Early the other morning I started my pickup (2000 Ranger) and left for the airport. I had been driving for about 10 miles when I noticed the battery indicator light came on. Over the next 15-20 miles my headlights dimmed and the voltage gauge showed a rapid decrease in voltage. Given the dangerous nature of driving w/o headlights I got off the freeway, parked/came to a stop (I could no longer accelerate) near an Embassy Suites hotel and caught their courtesy shuttle to the airport. When I got back, I had the battery checked, it was toast so I put a new charged battery in. My pickup started just fine, the battery indicator light remained lit, the voltage indicator read lower than ‘normal’ and dropped when I revved the engine. I drove around the block a few times and the voltage would drop whenever I accelerated and then stabalise while maintaining a constant speed.
I figured I could make it home and had a friend follow behind in case I didn’t make it. By the time I got home, the battery light had gone off and the voltage indicator read normal regardless of acceleration/engine rpm. I then tested the battery with a volt meter. It read 12.6 V with the pickup off and ~13 V on/revving.
This morning (several days later) the pickup started fine, the battery indicator light did not stay on after ignition and the voltage indicator read normal. That is until I revved the engine. Again the voltage dropped as the engine rpm increased. Any ideas?
Sounds like a bad alternator. 13 volts is not normal; around 14 give or take a little bit is more like it.
Hopefully whoever tested your battery put it on the charger for at least an hour before testing it.
Could be a regulator but many are built in to the alternator. Could also be a short in the armature, ie: replace alternator…
Check if the regulator can be swapped out. It’s probably less money than an alternator and probably less involved to swap. Did you check the connections from the alternator?
Make sure the alternator belt isn’t slipping. The higher you rev the motor, the more likely it is to slip under load. Check the underside of the belt for glazing and for proper tension.
Good advice from the previous posters. If the connections and belt tension are ok then the problem is most likely in the alternator. It is fairly common for them to have trouble while revving the RPM’s.