Alternator Problem

[TwinDaddy]

Hi All,
I'm new to the forum. I have a problem with my 99 Ford Ranger (XLT 4WD SC 3.0L). My battery light has been coming on intermittently when driving. When the battery light comes on, the battery gage needle drops toward "L" and my headlights and dash lights get dim. My windshield wipers get slow as well. The light will then go out and everything returns to normal.

I had the alternator replaced last year, and I brought the truck back to where I had the work done and they said the alternator was fine and it was the battery. I then took the truck back to the store where I bought the battery and they said the battery was fine, it was the alternator. The guy at the store said the alternator was flipping between +14V and -20V, which was corresponding to when the lights would get dim and the battery light was on.

Shop where I had the alternator work done said they've never heard of this.

Anybody know what's going on?

 

[MAKG]

-20V? That's a new one. Usually, they just drop out or brown out.

I'd suspect a failed rectifier. Part of the alternator. There is NO way the battery can do that. Assuming those guys know how to use a voltmeter.