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2500HD/3500 Accessories Engine, Truck & Drivetrain Accessories

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  #1  
Old 12-02-2018, 01:02 PM
CrashTestDummy CrashTestDummy is offline
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Default Intermittant dash light issue on '02 2500 HD

Been a while since I've been here. We have a '02 GMC Sierra 2500HD that's used mostly these days for towing the race car. The last couple of times we've been heading out, when you turn the key on the dash lights don't come on (It's usually o'dark-thirty in the morning when we roll out of the garage).

Last month, I believe the lights came on after I switched the truck off and did a restart. Today, however, that trick did nothing. I tried moving the headlight switch from auto to on, and still nothing. I even punched the dome light button on the headlight switch assembly, and while it did turn off and back on the dome light, it didn't affect the dash lights.

We went ahead and left with the dash lights inop, since it would be light soon, and I could always shine a light on the dash.

The first time I hit a turn signal, the dash lights came on. They worked the rest of the trip.

This afternoon, on the way back home, in the daylight, I noticed the compass in the rear view mirror was out. Not sure if this is tied to the dash light issue, but it is something I noticed.

Anyone have an idea where to start looking? Is there some sort of relay for the dash lights? I haven't dug into the FSM yet, and a search of 'dash lights' here yielded nothing very helpful. Has anyone experienced this issue before?

Thanks.
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2002 2500 HD D/A
No mods other than replace tires.
Race car tow truck
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Old 12-02-2018, 03:17 PM
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DmaxMaverick DmaxMaverick is offline
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Arrow

Sounds like the dim circuit in the headlamp switch. The compass dims, as well as the radio display. The late model dimmer circuit is solid state, not the rheostats of old. I haven't heard of this in a while, but it wasn't that uncommon early on. (those that would fail early, did). The switches were redesigned for 2003+.
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1985 Blazer 6.2
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Old 12-02-2018, 03:29 PM
CrashTestDummy CrashTestDummy is offline
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It's not dim, they're out. Then on, then out. I just got back from the shop to see if I could find any obvious relays, but could not. The instrument lights came on as usual, though, when I turned the key on.

I did notice that when I did this, the compass/thermostat in the mirror was not lit. I pressed the 'Compass' button and it came back on. Weird.
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Pearland, Texas
2002 2500 HD D/A
No mods other than replace tires.
Race car tow truck
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Old 12-02-2018, 04:22 PM
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DmaxMaverick DmaxMaverick is offline
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If the dimmer circuit is open (turned off or broken), the lights don't come on, like you describe. That may not be it, but it's the easiest to access and check (burned connector, dimming output voltage at the Dk Grn wire), and the least expensive to mess with. Otherwise, the IPC or BCM may have issues.
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1985 Blazer 6.2
2001 GMC 2500HD D/A
2014 Chevy Cruze Diesel - Fabulous car, no problems at all, but sold Nov. 2016 @ 55K miles.
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Old 12-04-2018, 09:51 AM
CrashTestDummy CrashTestDummy is offline
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That DkGrn wire is in the harness going to the light switch? I may dig in a bit this weekend and see, although we're otherwise having no other issues with the truck.
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2002 2500 HD D/A
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Race car tow truck
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Old 12-04-2018, 06:54 PM
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DmaxMaverick DmaxMaverick is offline
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The dark green wire at the headlight switch connector is the instrument panel lighting dimming source. I thought I recalled it being gray, but my diagram shows dark green, so check both. One of them should follow the dimmer control knob. If neither does, then the switch likely needs replaced.

Also, the light sensor (in the middle of the dash, between the defrost duct outlets) can also fail and cause a similar situation during "auto" mode, but failure is much less common. If the headlights come on when it's dark outside, that's probably not the problem. You can bypass the auto feature by turning on the headlights manually. The dimmer should respond, then. If the lights still don't work (or if you don't get power as above), the switch is likely bad.
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1985 Blazer 6.2
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dmaxmaverick@thedieselpage.com
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