This is a tech note for folks that are having an issue with the alternator light not going out after the engine starts on 74-83 models. The typical symptom is that the car starts normally but the alternator light stays on until you blip the throttle to 1500rpm or so. After you blip the throttle, then the light goes off and all is normal. If you put a voltmeter in the system, you will see that you are running on battery power until the alternator light goes off. The symptoms usually follow the replacement of the alternator or voltage regulator.
There seems to be quite a few folks that are just living with this quirk, but for those of you that find it annoying, here is the solution. There is a Porsche Technical Bulletin that deals with switching alternators or voltage regulators to internal ones. Here is a link to a pdf of the bulletin:
http://members.rennlist.com/emcon5/Tech_bulletin_1.pdf
On the 2nd page, there is a little note referring to this issue:
If battery charge indicator lamp does not go out after starting the engine, the exciter current is too weak because of 2 watt lamp. In this case, supplied resistor, part number 911 641 981 00 (91ohms, 5 watts) must be installed parallel to battery charge indicator lamp.
Unfortunately, the parts referred to in bulletin are not available and the instructions are a little more than vague. So, I have taken it upon myself to document this process so that others that choose not to live with this quirk don't have to.
The first thing we need to do is find the parts. For this task, I went to
Mouser Electronics. These are the parts that I grabbed:
TE Connectivity Terminals 250 RECP W/ DBL TAB
Part #571-726308-1
Link
Xicon Wirewound Resistors - Through Hole 91ohms
Part #280-CR5-91-RC
Link
You need 2 of the terminals and 1 resistor. The parts should end up costing less than $2.50. You may consider picking up an extra of each in case you screw something up.
Now, you want to solder the resistor to the outer terminals as shown.
I'm not a fan of having bare terminals behind the dash, so I hit it up with some liquid electrical tape (available in the electrical section of Home Depot, Lowes, etc).
While it is drying, we can go to the car and pull the alternator light socket out of the back of the gauge (be sure to remember which wires go on each terminal). We just need to slide our new doohickey onto the socket for the alternator light.
Just need to go slide it back into the gauge now. It seems to fit well with the resistor towards the bottom of the gauge.
Now, we just re-install the gauge and fire up the engine. Hopefully your alternator light will now go out right after starting!