alternator pulsating


Mar 17, 2002
Can anyone advise me which part in the aternator needs replacing when it pulsates. At night all my lights will pulsate in accordance with RPM's, slower at low RPM's and faster with higher RPM's. I'm thinking regulator but want to be sure. TIA
Put a voltmeter on the car somewhere. Radio Shack makes one that plugs in the lighter outlet.

Or else use a meter wired to the fuse block or lighter socket.

What are the voltages at the battery car off car idling.

And voltages at say 2500 rpm.

Lights on at 2500rpm.

That will help determine what is going on.

Yes it could be the internal regulator.

Any noises such as locking bearings/squealing belt when it dims?
I use the OTC4000E and volts always between 13.5 and 14.5 whether lights are on or off. I have no trouble with charging, it is just a little annoying. No bearing problems either.
No expert here, but try going to and type in Charging System in the search field. I just did and it said that a good alternator puts out around 13.5 - 14.5, but a weak battery will potentially make lights brighten with RPM. Food for thought....I'm sure a local parts store would load test yours for free. At least it's a start.

I've repaired two gm cars that did this-It indeed was the regulater. While in there, why not put a diode trio in there also. When they go out - (usually just one one the group) you'll get a faint glow of the alt light but still charge. Also never run your car at the exact moment another car is being started when jumping someone off. This imposes a load that the alt. is not designed to take. Good Luck