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.