I removed the
instrument and checked each bulb the one under the 10VDC on the meter was burned
out . Replaced it with a good one and re-assembled POD.
I also found a broken switch that the PO probably was unaware of that controlled the fuel pump. Then I messed with it the car would run and die. Another attempt at anti-thief gone wrong.
After all that was sorted out the DVM indicated 12.45 VDC when running and slowly the voltage went down. With the lights all on the VDC went to 11.45 VDC.
At this point I suspected that the car was running off the battery only. I gave it some gas and the Voltage jumped to 14.25 VDC.
I think something is still amiss. BTW I still did not see the Alt Light come on.
Quite a bit of difference between a 1979 and a 1993 - it helps to know what we are discussing...
The excitation voltage in the 928 (most years) is furnished from the ignition switch to the alternator thru the alternator warning light and a parallel-mounted 68-Ohm resistor. If the resistor or the bulb has failed, or if there is a poor connection in the circuit, the excitation voltage will be low, and the alternator will not start charging until the engine is revved to 1500-1800 RPM in most cases.
When you turn the ignition switch on, the alternator warning light should be on. If not, there is a wiring fault, a faulty bulb, etc.
Two common places for problems in the wiring between the dash and the alternator D+ are:
the multi-way connector by the jump start terminal and at the connection between the wire and the termination onto the D+ post.
Wally is right on the circuit. However I found on the 84 I had it was more a connection issue rather than a failed part. The resistor was corroded and was out of the circuit. The corrosion was not evident. Removing the resistor and cleaning all the contacts made a huge difference in both the alternator working and the operation of the cluster.
However, the only difference between the resistor in the circuit and not is requiring the engine to rev before the alternator kicks in. After the alternator kicks in it will charge. The resistor and light bulb are in parallel and the resistor is in the circuit to insure the field on the alternator will not open up and run away causing the alternator to fail.
If the battery is dead then there may be an issue of a dead battery or some bad grounds and most likely if it is a older pre 85 car an under capacity alternator. For the older cars the best bet is to upgrade to a Delco. It will solve a lot of electrical issues with the car. There is a good thread on Renn List about upgrading the alternator. It is almost a bolt up with using the Porsche pulley and putting in a spacer in for the offset.
I am going from old, old memories, so I hope that this info is correct. Replacement will require some delicate soldering.
What you want is a "grain of wheat" #2721 bulb, 12 volt, 1.2 watt. Be aware that a "2721 bulb" may be a bare bulb, or a bulb in a twist socket, and that a socket-mounted bulb might be either wire leads (which you can use) or a wedge-base bulb (which you can't use). Also be aware that there are many different wattage grain-of-wheat bulbs that will fit, but you need the 1.2 watt (100mA) version.
Sometimes, you can find these at SAAB dealers (if any are still in business) or other Euro dealers. Never tried a VW dealer...
http://www.pap-parts.com/products.asp?dept=2761 offers these - I have never bought from them, so YMMV.
At one time, Advance Auto Parts had these on cards in the euro parts section. Radio Shack had some at one time - watch the wattage rating on these.
If you get really desperate, 12 volt replacement Christmas tree bulbs are readily available at the moment, but finding a 1.2 watt version might be difficult.