How alternator light works, a more detailed description Options

[Tom]

After reading many accounts of how this circuit works, I felt compelled to investigate further as I did not understand how two positives would cause a light to operate. They won't. One must be somewhat negative to complete the circuit. Internet searches turned up the same basic explanation, still was not buying it. I think it was being oversimplified.
This is how I think the alt light works:
When the key is on and engine not running, there is 12 volts + at the alt light power side coming from the fused side of fuse #9. The other side goes to a junction on the relay board with D+. With the key to off and a meter connected between D+ and ground at the relay board, the reading is 12 ohms. As soon as the key is turned to on, the reading jumps to 12.5 meg ohms and the light comes on. If the wire for D+ to the alt is removed, the reading stays the same and the light stays on. Removing the VR caused the reading to jump to infinity and the light goes out.
For the light to work, there has to be power to one side of the light and some resistance reading to ground for the other. Looking at the wiring diagram, one can follow the blue wire to the junction at the relay board at D+, then up thru the VR to a set of relay contacts, then down thru a ( resistor ?, not sure) and then down to the DF connection and on to the rotor where the current will produce a magnetic field. After the rotor, it goes to ground. When the alt spins enough RPM's, a voltage is produced and fed back to the VR, causing the relay to open and removes the ground path for the alt light.
I could be entirely wrong here, but this is what I see and my readings more or less confirm it. If you see an error in my thinking, please post and let me know.
Thanks,
Tom

[930cabman]

Excited

Still having an issue, but seems as though the alternator comes into charging earlier. I am still unable to get the GEN light to light under any condition (Lamp will light with 12v applied). Red/white wire has switched battery voltage. Blue wire I cannot get anything, no voltage, no continuity to ground, nothing.

But, it seems as the system comes online quicker. Alternator was bench tested and was working according to my local rebuilder. Hate to source a new alternator, but kinda looking that might be the answer.

[Superhawk996]

Excited

Still having an issue, but seems as though the alternator comes into charging earlier. I am still unable to get the GEN light to light under any condition (Lamp will light with 12v applied). Red/white wire has switched battery voltage. Blue wire I cannot get anything, no voltage, no continuity to ground, nothing.

But, it seems as the system comes online quicker. Alternator was bench tested and was working according to my local rebuilder. Hate to source a new alternator, but kinda looking that might be the answer.

Do not put a new alternator into it until you’ve measured everything with a DMM with engine running.

When you say blue wire has nothing on it is that with engine running?

If that answer is yes, you need to trace continuity of the blue wire back to the alternator and where it is connected to on the alternator.

You also need to determine what wattage bulb you have for the generator light. When the change was made to internal regulators that bulb changes, too much resistance through that bulb limits alternator field excitation at start up.

I haven’t seen anything posted that would indicate you need a new alternator.

@930cabman

[930cabman]

Excited

Still having an issue, but seems as though the alternator comes into charging earlier. I am still unable to get the GEN light to light under any condition (Lamp will light with 12v applied). Red/white wire has switched battery voltage. Blue wire I cannot get anything, no voltage, no continuity to ground, nothing.

But, it seems as the system comes online quicker. Alternator was bench tested and was working according to my local rebuilder. Hate to source a new alternator, but kinda looking that might be the answer.

Do not put a new alternator into it until you’ve measured everything with a DMM with engine running.

When you say blue wire has nothing on it is that with engine running? Yes, engine running

If that answer is yes, you need to trace continuity of the blue wire back to the alternator and where it is connected to on the alternator. A PITA with carbs, shroud, ...

You also need to determine what wattage bulb you have for the generator light. Stock wattage as far as I can tell. It will light with 12V applied. When the change was made to internal regulators that bulb changes, too much resistance through that bulb limits alternator field excitation at start up. So, an alternator will use a different bulb?

I haven’t seen anything posted that would indicate you need a new alternator.

Maybe I will continue to use the decoder ring (IMG:style_emoticons/default/aktion035.gif)

@930cabman

[Superhawk996]

When you say blue wire has nothing on it is that with engine running? Yes, engine running

If that answer is yes, you need to trace continuity of the blue wire back to the alternator and where it is connected to on the alternator. A PITA with carbs, shroud, ...

You also need to determine what wattage bulb you have for the generator light. Stock wattage as far as I can tell. It will light with 12V applied. When the change was made to internal regulators that bulb changes, too much resistance through that bulb limits alternator field excitation at start up. So, an alternator will use a different bulb?

I haven’t seen anything posted that would indicate you need a new alternator.

Maybe I will continue to use the decoder ring (IMG:style_emoticons/default/aktion035.gif)

@930cabman

Can’t help you if you choose not to troubleshoot the blue D+ wire. That wire should have a ground on it at start up and then goes to 12v once the alternator has been excited.

With respect to a bulb change. Seems crazy but if that charge lamp bulb doesn’t flow enough current - there is no excitation. See Porsche bulletin below