Alternator stops working at higher rpm

[meatshieldchris]

So I've noticed my voltimeter needle fluctiate a bit comming off of idle, figured it was just idling low since everything came back to normal off-idle. Yesterday, the voltimeter dropped and stayed low, and the battery light came on. Managed to make it the 65km home, and was pleasantly suprised that the 2.9L standard with full lights barely knocks the battery charge down in a 1 hour drive...

I did a little more testing with a digital multimeter hooked up to the battery and run into the passenger seat. I figured out that the alternator quits charging higher than about 2000 rpm, and will kick in again once you bring it back down to 1500 rpm. cruizing at 1800 rpm, it will slowly climb to 14.5 volts, at 1200 rpm it's down to 12.5 volts, and at idle it's showing 11.85 (this is all with the headlights on). Higher than 2000 rpm, the battery light comes on and the voltimeter reads 11.85. Let it drop, the battery light shuts off, and it resumes charging. Engine off with headlights on is 11.85V. Engine off, lights off, it's 12.5v.

So, alternator's dead right? Nothing else that could cause this?

 

[dennis461]

brushes

Sounds like brushes are worn to the limit.
Easy fix if you are careful...

 

[Bob Ayers]

+1

 

[srteach]

Voltage at idle should be about 14V, and should maintain 14V unless the battery is dead or the alternator is going bad.
Loss of voltage at higher RPM usually means you are floating the brushes. Brushes need replacement. If you know what you are doing, replacing them is inexpensive. Otherwise, buy a new alternator.

 

[Randy960]

i had the same thing happen. replace you brushes before they chip and wear through the contacts. they cost like 9 bucks at advanced, but i cought it too late

 

[meatshieldchris]

Took apart the rear half of the alternator to take a look, and you're all correct, one brush is pretty well completely gone, and one slip ring is pretty worn. How much wear is acceptable? One slip ring looks good, the other has a 0.021"r groove worn into it (see pic for that and the brushes). Do I call this one dead and gone and go get a reman?

 

[meatshieldchris]

I have access to a lathe, can I just remove a little on the slip rings till they're flat?