Yesterday I finished up replacing the alternator on my 1979 911SC Targa, after the mine suffered catastrophic mechanical failure while driving home from an auto-cross. The voltage regulator is about 18 months old, so only the alternator was replaced.
After everything was buttoned up, I fired up the car and started checking voltages. I had my grandson hold the throttle open between 2,500 and 3,000 RPM. I have a digital voltmeter plugged into the cigarette lighter location, and it showed 13.8 volts, which is an improvement over the old one. However, when I put a multi-meter at the battery, I only showed about 10 volts. I checked this with the engine at various speeds, and it always showed around 10 volts.
If it helps to know, I have a “red top” Optima battery. The battery was 100% charged before starting the car after the replacement of the alternator.
I have checked grounds and I am 100% on the ground strap on the transaxle.
While looking at the wiring diagrams in the Bentley Manual, I did notice I have an extra ground wire (See black wire in below picture). When I pulled the engine a couple of years ago, I noticed this wire was looking rough, so I replaced it with the assumption that it was supposed to be there. After all, the condition of it seemed appropriate to the age of the car.
Lastly, I am assuming that since the volt meter in the car is showing just under 14 v, that the alternator is outputting correctly and the voltage regulator is good.
The three questions are:
1) Does anyone have any thoughts on why the voltage at the battery is so low? Would the starter have anything to do with it? (it is the only thing between the alternator and the battery)
2) Do I need the extra ground wire which goes from the alternator to the crankcase?
3) If “no” to #2, then will it hurt to leave it until the next time I pull the alternator?
