1969 911E - Alternator Light

I just picked up my first longhood and couldn’t be more excited. However,the red alternator light came on today and I’m hoping someone can help. I was driving the car this morning with no light and at a stop light my idle dipped to ~800 and the light flickered. It went out as I increased the revs.

It eventually came back on a few minutes later and didn’t go out. After some searching, I’ve done a few things that hopefully help to diagnose.

1. The light does not stay on when I turn the ignition off.
2. The light does come on in the accessory position before cranking.
3. The light stays on with seemingly consistent brightness throughout the rev range.
4. Battery voltage when off: 12.38V
5. Battery voltage at idle and at 3,000 rpm: 12.06V

Finally, here’s a picture of what I’m dealing with - not sure what if any hacks have been made. Sincerely appreciate any help.

Attachment 596753

Battery should be about 12.6 to 12.7 when new and fully charged. Alternator should be at least 13.5 when running at idle and above.

With 12.06 volts when running, you are continuously discharging the battery and the engine will eventually quit in the not too distant future.

The red light is doing what it is supposed to do-warn you that the voltage is too low.

Almost certainly, the voltage regulator has problems or the alternator has failed. Do not disconnect the battery while running the engine as the alternator can be damaged

Congrats on your new long hood.

Your measured voltage at 3000rpm is abnormally low, should be about 14V or so. So your alternator is not charging properly or your voltage regulator could be faulty. There is a test to differentiate the two but I am not smart enough to tell you how to do it in a safe manner, it involves some risk if not done properly...On a new to me old car with little history to go on I would address both the alternator and the regulator

Your voltage regulator is a replacement and the wires have been spliced, one possible reason is a motorola type harness was there originally (common for 1969) and someone spliced in a bosch type plug for the replacement regulator.

One solution would be to remove the alternator, take it to a shop for testing and rebuild. Replacement alternators with built in regulators are available, they require some minor wiring modifications and some do not like to place the regulator in the hot environment on top of the engine.

It sucks to just replace stuff, but a new solid state regulator is not too expensive.

Thank you both very much for your insightful responses. I suppose, rather than perform a risky test, I’ll just replace the Voltage Regulator first. Is an exact replacement of this part the right direction? I see many kinds are available…

https://www.stoddard.com/90160320602-sic.html

Thank you again!

First thing I would do is ultra clean and shine every wire connection.
One by one
Including getting to to the wires behind the alternator.
And especially every ground.

And see where that effort leads.

Thank you - will do some cleaning up. Should I be focused on the area shown above (driver’s side of engine bay) or are there other areas that could potentially contribute?

Start with the battery, clean both posts/terminals and the ground attachment to the body. Your car had two batteries originally, so lots of em have been modified by now, some better than others. Make sure all looks well in the front trunk battery area.
Next major ground is front right transmission to body. There is a ground stud behind the fuel filter seen in your photo above with perhaps 5-10 brown wires attached. Also, you could clean the terminal at the very left of you photo, the red/white stripe wire probably goes to your alternator. Make sure your voltage regulator terminals are clean and the plug firmly seated. Your VR looks just like your stoddard pic so might guess it is not too very old.
There is a ground straps and several significant connections on the back of the alternator, but obviously you have to remove the alternator to get to them. At that point get the alternator tested.
Of course, your VOM probably has a setting to allow you to test the continuity of all these wires if you wish.

https://patrickmotorsports.com/produ...a53cacca&_ss=r

While I find the time to check connections and wait for a new VR to show up, I thought I’d post a pic of the driver’s side battery box. Looks particularly messy with some aftermarket additions (those relays on the bottom left?? Also this fuse?). Not sure if of any relevance to my issue but figured more info is better than less.

Attachment 596911

Attachment 596912

I would remove the alternator and voltage regulator and take them to an alternator/ starter shop to have them tested. Takes about an hour but you need the tool to remove the fan belt. Check the articles from the 101 projects book on Pelican Parts (online) for details.

Edit, this one:
https://www.pelicanparts.com/techart...eplacement.htm