lighting problems

I have a lighting problem I have never seen before in any vehicle I have ever owned. I have a 99 windstar se. At night when the headlights are on, every so often they will get much brighter, and the dash lights will get much dimmer at the same time. Then after a while the headlights will go back to normal (dimmer) and the dash lights will get brighter like they should be. This is very annoying, as this happens every few minutes. What could possably be causing this?

Doesn't anyone have any idea? :dunno:

I notice my head lights will get a little brighter and dimmer but thats normal alternator engine load. when you hit the gas it gets brighter.. At pace speed it seem to dim down.Other than that I think it normal... To be on the safe side you can have your Alternator and Battery checked for free at most auto parts places...

This does not happen from steping on the gas or letting off the gas. It just randomly happens every few minutes. When the headlights get bright, the dash lights get very dim at the same time. When the headlights go dim (back to their normal brightness) the dash lights get bright at the same time. If it was the alternator or battery wouldn't they all get bright or dim together, instead of one getting bright and the other getting dimmer at the same time?

This does not happen from steping on the gas or letting off the gas. It just randomly happens every few minutes. When the headlights get bright, the dash lights get very dim at the same time. When the headlights go dim (back to their normal brightness) the dash lights get bright at the same time. If it was the alternator or battery wouldn't they all get bright or dim together, instead of one getting bright and the other getting dimmer at the same time?
As ever mentioned, and to eliminate that possibility, try to check your charging system ( battery and alternator). If nothing is wrong there, this could be a problem with the FEM. The front electronic module control the lights in front of the vehicule and also the lights in the cluster.

Not sure what year your van is but like catvents said the FEM or charging system could play into this.. Also as your Battery get older crazy things happen on these vans as it did on mine. When my original OE on my 2001 started to go bad last year my inside lights would flash my headlights would flash and then nothing but would start. I thought I had some bad electrical issues turned out the battery was starting to die..So I would first have battery and alternator checked out..Then go from there. Besides thats free.. And most auto parts places have a machine they can hook up so you wont have to take the battery out. But If you are unable to locate the problem there. I would pull the battery and have them to put a load test with a charge to make double sure. You can check the alternator with a volt meter. My 2001 hits about 13.5 goes up a little at fast idle..hope this helps...