No amount of searching that I did could help me find the solution to my problem, so I'm posting a video here too see if anyone would have some insight.

I suspect the DRL module or the BCM. I just don't know.

If there is anything I can clarify, please let me know.

YouTube - ‪Auto headlamp and DRL problems.‬‏

 

Bump?

Any idears?

 

For those that can't view youtube at work (like me), could you type out a description along with any modifications you have done? Have you swapped out any of the relays with known good relays?

Also, there isn't a standalone DRL module. Control of the DRL relay is handled by the BCM (aka TBC - truck body computer).

Attached is a wiring diagram for your truck:

 

Thanks for your reply, and I will try and make this as unconfusing as possible.

My automatic headlamps have been disabled by the resistor modification. Despite this, with the headlamp switch off, my dim headlamps come on when the key is turned to the on position. Using the turn signal stalk, I can alternate between dim and bright as though my headlamps are on. During all of this, my instrument cluster illumination, brake lamps, front parking lamps and licence plate light all remain off. The come on when I manually turn the headlamp switch to the on position.

If I replace the black DRL fuse in the underhood block, the normal DRL's (the reduced power high beams) come on, while the dim lamps remain on.

I have switched all of the relays in the underhood block with known good relays, and the problem remains the same.

What I am attempting to do is retain the normal DRL's (the reduced power high beams) and to get my dims to only come on when I switch them on.

Due to some other issues I'm having, this may be a problem with my PCM from the sounds of it.

 

I think what you are calling a fuse is actually an ATM size diode (electrical device that allows current to only flow one way). Here's something worth trying. Flip it around so the arrow points in the opposite direction. If the electronic version of the '98 owners manual is correct, the arrow should be pointing towards the firewall of the truck.

 

Well, you were correct in stating that I was talking about the diode. It did only work with the arrow pointed toward the firewall.

I went out tonight to fool around with it, and fixed the problem quite by accident.

I was looking at the fuse block when I noticed that the 15amp fuse for the fog lamps was missing (I don't have fog lamps). I figured why not, and stuck one of the spare fuses in there, and voila! The headlamps went out, and the DRL's remained on. I flicked the headlamp switch on, the headlights came on, and the DRL's went out.

Looking at the wiring diagram, and I can't figure out why the missing fog lamp fuse would have caused the dims to remain in a closed circuit, but replacing the fuse opened it.

Go ahead and lock this up if you like!

 

Actually, that makes perfect sense. The FOG LP fuse powers the switching side of the headlamp grounding relay and is used regardless of whether the vehicle came equiped with fog lamps or not.

 

Something to do with making sure the fogs only come on when the headlamps are on I presume?

 

Just how they are wired. Nothing really to do with the operation of the fog lamps. Look at the wiring diagram I attached earlier. Gotta love GM and their wacky way of wiring things!