Help finding High beam signal wire

Metal.Dave · January 30

Hello fellow GM owner! I got a 2021 Sierra and I am trying to find the high beam on signal wire coming from the multifunction switch (turn signal / high low beam stalk switch). There has to be 2 signals wires, 1 functions to momentarily flash high beam, 2nd one functions to constant on high beam.

I am assuming it functions on the principle of + comes into the stalk switch, when triggered it sends the + signal to the high beam relay. (this assumption may be wrong)

Truck is off- all acc are off, most of the wires should be dead with no +signal (from the way I understand it)

I took off the steering column plastics to expose the harness going to the multifunction stalk switch and needle probed each wire with a multimeter looking for a switched + signal when the high beam is triggered. There is about 12 wires there, nearly all had a constant + 12V, a few had no signal.

I followed the steering column harness down to the footwell area where it goes into a BCM, green connector, probed every wire on that connection, nearly all had a constant + 12V, a few had no signal, and one wire triggered +12V on the high beam flash switch, but then it remained constant on 12V+ even after the high beam was off, no voltage bleed out/ drop to indicate it was just a capacitor charge, and there shouldnt be capacitor on the high beam signal wire anyways.

I am completely stumped! Why does almost every wire have a 12V+ charge when everything is turned off? Why cant I find the the 2 high beam signal wires?

Is my understanding wrong of how the electrical system works on these trucks?

And yes I specifically need to find the signal wire coming from the stalk switch, and no I cannot use the high beam + load wire after the relay as the signal wire instead.

Edited January 30 by Metal.Dave

newdude · January 30

The stalk is the grounding side of the circuit. You push forward for highs or pull towards to flash, its completing the ground to the BCM and then the BCM applies ground to the relay.

High Beam Headlamps: When the low beam headlamps are ON and the turn signal/multifunction switch is placed in the high beam position, ground is applied to the BCM through the high beam signal circuit. The BCM responds to the high beam request by applying ground to the high beam relay control circuit which energizes the high beam relay. With the high beam relay energized, the switch contacts close allowing battery voltage to flow through the high beam fuse to the high beam control circuits to there respective high beam solenoid actuators located within the headlamp assemblies. With the left and right high beam solenoid actuators active, the solenoid shutters open in each headlamp assembly exposing the remaining portion of the headlamp that was covered by the shutters illuminating the high beams at full intensity.

Circuit 524 white wire is high beams on signal, circuit 307 yellow/brown stripe is high beam flash signal.

Edited January 30 by newdude

newdude · January 30

Base and SLE headlight circuits from BCM to lamp. Circuit 1969 brown/violet strip is high beam control from BCM to the KR48 high beam relay.

SLT/AT4/Denali headlight circuits from BCM to lamp. Circuit 1969 brown/violet strip is high beam control from BCM to the KR48 high beam relay.

Edited January 30 by newdude

Metal.Dave · January 30

Appreciate the response @newdude !

So I was testing it wrong after all....

Ok to clarify- I should be testing for continuity to ground on Circuit 524 white wire, and when high beam on is triggered I should get a closed ground circuit?

newdude · January 30

2 minutes ago, Metal.Dave said:

Appreciate the response @newdude !

So I was testing it wrong after all....

Ok to clarify- I should be testing for continuity to ground on Circuit 524 white wire, and when high beam on is triggered I should get a closed ground circuit?

Based on how I read it, yes. Key on engine off and lights on, if you trigger forward on the stalk you should see it toggle to ground from open. Should see some form of resistance on there when commanded, should be open circuit when off.

Also...

Quote

And yes I specifically need to find the signal wire coming from the stalk switch, and no I cannot use the high beam + load wire after the relay as the signal wire instead.

What exactly are you trying to do? BCM won't like extra stuff on circuit input side.

Metal.Dave · January 30

I am trying to install a yellow/amber LED bar high beam so I have yellow only high beams controlled by the high beam stalk. (for heavy snow and foggy nights).

Plan is to see if that is possible when the headlamp switch S30 is set to running lights only

In running lights only position, the high beam on trigger does not turn on high beams, but I am hoping that if I tap into the signal wire from the stalk switch (before BCM) I will still get an active circuit. Im not all that good with wiring diagrams so cant tell if circuit 1851 / headlamp switch may prevent this from being possible.

