AZSAIL

I have recently aquired a 1983 928 that sat for 6 years. I have been able to get it running again but the alternator was bad so I got a rebuilt & put it in and no juice came out. It turned out that the exciter wire was only putting out about .5 volts. My rebuilder tells me it should need about 6 volts but not a full 12 volts. I jumped the exciter with low voltage and the alternator started charging. I have checked the harness to the main engine electrical connection on the right fender well and the voltage to the connector is only .5 volts. Where does this wire come to the connector from? I have the checked wiring diagrams and they go to another page and I can't find which relay steps the voltage down from 12 volts. The wires in the engine compartment not in very good condition (cracked and peeling insulation) so I plan to jump around the harness if I can find the source.

Thanks in advance for any help,
Tom Errickson

83' 928, 71' Lotus 7 S4, 00' Chevron Super 7, 77' 280Z V8, Van Dieman RF90 F2000,

Jim bailey - 928 International

The powerflow is battery to starter starter to jump post

AZSAIL

Yes, I understand for the main heavy charging cable but what about the small (about 12 gauge) wire? It looks like it was blue or maybe green that is in the harness with the main charging cable.

Sean79 5spd

In my 79 the exciter wire goes into the dash and is connected to the charge light. When the car is not running power comes from the battery through the charge light to the alternator to excite and light the charge light. Once the the alternator is running it self excites and provides power back through the excitor wire which extinguishes the charge light because there is no longer any potential. E.g. equal volts on each side of light. I belive there is resistor in the dash which reduces the excitor voltage.

Someone like Wally P could do a much better job of explaining this.

Sean

Jim bailey - 928 International

There is a Porsche Tech bulletin which calls for replacing the resistor on the back of the instrument cluster with a 928 641 981 00 a 68 ohm 5 W resistor which increases the exciter current

SharkSkin

If the dash light with the little battery icon is not illuminating then you probably have a bad bulb. If you do not have the added resistor per the tech bulletin that Jim referenced then a burnt charge light bulb will prevent the alternator from charging.

AZSAIL

I have not seen a charging light on the dash, only the volt meter. Does an 83' have a light? I might be lucky and it is only a loose bulb. I am at work now and will look closer tonight.

Thanks!

Jim bailey - 928 International

Yes that warning light is basically what triggers the exciter circuit

SharkSkin

The light should illuminate when you first turn the key to "on", before starting. You can test the circuit by grounding the thin wire that normally connects to the alternator. Play it safe and ground it through a ~5A fuse. If, when you do this after checking/replacing the bulb, the light in the cluster illuminates, then the circuit is good.

fbarnhill

Yep,
This happened to my old 83s. The wrench replaced the bulb as well as the resister. It worked like a charm after that.

AZSAIL

Thanks everyone for your insight, problem solved! I finaly got to take the shark more than around the block now that the battery charges. I'm looking forward to putting on a lot more miles.

Tom Errickson

SharkSkin

So what was it, just the bulb?

Johnu33301

Hi All

I have a special porsche GM high output Alt that only has one plug in for the power anyone now or have a wiring diagram to help with this. Its an 82 928.....

SharkSkin

It will most likely work, but the single wire hookup will not work properly with the charge light in the dash. The charge light will always be on unless the bulb is burnt or you pull it out. Where did you get the alt?