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Posted by: yeahmag May 4 2022, 11:22 PM

I'm having an intermittent problem where over 3500 RPM (cruise speed for example) the alternator light will come on. If you LOWER the RPM the light goes out and has never come on at idle even with a full electrical load. I've done the following to try and figure it out:

• I've swapped the VR once (I have a few more to try)
• I've checked voltage at the battery while at idle and running in the driveway
• I've checked for AC voltage at the battery, which as I understand it would indicate a bad diode.
• Checked belt tension

I plan on checking:

• Battery cable at starter
• Ground strap off transmission
• Verify the relay board is sound below the VR

Can anyone think of something I may be missing here? I know my Optima Red Top is getting old, but I can't see how that would fail in this way. Got a race on Saturday and need this sorted! Help!

Posted by: yeahmag May 4 2022, 11:36 PM

The only other thing I can come up with is the brushes are getting week and the centrifugal force imposed on them is pushing them away, not letting them do their job. Just spitballing here...

Posted by: GregAmy May 5 2022, 05:19 AM

Bad alternator diode. See posts 15 and 23.

http://www.914world.com/bbs2/index.php?showtopic=326479

Posted by: yeahmag May 5 2022, 05:34 PM

I just did the test on the D+ side of the light (blue wire) and it goes to 5V. It doesn’t go high…

Posted by: Spoke May 7 2022, 06:07 AM

When does it go to 5V? It should be at battery voltage so there is no current through the GEN light.

Did you check the voltages of the VR and battery?

D- to chassis
D+ to chassis
DF to chassis
Battery voltage to chassis

Do these tests with headlights on at idle and 3500RPM.

If D+ is equal to the battery voltage at low RPM then drops with high RPM, it might indicate some sort of rotational issue.

Not on a 914 but I had a car where the alternator would charge at idle then virtually stop working at higher RPMs. Removed the alternator and checked the resistance across the brushes and armature. Got a resistance value then when I spun the rotor, the resistance went way up. Took the alternator apart and cleaned the brushes and commutation ring and the alternator worked perfectly after that.

Posted by: PatMc May 7 2022, 08:55 AM

tighten the belt a bit and see if it changes

Posted by: yeahmag May 7 2022, 10:08 PM

Had an autocross this morning and the alternator completely failed on my way to the event. Turned around and made it home under battery alone with no problem. Bruce is getting me a rebuilt unit this week.

Posted by: bdstone914 May 7 2022, 11:11 PM

It should be ready by Monday or Tuesday.