Battery light comes on at high RPMs, but voltage *drops* ?

Question

In my 99 Taurus, I've noticed lately that the battery/alternator light comes on when the engine goes over 3500 rpms. I've noticed no problems other than the light coming on, but I was assuming that the voltage was going too high so I avoided doing that.

So today I hooked up a multimeter and measured the voltage at the cigarette lighter as I drove. Normally it's around 13.8 volts, which is normal.

However, at 3000 rpms, the voltage dropped to about 12.5 volts. And at 3500 rpms, it's at 11.8 volts. This is with the engine running, driving down the road, A/C and lights off. Going back to 2500 rpms, the voltage goes right back to 13.8 volts.

I was thinking that the voltage regulator on the alternator was failing, but I'd expect the voltage to go too high when the RPMs went up -- not too low. Any ideas?

And with the engine turned off, the voltage is about 12.8 volts -- as expected for a mostly charged battery.

N8N · Accepted Answer

Any evidence of belt slippage?  How tight does the belt feel?Tensioner going bad, maybe?nate

Rich · Answer

I second the belt.  The symptoms you describe are a classic belt slipissue.  Check the tensioner and replace the belt.

Donald Lewis · Answer

Better to measure it accross the battery.

Not necessarily at all.

Needs an alternator. Not uncommon for them to fail this way. Possibly a rediculously loose belt, but it would probably be screeching in that case.

GO 100% NEW! Rebuilt is junk. O'Reilly has 100% new for a reasonable price.

Don

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Bob Flumere · Answer

Belt slipping most likely would not turn the warning lamp on and the battery would be lower.

So... what you have going on there is:

A flying open circuit (centrifugal open) in the alternator rotor.. or running low on brushes..

Not that unusual a symptom.

Deal with it right away so that you don't do damage to any solid state components (or end up w/a dead bat.)

Let us know what fixes it !!

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lugnut · Answer

If the light is out and the voltage fine below 3500 and goes to hell above 3500, I would suspect the brushes are shot. and/or the slip rings are damaged. It is not likely that anything less than an alternator rebuild or replacement will correct the problem. If the belt is tight with the engine stopped, it is not likely to loosen as the engine rpm rises. The tensioner is designed such that it is on the slack side of the belt system to maintain tension. I would be more suspect of the drive belt system if this were happening at low speed and getting worse at higher speeds, but yours seems to be fine a low speeds.

Lugnut

KG · Answer

Belt possible, but I would bet on alternator brushes bouncing at higher rpm. How may miles on the alt?

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Steve · Answer

Probably an alternator mechanical problem- when its spinning fast the rotor is distorting a little, or the windings on the rotor are broken and the force causes it to stop making contact. Could also be an issue with the slipring brushes chattering at higher speed if the sliprings are getting worn.

Battery light comes on at high RPMs, but voltage drops ?

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