do. Turn the key to #2 position and observe the bottom of the voltage gauge. There should be a red light showing there is no charge coming from the alternator. If the bulb is out, this is most likely your issue as the bulb is inline with that circuit. Having to rev the engine gives enough of an electrical charge to by pass and excite the circuit. Additionally, there was a resistor linked to the gauge as well that goes bad every now and again, so as a safe measure, (WYIT) replace that when you replace the bulb.

What the guys are talking about above is if your needle creeps back down to 10v constantly.

I just saw this coincidentally https://rennlist.com/forums/928-forum/391145-alternator-question-82-euro.html

89 928 turbo

Thanks,

Keith's description sounds a little closer, once it has settled above twelve, it stays there until I turn it off again....

anyway, I'll check all that stuff and let you know!

Thanks for the direction...

Jt
89 928 Turbo

atb

JT,

What's the deal, Thurston out of town?