Chevy and GMC Duramax Diesel Forum banner

1 - 20 of 21 Posts

·
Registered
Joined
·
1 Posts
Discussion Starter #1
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? :)
 

Attachments

·
Word Up
Joined
·
3,517 Posts
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
 

·
Word Up
Joined
·
3,517 Posts
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...
 

·
Registered
Joined
·
69 Posts
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.

 

·
Registered
Joined
·
69 Posts
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.
 

·
Registered
Joined
·
97 Posts
Why did GM make it so difficult? Thanks for the information.
 

·
Registered
Joined
·
417 Posts
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!
 

·
Registered
Joined
·
69 Posts
I'm not sure where you get that the headlight low/high circuit is part of the fog light circuit. They are powered and switched separately. The only interface is within the module. Disconnecting the wires as I proposed will made the module think the fog lights are in the off position.
 

·
Registered
Joined
·
712 Posts
Maybe the confusion is becasue the schematic shows a Dark Green/White wire (Versus Dark Green/Black) that goes from the BCM to the Fog Lamps' replay coil. Or, am I missing something here?
 

·
Registered
Joined
·
417 Posts
My concern is that the Dash Indicator lamp for the fog lights is also driven by the BCM. Bypassing the BCM would disable the dash indicator light. Your diagram is sound!
 

·
Registered
Joined
·
150 Posts
keep us posted as to how this turns out. I just put in BFXenon 3000K Fogs in and would like to be able to run them with the high beams on as well as with the low beams.
 

·
Registered
Joined
·
417 Posts
OK Guys;
I took the dash apart today and checked the fog light switch. Bypassing the BCM (Switch to relay,) Will NOT work The fog light switch is a MOMENTARY switch. It only closes while the switch is depressed.
The next trick to try will be fooling the BCM to think that the high beams are not on! The other possibility would be to run a separate switch for the fog lights or have the BCM reprogrammed.
Sorry for the bad news guys!:mad::confused:
 

·
Registered
Joined
·
69 Posts
(forehead slap)...

As soon as I started reading your post, I realized the issue. Thus far, I had not appreciated the fact that the interior light cancellation button above it is a on-off switch but the fog light switch is in-fact a momentary switch.
 

·
Registered
Joined
·
417 Posts
exactly. I must admit after pulling it apart and checking the wrong switch, I thought the BCM had gone bad. I pulled the whole thing apart only to realize I was pushing the nterior light cancellation button. Too funny
 

·
Registered
Joined
·
218 Posts
Any other ideas? I've been trying to think of a way to make this work for a while and can't come up with anything.
 
1 - 20 of 21 Posts
Top