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How To Bypass The Hi-Beam = No Fog Circuit


Charter Member
SM 2003
Full Member
Jan 28, 2002
Larkspur, CO
The Good:
You can bypass the BCM controlled high beam/fog disable circuit in five minutes, for less than a buck.

The Bad:
You lose your high beam indicator lamp.

The Ugly:
You have to cut some circuit traces on the instrument cluster's circuit board and do some micro-soldering to get the high beam indicator lamp working again.


First the good:

Get yourself a 3M Scotchlok type 558 IDC (insulation displacement connector). ?The 558 has one pass thru port and one butt type port. ?It allows you to splice a stub of wire onto an existing line.

If you can't find one locally, I'll send you one for every one dollar you donate to the Chevy Avalanche Fan Club. ?Hurry, supplies are limited. ?;)

Next, pull off the knee bolster panel beneath the steering wheel. ?It is held in place with two screws on the bottom. ?Use a 7mm socket to remove them. ?Once the screws are removed, carefully pop out the top edge and set the panel aside.

Below the steering wheel, you'll see the three Body Control Module (BCM) connectors. ?Conn. 1 is brown, Conn. 2 is light-blue and Conn. 3 is purple.

On Connector 2 (light-blue) look for the purple wire along the top row, fifth from the right (position C2-B5). ?This is the "Headlamp Dimmer Switch High Beam Signal" input to the BCM. ?When this signal is at +12VDC, the BCM knows your low beams are on. ?When it is grounded, it knows your high beams are on (and turns off the fogs). ?Now here's the scary part... CUT IT. ?Make the cut two inches away from the connector.

Now look for the white wire (careful, there are two on this connector) that is on the bottom row, second from the end (position C2-A11). ?This is the "Headlamp Relay Coil Supply Voltage". ?Slide the Scotchlok over this wire, then insert the purple wire from conn. 2, pin B5 into the hole in the connector and push until it butts up against the end. ?Verify wire positions within the Scotchlok and crimp. To resolve the BAD and UGLY outlined at the end of this docement, stick the anode (non-striped side) of a small diode into the Scotchlok before you clamp it. Too verify a good connection, you might want to solder the diode lead to the purple wire.

What you end up with is something that looks like this (note the other half of the purple wire floating in the left of the pic):


What the mod does is allow you to go from this (fogs with low beams):


To this (fogs with high beams):


Now, in order to rectify the BAD and UGLY, you need to remove the instrument cluster. ?If you're still with me this far, my guess is that you already know how to do this.

Once you get the cluster broke apart... ahem, "disassembled", you end up with this (you don't need to take it apart this far, I was curious as how to hook up a tranny temp gauge):


What you need to do next is flip the circuit board over and cut the two circuit traces leading to and from the high beam indicator lamp. ?You can just make them out in the picture below. ?Next, I soldered some 3 ft. lengths of 30 ga. (not sure, but this stuff is small) to each solder trace where the lamp plugs in. ?I advise that when you plug the lamp back in, ohm from wire end to wire end to verify continuity through the bulb. ?Here's the only picture I have of this:


Next, take one wire from the high beam indicator lamp you added in the instrument cluster and solder it to the the diode connected to the Scotchlok connected to the white wire at C2-A11. I suggest using some shrink tubing to cover the entire connection.

Finally, tie the second wire from the high beam indicator lamp to the purple wire. I soldered the connection and covered with heat shrink, but a butt-splice crimpon will work too. ?

Also, it probably wouldn't hurt to use a length of heat shrink to act as a wire loom to protect the two wires along their route from the instrument cluster, down to the BCM (I used teflon coated wire). ?Here's a picture of what I ended up with:


As always, comments and criticisms are welcome and of course, you do this at your own risk.
Tim - this is a GREAT post

However - I know my limits (at times) and this is not one I will try :)
Good mod Tim!

I still think the RJ customs fog light controller has it's value though. ?Easier hookup, and still a functional high beam indicator lamp. ?
Worth the $20...
Now members have a choice. :0:
zimmsAV said:
Good mod Tim!
Hey, thanks.

I still think the RJ customs fog light controller has it's value though. ?Easier hookup, and still a functional high beam indicator lamp.
Hmmm... Never heard of RJ customs. Do you mean R&D? I was under the impression that their foglight controller simply turned on the fogs with the headlights. Here's the pic of an R&D hookup >:D...

I'm probably wrong, but I don't see how this mod could be done without cutting the purple wire and tying it high to fake out the BCM. You can certainly do it with underhood mods though.
You're right - R&D. I don't know where I got RJ from... :rolleyes:

Yeah there fog light controller just turns the fogs on with your headlights, but doesn't there headlight controller let you have your high, low, and fogs on at the same time?
Wires :eek: Wires :rolleyes: Wires :( Everywhere but not a sec. to blink..Great post but I decided to stick with the very easy Mods..Full time DRLs and HI Low beams at the same time ...the only thing I can't do is have my Hi's
on with my Fogs....I can live without that ;D I mean how often to you really need your:
High Beams
Low Beams
Certainly choices to be made by all ??? ??? ???

Keep on Moddin'
All I wanna know is who do I have to pay to do this MOD for me? :D

zimmsAV said:
Yeah there fog light controller just turns the fogs on with your headlights, but doesn't there headlight controller let you have your high, low, and fogs on at the same time?
My understanding was no:

RPHagan said:
Yes I know where you're going with these 2 mods. ?I had the same thought when I bought them about a month ago. ?However, be advised that while the low beams stay on with the high beams, and the fogs come on with the lows, the fogs do not stay on when you go to high beams. ?Moreover, the means to make this happen is not immediately apparent upon installation because one goes in the underhood fuse block, and the other goes behind the dash by the steering column.

Still, they are cheap and easy mods and they are pretty cool in their own rite.

Also, I agree with g, there is no way that these will affect the warranty, even in the off-chance that a dealer became aware that they are on-board. ;D ;D ;D


btw... Tonight, I added that diode to my circuit, went back to the original wiring scheme and all is well with the high-beam indicator lamp. ?Before adding the diode, I measured a negative 5VDC across the lamp when the lights were off and the dimmer switch was set to low-beam. ?This caused the high-beam indicator lamp to illuminate about 50%. Blinking the dimmer twice, cleared it, but that was annoying. ?The original post was modified to reflect the change.

I'm considering another lighting mod that would turn on the puddle lights in the mirrors, the DRL's up front and rear facing fogs at reduced output (if equipped) when hitting "unlock" on the keyfob. ?That should light up the area around the vehicle pretty well at night.
I've linked this thread into the FAQ post, you've got some good info in here, but I think you'd need a big set of kuhunas to do that mod. :cool:
Skidd said:
but I think you'd need a big set of kuhunas to do that mod. ?:cool:
Clank... clank... clank... ;D Just kidding. However, I've had to do this kind of thing more than once when I screw up a design and the prototype comes back DOA from manufacturing. :-[