brianrheffron

I am new owner of a 1989 928 S4 auto based in Glasgow, Scotland. My other interesting cars are a home built Triumph Dolomite Sprint period replica race car and a fifty year old standard VW Beeetle. First of all let me say that I have found the Rennlist forum by far the best source of 928 information on the net and am really impressed by the technical knowledge of your members. In time I hope to be able to contribute.
Being new to 928s I am not sure what is normal behaviour for the car and am a bit puzzled by the following. With a fully charged battery showing 14 volts (tested at the battery) when I turn on the ignition the voltmeter on the dash reads about 10. When I start the car the reading stays at 10 untill I rev the engine up to about 2,300RPM at which point the guage jumps up to 14. At the same time all the lights brighten noticeably and if I'm using the wipers they speed up. Thereafter the guage reads 14 even at idle. I interpret this as indicating a couple of faults. The voltage drop between the battery and the dash is too great and also that for some reason the alternator is not charging at low revs on start up. Can anyone tell me if this is normal or not. If not what should I be checking?

Tampa 928s

You have a exciter wire that provides the low idle voltage that goes from the small post of the alternator. This wire goes into the dash across a resistor and through the Alternator lamp. If your bulb is burned out or the resistor is loose or contacts are corroded that will cause it. Does your bulb go on when you turn the key this should eliminate the bulb. This is on an 86 not sure if your year has the same but I think it does.

Mike Frye

First off, WELCOME!

At first I thought it wasn't a problem at all, because the voltmeters are notoriously inaccurate in these cars. A digital VOM will tell you much more accurately what you're looking at (check at jump post).

After re-reading your post and finding that the lights get brighter also, my money would be on something in your alternator or maybe just a dead battery (or dead cell in the battery?).

If there's nothing loose like the alternator belt or a wire on the alternator, I'd try a different battery.

Dennis Wilson

Mike,

Some of the new alternators are self exciting (does that sound bad) at about 2.5K rpm. It sounds like the exciter circuit is deficient as previously mentioned.

Dennis

brianrheffron

Thanks for the input. My battery is brand new so I have assumed that it is not at fault. Once the engine has been revved and the alternator cuts in the output measured at the underbonnet post using a digital meter is 14v approx from idle upwards so, once excited, the alternator seems fine. From the information posted and from another similar concurrent thread I think the problem lies with the alternator warning light or the exciter circuit. Am I right in guessing that the alternator warning light is behind the battery symbol to the bottom right of the dash voltmeter. When I turn on the ignition I do not see any warning light there so that will be the first thing to replace.

Dennis Wilson

That is the light that should come on when the key is in the on position and engine not running. Also, the parts and tech manual has an article about changing the resistor to a new value when going to an upgraded alternator. I don't have the article here at work but you may want to check it to see if it applies to you before removing the pod to change the warning light.

Dennis

the flyin' scotsman

Brian.........welcome to the gong show; always nice to hear from a fellow Scot.

The in car voltmeter is not accurate and the tests you've performed seem to suggest that the battery and alternator are in working order. Have you measure the voltage under the bonnet when the in car voltmeter is reading 10v? this may confirm the exciter cct.

Also, there's a poteniometer in series with the in car vmeter; that may cause your inaccurate readings.

Mike Frye

Brian,

I'm not sure where the warning bulb is in the '89, but there should be a diagram in your owner's manual. I found out about a bad warning bulb only after I'd had the pod out and back in, so I ended up going in for it later. You should compare the diagram in the owner's manual to your dash with the key on so you can see if any are out.

Whitesands

Mine did this, check the muilty pin plug by the jump start point under the hood, mine had dirty contacts, after cleaning, oh and I also spread the pins a little, the fault disappeared, if the plastic cover for the jump point is missing get one.