Originally Posted By: JetStar
The truck may have a headlight control module that switches and monitors the bulb current. Its what gives you a dash warning if a bulb goes out. I could be much more complex than a switch, wiring, and some relays. I seem to remember something about this when I had my 2004 Ram. I recall it flickering the headlights when the ABS would go through its BIT (built in test). IIRC it was first excursion through 5 MPH at each start-up. On the forums back then some guys were having issues with headlights and it turned out to be a $300 headlight control module.
I've seen 2 kinds of flicker.
1. momentary drops: guys that try to convert to HID's run into this. something about how it tests for a bad bulb is it cuts power for brief moments. a filament bulb won't go out during the quick drop but an HID shuts off and cycles. They don't like to use a resistance shunt in the test circuits because of the potential miniscule drop in voltage. So they do something else--- which causes the flicker. I've never looked into the theory of operation.
2. ongoing unsteady voltage. 3 of the 4 chryco products I've owned has had lumpy voltage. There's always this very mild, random fluctuation that if you really pay attn happens in all lamps, including the cluster. I've tried batteries, auxiliary grounds, fuses, filters... none made a difference, and none left me stranded from a voltage issue.