2004 GMC 7500

schrade · Jan 28, 2016

Truck at work lost daytime running lights and low beams. High beams still work as do rear and clearance lights, does anyone have any ideas as to what the problem might be.

DeltaRed · Jan 28, 2016

If fuse is not an issue(always check fuses first),then I'll bet the light switch is bad. My '68 C50 did the same thing.My local NAPA had a replacement on the shelf.

MARK ROBKE · Jan 28, 2016

Could actually be bad bulbs-see if you have voltage at the bulb.I've seen both out at the same time more than once.Mark

4play · Jan 29, 2016

After checking the Low beam bulbs, I would next look at the dimmer switch and test it, with or without DRL, the headlamp power circuit goes through the dimmer to split power to either DRL/LOW or HI.

There are 3 wires for the Dimmer,
1. power in ( Yellow )
2. Low/DRL out ( Tan )
3. HI out ( green ).

4play · Jan 29, 2016

Don't worry about the headlamp switch right now, the DRL circuits don't go through the headlamp switch. The only common components are the dimmer switch and possibly DRL relay. The low beam circuit passes through the DRL relay (un-energized side of relay), this relay would have to had a very odd or rare failure or be missing for it to take out both functions.

GVSII · Jan 30, 2016

(quoted from post at 20:05:00 01/28/16) Truck at work lost daytime running lights and low beams. High beams still work as do rear and clearance lights, does anyone have any ideas as to what the problem might be.

Geeze,don't all G M vehicles deliver with light problems.Around here 3 out of 4 will have 1 or more lights out.Just the way it is!

Owen Aaland · Jan 30, 2016

Unfortunately you are almost on your own when it comes to electrical problems on those (2003 - 2009) medium duty trucks. What is shown for wiring diagrams in the repair manual is often not how the truck is actually wired. Rather than make up a new manual for the new truck model in 2003 GM tried to use the old manual and just make changes where things are different. Unfortunately many of the electrical changes are not correct in the manual. You will also find that the fuses in the dash fuse panel do not match what is shown in the manual. To make it even more of a challenge the placard on the fuse panel cover also does not match how the truck is wired.

Besides being powered through the fuses some of those trucks are wired so that the switch controls a relay rather than directly powering the lights.

2004 GMC 7500

schrade

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DeltaRed

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MARK ROBKE

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4play

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4play

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GVSII

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Owen Aaland

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