LMM Fog Light Circuit Diagram? - Chevy and GMC Duramax Diesel Forum
2007.5 + (GMT 900) Non Powertrain--New Body Style-- Discussion 07.5 - 2010 Topics Not related to the Powertrain, interior, suspensions, body, wheels, tires, ect.

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post #1 of 21 (permalink) Old 02-28-2010, 05:05 PM Thread Starter
 
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LMM Fog Light Circuit Diagram?

So here’s a question for anyone that might have access to GM wiring diagrams because it’s probably going to take that level of detail to make this happen. The techs at the local dealer are either stumped or just don’t care.

My truck is a 2008 GMC Sierra 2500 SLT Four Door Diesel 4x4 and I want to control the fog lights via the fog light switch on the dash independent of the position of the headlights (high or low beam). Before anyone starts asking the “why?” question…and other than “because I want to”…I am using one of the wires that used to power the fog lights (yes, they are gone) to turn on a relay that powers something else.

I’ve seen the resistor mod and that essentially hotwires the fog lights to be on anytime the headlights are on. I’m going to reference someone else’s picture for this next part (see attachments). The top left pin of the fog light relay (where the diode jumper connects) is a switched ground that actually turns the fog lights on and off. When the fog light switch is off, it is an open ground. When the fog light switch is on, it goes straight to ground. When the high beams are on (regardless of the position of the fog light switch), it is an open ground. I need to figure out how to keep the high beams from affecting the functionality of that pin so only the fog light switch affects it.

I pulled the fuse panel on the driver’s side near the battery. It has a PCB inside it so the opening and closing of that ground may occur somewhere in there. If that’s the case, I can blow out a trace as long as it is on the outside of the PCB. I’m hoping it is a wire I can cut or a leg of a circuit I can bypass somewhere in the wiring harness or under the dash.

Any thoughts?
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post #2 of 21 (permalink) Old 02-28-2010, 05:11 PM
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if i tried to do this

i would my truck up

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post #3 of 21 (permalink) Old 02-28-2010, 05:17 PM
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been thinking about it this for a while and i think what im goin to do is bypass the fusepanel run separate wires from fogs to a relay and then wire it to the fog switch, pull the fog relay out of the panel... easiest way i think to go about doin it. just hopin that the fog light will come on in the dash when i try it


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post #4 of 21 (permalink) Old 02-28-2010, 05:25 PM
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I think what happens is when highbeam comes on it opens ground on the fog relay making the switch inoperable and lowbeam closes ground so it works, some how you have to isolate the fog wiring from the headlights maybe from under the fusepanel im not sure...


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post #5 of 21 (permalink) Old 03-01-2010, 07:47 PM
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The logic resides in the BCM. Here's the Headlight Circuit Diagram. Check the volatage going into the BCM (lower left side of dash) on the orange wire 192 going into position 21x3 and coming out on the dark green/black wire 1317 from position 11x4. If you bypass the BCM by connecting those two wires directly, you should get what you are aiming for.


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post #6 of 21 (permalink) Old 03-01-2010, 08:30 PM
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thanks for the pic, i think thats the answer... but would you cut and connect the wire or jump it? im thinking if you cut it and connect it, it will incomplete the circuit no?


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post #7 of 21 (permalink) Old 03-01-2010, 08:50 PM
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Jumping it would send power into both sides of the BCM whenever the switch is "on". That's not usually a good thing. As flaky as the GM BCM's are, I wouldn't put power into both sides of the BCM on any circuit.

By bypassing the BCM disconnecting both wires (or cutting) and hooking them together, you eliminate the BCM and wire the Fog Light Relay control strictly off of the 12V output from the switch. HOWEVER, having not explored this circuit myself, I would first test the input and output voltage on the circuit at the BCM to ensure that both Voltages match when the fog lights are "on". If the voltages match, I'd start the surgery. Be sure to only cut the wires several inches away from the BCM so that if it does not perform as advertised, you can rebuild the original circuit.

2008 GMC Sierra 2500HD SLE2 LMM 2WD Crew White/Ebony
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post #8 of 21 (permalink) Old 03-01-2010, 09:11 PM
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im gonna try that...but i think the highbeam switch will still break ground on the fog relay... maybe not........


"EDIT" I get it....... as long as the power is the same yes it will work for sure.


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Last edited by cainer70; 03-01-2010 at 09:19 PM.
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post #9 of 21 (permalink) Old 03-01-2010, 10:34 PM
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Why did GM make it so difficult? Thanks for the information.

2009 2500HD Extended Cab 4x4 - bone stock for now but not for long!
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post #10 of 21 (permalink) Old 03-11-2010, 12:39 PM
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Great job Convoyduel;
You guys may be able to dis assemble the wire connector, remove the two wires (orange wire 192 going into position 21x3 and coming out on the dark green/black wire 1317 from position 11x4.) At this point you should be able to test it by connecting the two wires and hitting the switch. If all works right the fog lights will light up. The dash Fog light indicator may not work without the BCM adding it's two cents, but hell its woth attempting. If you don't like the results put the wires back into the harness!
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