On my sisters 91 Explorer, I have noticed that every now and then, the voltmeter gauge starts to flicker like crazy, which in turn flickers the headlights and occasionally bogs down the engine.

I've tested the battery, but have not had a proper chance to check the alternator fully. The connections at the battery look good, slightly dirty but not enough to cause these issues.

Turning the lights on does not start the problem. Nor does cranking the A/C or heat on full blast. No amount of power draw will cause this issue to start or stop. It basically just comes on it's own.

I was told that it mostly happens when she has her foot on the gas that the engine starts to bog down, and the flickering mostly at idle.

It has a 4.0L engine (and all the electrical components and computers) for a donor 95 Explorer.

Anyone have an idea for what might be causing this?

 

First check to make sure the serpentine belt is tight and not excessively worn. They can slip and you won't know it if they're loose or worn too much. Then test the alternator. It sounds like one of the diodes has gone out in the voltage regulator part of the alternator. Some parts stores sell the voltage regulator by themselves if you feel like taking the alternator apart to put in a new one. Or just get a new or rebuilt alternator to put in. If the belt is starting to look old it might be a good time to change it too. Check the belt tensioner for being in good condition also. Let us know what you find.

 

I will do. Since we did the engine swap, I'm hoping we still have the other alternator lying around.
Now, I just need to sort through the endless amount of parts from all of our projects to find it.

 

Sticky/worn brushes cause the output to flicker, especially a low speed