Alternator warning light

[Bill Richards]

I am tidying up the wiring in my 1969 911T and found that someone has included a diode in the wire from the alternator to the warning light.
It is not shown in the wiring diagram.
Any ideas as to what it might there for?

[Robin]

strange

A diode does not seem to make sense on the alternator light. This light works by being fed 12 volts in both directions - from the alternator as it charges and from the battery - so while the alternator is working correctly you have +12 and -12 Volts giving a potential difference of zero so light is off. When alternator gives up you have only +12 volts from battery and 0 volts (or low) from alternator so potential difference is positive and the globe light up.

A diode restricts current to one direction so I would think it would stop this light doing its job. I would suggest you go back to as per the factory wiring diagram. Unless your car has some sort of modification that would need this diode (not aware of anything that needs this).

[Cairns888]

I read about this a few years ago.
Is it something to do with keeping the battery voltage from lighting the bulb?
I will have a look for the article as my brain has a hole in it a leaks out fast.

[72_sporto]

Hi Bill,

Are you sure it's a diode? I have a K3 alternator on my car which required a black resistor to be fitted to the Alternator light in order to operate correctly.

[9er]

Resistor sounds correct, to stop the annoying glow that occurs at low idle with some set ups.

[jason]

Resistor sounds correct, to stop the annoying glow that occurs at low idle with some set ups.

Do you have any details on this?

[72_sporto]

Hi Jason, quite a bit written about this. You might like to look at-

http://forums.pelicanparts.com/porsche-911-technical-forum/667753-alternator-light-doesnt-go-out-after-startup.html

A lot of guys just put up with it- not realising the battery isn't charging.