Alternator light stays on until 1200-1400 RPMs

[gcdpetaluma]

My 1966 911 alternator light does not turn off until approximately 1300 RPMs. Please suggest some trouble shooting tips to begin the hunt. Thank you.

[ibmiked]

If it's a new alternator then you likely need the resistor added to the alternator light. If you Google it, you'll see that it's a known fix for this issue on 911s.

[John912]

I have the same issue that was discussed a few weeks ago regarding my 66 911. Since the light always goes out and it's charging, I left it alone (also, could not locate the resistor)

https://www.early911sregistry.org/fo...Charging-Issue

[Orange71S]

My '71 S does this but only when the engine is cold. I've never worried about it, or tried to fix it.
Jay

[911int]

Normally one side of the alternator light receives + from the ignition lock. The other side is connected with the alternator +. As long as the alternator does not produce this connection is the minus for the bulb and the circle is closed. After the alternator delivers energy you have two times + at the alternator light and it turns off. Modern alternators need to take a initial step to produce. After taken that step they deliver power also at lower rpm. Perhaps you can try to start the engine, give a short gas shock up to 2000 rpm and afterwards have a look if the light will keep turned off.

Perhaps this is the reason.

Michael

[ibmiked]

I'm not sure why you wouldn't want to fix the root of the problem, which is that the alternator + wire needs a resistor at the bulb. This is a known issue with an effective fix.

[John912]

I'm not sure why you wouldn't want to fix the root of the problem, which is that the alternator + wire needs a resistor at the bulb. This is a known issue with an effective fix.

Do you know where to get the resistor? Looks simple enough to install. 911 641 981 00 is no longer available through Pelican. Are there other Porsche sources, or is resistor common item available elsewhere?

[ibmiked]

Hi John,

I bought this diode kit from Amazon and then connected it to the posts on the alternator light:

https://www.amazon.com/gp/product/B0...?ie=UTF8&psc=1

If you scroll down to post #6 there is an excellent description of why this is needed and how it works:

https://www.early911sregistry.org/fo...light-on-again

You need to be sure to orient the diode in the correct direction. I soldered normal wire connectors on mine and then heat shrunk it. Here's the diode from the Porsche bulletin image blown up to see better how it's done.

[John912]

Great info, much appreciated. I will give it a go. Before I do, some questions:

I assume I continue to use 2w bulb?
Which of the diodes in the selection pack you sent (PN) do I use?
The bulb has a solid blue wire from VR and blue/yellow 12v power wire. Which end of the diode goes to which wire?

I only want to crawl into the frunk once


Thanks for the guidance!