Alternator glow at high rpms?

billh1963 · billh1963's Garage

On my '76 911 2.7L, I've noticed that the alternator light starts glowing at around 5000 rpm and shines brighter the higher the rpm.

The belt is new and is tight so that isn't the issue. Is the glowing an indication of:
1. Alternator going bad?
2. VR going bad?
3. Something else I haven't been able to deduce from googling?

Opinions are appreciated...thank you

Targalid · 01-10-2016, 10:58 AM

I like to have a cigarette lighter volt meter to help diagnose problems with the charging system. I have one like this with the ability to change the viewing angle. My guess is that you are not charging at higher rpm for some reason but verify first:
http://www.amazon.com/INNOVA-3721-Battery-Charging-Monitor/dp/B000EVWDU0/ref=sr_1_3?ie=UTF8&qid=1452455932&sr=8-3&keywords=cigarette+lighter+volt+meter

DanielDudley · 01-10-2016, 11:06 AM

If the belt is slightly loose, it will start slipping at higher RPMs Recheck your tension. If you can move the fan blade with the engine off, and the engine does not try to turn, it is slipping.

manbridge 74 · 01-10-2016, 11:46 AM

If your belt is in good shape try and dress each pulley half where you see belt contact. I use red Scotchbrite. Running the belt with proper tension or even slightly less will be easier on alternator bearings.

patz · 01-10-2016, 12:15 PM

Check the belt tension, the right number of shims AND the voltage at rest, idle and 2500 rpms. Report back.

rick-l · 01-10-2016, 02:13 PM

If you have a meter put it on AC and measure the voltage at the battery and see if you see anything weird.

patz · 01-10-2016, 02:14 PM

DC, AC is in your house.

rick-l · 01-10-2016, 05:48 PM

In addition to the DC measurements look at the AC.

One of the reasons the alternator light is glowing could be the output of the diode trio (half wave rectifier) is greater than the output of the three phase full wave rectifier. This could be caused by a shorted diode on the high side of the rectifier and it is only rectifying two of the phases. This would put a lot of AC ripple on the output.

Porboynz · Porboynz's Garage

Worn brushes

billh1963 · billh1963's Garage

Will be checking everything suggested this week and will report back.

Thanks you

billh1963 · billh1963's Garage

Just realized I never reported back. Cause was a bad diode in the alternator!

stevej37 · 05-12-2017, 05:47 PM

Good you found the prob.
Been checking the site for the results everyday for 14 months now!

billh1963 · billh1963's Garage

Quote:

Originally Posted by stevej37

good you found the prob.
Been checking the site for the results everyday for 14 months now! Smilewavy

lol

Canada Kev · 05-13-2017, 03:08 AM

Quote:

Originally Posted by patz

Check the belt tension, the right number of shims AND the voltage at rest, idle and 2500 rpms. Report back.

Congrats on solving your issue; I never would have thought to check that. But with the idea of helping others in the future with diagnosing voltage regulator issues, I second the use of a cig. lighter voltage meter instead of just checking at various rpms as that might give a false negative.

When I had a VR fail, it passed that test but suddenly spiked the volts a few blocks from home when driving down the road.

Tremelune · 05-13-2017, 05:58 AM

Thank you for reporting back...I hate it when I stumble on a thread with the same problem I'm trying to solve, and the resolution is never revealed...

Original poster, what did you see?!