Interior dimmer switch controlling headlights...strange

[bdp1978]

Hey fella's, this question pertains to my fathers '02. I have been researching the OS on this for the past couple days off and on and havent found a similar issue.


On my Dad's TB if he uses the switch knob that controls the brightness of the instrument panel etc. he can actually turn his headlights dimmer and shut them off. When this happens it also triggers the parking lights to remain on after the truck is shut off. he has had to disconnect the battery twice since this began occurring.

I read through about the #46 relay so he swapped mine in and tried it. With my relay in the parking lights operate normally but the headlights will still dim if he turns the instrument panel down and will shut off completely if he turns it all the way.

Any ideas on this??? The closest I can figure is that it may be a BCM issue but it seems strange?

[The Roadie]

Yes, the BCM runs the dimming signal for both the headlights and the instrument panel. There's a cross-connection somewhere in the harness. Could be a bear to locate, though. If you had an AlldataDIY, you would have access to the schematics, although anybody with the shop manual could help you less efficiently. Do you have experience in the frustrating world of wiring harness tracing?

[bdp1978]

My wiring background is limited to motorcycles, but my dad is a certified master mechanic so he has some. He def. doesnt get excited about vehicle electrical systems though and the TB's dont exactly have the simplest of them.

[bdp1978]

I guess I should also ask, I understand the dimming issue as it pertains to the use of DRL's vs. normal Headlights, but the switch completely turning off the headlights? Is that attribituted to the BCM as well?

[The Roadie]

All of the switch signals are merely "advice" to the BCM who actually is the ultimate light control device. The BCM might reject the switch's advice under certain conditions, or override the switch if it wants the lights to blink or whatever for a security event. The BCM and PCM can also command about five levels of load shedding to keep the engine running as long as possible in case the alternator fails and you're running only on a dwindling battery energy supply.