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Posted by: yeahmag Mar 21 2022, 11:22 AM

Got a strange one... Alternator light started flickering on my way to this weekends autocross. This was on the highway at steady rpm of around 4K. I hadn't seen it flicker like this before: sometimes on dim, sometimes on for a few seconds at a time, sometimes flickering.

I figured I'd just carry on and hope I could make it through the event. As I pulled off the freeway on to surface streets the light went off and didn't come back on for the duration of the event. Packed up and headed home and only had a brief flicker or two while on the highway. I've never seen it come on while on surface streets or at idle.

I figured I'd check the alternator for AC voltage (failed diode) and maybe change the belt (which I personally run on the loose side and is probably aging out). Anything else I should look for? Almost feels like a marginal VR, but I've never had one fail in this way.

Thanks!

-Aaron

Posted by: Spoke Mar 21 2022, 11:41 AM

There should be no slippage between the engine and the alternator. It should be tight enough that you can turn the engine using the bolt on the alternator pulley. Try tightening the belt a lot and see if it works better or not.

Posted by: emerygt350 Mar 21 2022, 12:18 PM

I have read that too tight a belt can do bad things to a type iv. Can't remember exactly what bad things, but bad things.

Posted by: yeahmag Mar 21 2022, 12:20 PM

I'm leaning heavily towards the VR at this point. Seems to be a common failure mode.

Posted by: emerygt350 Mar 22 2022, 05:51 AM

I recently installed a hella solid state, very impressed with it. 13.8 all day.

Posted by: yeahmag Mar 22 2022, 10:41 AM

Interesting. Mind sharing where you got it from?

Posted by: 914Mels Mar 22 2022, 10:50 AM

If it's an old alternator with lots of miles your brushes may just be worn to the point of losing the spring tension that holds them tight to the slip rings.

Posted by: yeahmag Mar 22 2022, 07:40 PM

Totally unknown on the age and mileage. Always been rock solid. I would think (and I could be wrong here), poor brush spring tension would go away at higher RPM.

Posted by: 914Mels Mar 22 2022, 08:19 PM

The shorter the brushes get the less the spring tension is pushing them down. Higher rpms will kick them up off the rings.

Posted by: Spoke Mar 23 2022, 11:57 AM

You could run some simple tests with engine running about 2k RPM.

On the alternator connector on the relay board, measure:

D+ to chassis ground (not the engine); should be the same as the battery
DF to chassis ground; should be 5-10V
D- to chassis ground; should be 0V
Battery POS to chassis ground

The voltages will let you know how well the battery, alternator, and wiring is doing.

Posted by: vitamin914 Mar 23 2022, 12:48 PM

This is where I got mine a few weeks ago...

https://www.pelicanparts.com/More_Info/0021547406.htm?pn=002-154-74-06-M44

Posted by: emerygt350 Mar 23 2022, 01:07 PM

I think I got it from there as well.