Unlit alternator light
 

Hi, I have a 1982 911 SC Targa. What would cause the alternator light not to come when I turn on the key? The oil light still comes on. Thanks, Dave

Try the alt. light bulb in the oil light socket. Could be just a bad bulb.

or the diode is shot. if it's the diode or the light burnt out the battery will not charge correctly

I think the alternator light bulb is of a higher wattage than the other instrument bulbs to excite the stator/coils in the alternator. But you can still use a instrument light to test the bulb it's just that you'll have to rev up the engine to get the bulb to go out. If the swap test fails your looking at a possible regulator/alternator problem. I'd pull the gauge out and test for power and ground at the bulb socket if you have no ground it's most likely the alternator or regulator. When you turn on the ignition power is applied to the bulb. The bulb get's a ground from the regulator when the car starts there's 12 volts instead of ground at that wire. That's when the bulb goes out.

Does the alternator light really have any effect on the charging? I replaced the light in my 82 SC with an LED, and everything continued to work just fine! Just wondering!

Yes that circuit with the blue wire does something with the field on the alternator. If the circuit is broken the alternator will not charge the battery. I chased that problem several years ago for several weeks, and it was a bad diode in that circuit. There is a trouble shooting checklist in the manual that explains it.

Oh and the diode is no longer available by the way. If yours is bad there are a couple threads on here that discuss how to make a new one. Or you can just bypass the diode. If you by pass it everything will still work only difference is the alt light will come on for a second when you shut the engine off.

Thanks everyone for all the information. At least I now have a few things to check out. Dave