Motorcraft alternator = "pulsing" headlights...

I have a Motorcraft alternator w/external voltage regulator. When the Jeep is idling, the headlights have a timed "pulse" of dimming/brightening. Each total pulse is about 1 second long
(bright=.5 sec./dim=.5 sec.).

When driving, the lights are normal (at least I think they are).

I've been told that this is very common, but have never been told why. So, I'm asking...why?

Shy of changing the alternator, is there a remedy for this? Thanks.

Rich

 

Pulsing like that sounds like the voltage regulator isn't regulating the voltage very well at low rpms. bright = high voltage, dim = low voltage.

Have you hooked up a voltmeter to see what the voltage range is?

The issue could be in either the voltage regulator, or the alternator. If you, or a buddy have a spare regulator, I'd swap it in and see if it makes a difference.

I wouldn't buy a regulator to test it though. For the price of a regulator, you could buy a new GM 10 or 12si, or pick one up from the junkyard. I just picked up a cs-130 100 amp alternator from pick a part on half price day for $18.

 

I've watched the volt gauge in the dash, and the needle doesn't jump. Perhaps it's not sensitive enough to pick up the fluctuation. I'll have to use a voltmeter.

Thanks.

 

Doubtful you'll see it without a 'scope. Most meters average over roughly a few hundred milliseconds, whether analog or digital.

It's unfiltered rectified 3-phase AC, so the raw waveform will have a big ripple. Presumably that's what you are seeing at low speed. The regulator could drive the alternator field up and down in response to the ripple, but that would depend on the gain and time constant of the regulator.

 

This problem can also appear when you don't have sufficient grounding between voltage regulator and the alternator. I would try adding a new ground wire. This was a very common problem with Mopar type charge system when the engine had been removed for repairs and the ground strap from engine to body was not connected.

 

That makes sense electrically. Added resistance in the ground path would reduce the gain of the regulator (ie the voltage feedback would be weaker).