Alt light flickers at idle and goes bright off idle

[Winger]

I found a dead battery today. Totally dead and it won't take a charge. I replaced the battery. Car starts and idles fine. The RED charge light is flickering at idle. When I raise the idle above 1200 RPM the RED charge light gets bright and stays bright at higher RPM's then at idle the RED light starts to flicker. Battery voltage with engine off is 12.4 and 13.5 at idle. Hoping one of you Pro's can help point me in the right direction to cure this issue.

[PCA7GGR]

Voltage Regulator?

[dr914]

loose alternator belt?

[Winger]

I swapped out the VR with two known good ones light still on.
Alt belt is tight also light still on.

[Dave at Pelican Parts]

Check the voltage across the battery posts. Not the clamps, the posts themselves. Should be about 12.6V with the engine off. At idle, should be above that, on the order of 12.75-13V. At higher RPM, it shouldn't go above 14.5V.

The light coming on bright at higher RPM sounds like either the regulator is failing (but you seem to have checked that) or the diodes in the alternator may be failing.

First of all, how do you know the "known good" VR is good? The possibility that yours plus two more are bad is very small, but still at least worth acknowledging.

Has the wiring been messed with at all? Has the alternator light been replaced since the charging system last worked? Anything else like that happened since then?

Failing that, the alternator seems the most likely suspect.

--DD

[GregAmy]

Alternator. Happened to me.

Remember, that bulb is just a differential voltage indicator; if the voltage between the D+ and battery differ significantly, the light comes on. Your symptoms indicate a higher voltage differential with higher RPM.

In my street car, I had a glowing GEN light that would be dim at idle and get brighter with RPM. I did some voltage checks and found that the B+ battery voltage was ~13.5V so it was charging fine, but the D+ voltage was full alternator output, from ~14.5V at idle to ~17+V at 3000 RPM. If D+ is significantly higher than B+, then the high side of the main rectifier or something else between the stator winding and the battery (B+ terminal hardware, battery strap not tightly connected, etc.) is probably busificated.

That is such a dog-forsaken job that if you find the voltage at the battery is around 13.5 while revving then I think you can safely drive the car while you prep and plan for an alternator swap. I drove it like that for a few months until I got motivated to replace the alternator (the only thing the D+ was going to was the GEN light). A new voltage regulator did not resolve it, but a replacement alternator did.