If possible, I will use it to power the lowest power draw relay I can find, that relay will energize power to LED bar.

Because the high beam-on stalk movement is like a momentary switch (stalk returns to neutral position, and does not stay engaged like the older cars did) I may run into more complications as Circuit 524 white wire may not be a constant closed circuit when high beam on is triggered, and may be a momentary signal which the BCM then controls to constant signal until circuit 307 yellow/brown stripe high beam flash signal is triggered which is also the high beam off function.

asilverblazer · January 30

On my 2018... If you tap the high beam circuit 524 between the signal stalk and the BCM and use that as a ground to trigger a relay to the lights everything should work fine. The stalk provides continuous ground to the BCM when pressed forward vs pulling back to momentarily flash high beams. The light bar will work if the high beam switch is pushed forward no matter what the other lights do (based on the BCM's control). If on a 2021 the 524 provides only a momentary ground to signal the BCM to continuously power the headlights until it receives another momentary ground to switch them off, then it won't work.

In which case I would start looking for a low voltage controller of sorts that can use momentary signals to trigger constant outputs.

Another JR · January 30

4 hours ago, newdude said:

The stalk is the grounding side of the circuit. You push forward for highs or pull towards to flash, its completing the ground to the BCM and then the BCM applies ground to the relay.

High Beam Headlamps

When the low beam headlamps are ON and the turn signal/multifunction switch is placed in the high beam position, ground is applied to the BCM through the high beam signal circuit. The BCM responds to the high beam request by applying ground to the high beam relay control circuit which energizes the high beam relay. With the high beam relay energized, the switch contacts close allowing battery voltage to flow through the high beam fuse to the high beam control circuits to there respective high beam solenoid actuators located within the headlamp assemblies. With the left and right high beam solenoid actuators active, the solenoid shutters open in each headlamp assembly exposing the remaining portion of the headlamp that was covered by the shutters illuminating the high beams at full intensity.

Circuit 524 white wire is high beams on signal, circuit 307 yellow/brown stripe is high beam flash signal.

And people wonder why our trucks are so expensive …

Metal.Dave · January 30

31 minutes ago, asilverblazer said:

On my 2018... If you tap the high beam circuit 524 between the signal stalk and the BCM and use that as a ground to trigger a relay to the lights everything should work fine. The stalk provides continuous ground to the BCM when pressed forward vs pulling back to momentarily flash high beams. The light bar will work if the high beam switch is pushed forward no matter what the other lights do (based on the BCM's control). If on a 2021 the 524 provides only a momentary ground to signal the BCM to continuously power the headlights until it receives another momentary ground to switch them off, then it won't work.

In which case I would start looking for a low voltage controller of sorts that can use momentary signals to trigger constant outputs.

If its a momentary signal, A latching relay should do the trick (i think thats what they call it)

asilverblazer · January 30

2 hours ago, Metal.Dave said:

If its a momentary signal, A latching relay should do the trick (i think thats what they call it)

I don't have experience with those - but energize trigger circuit momentarily to latch relay, then energize circuit momentarily again to unlatch should work.

But can you imagine if the two functions ever got out of sync (for an unknown reason of unknown likely hood) where you flip the high beam stalk forward and end up toggling between the lightbar and high beams... a secondary momentary switch/button that disconnects the common ground path between the two functions while supplying an alternate separate ground to the lightbar (without sending that same ground to the BCM) to re-sync them might be something to include in the plan.

Metal.Dave · January 31

3 hours ago, asilverblazer said:

I don't have experience with those - but energize trigger circuit momentarily to latch relay, then energize circuit momentarily again to unlatch should work.

But can you imagine if the two functions ever got out of sync (for an unknown reason of unknown likely hood) where you flip the high beam stalk forward and end up toggling between the lightbar and high beams... a secondary momentary switch/button that disconnects the common ground path between the two functions while supplying an alternate separate ground to the lightbar (without sending that same ground to the BCM) to re-sync them might be something to include in the plan.

Imagine how convenient it would be to toggle between white or yellow high beams!! That's a hidden extra feature not a glitch!

But yeah I will put in a secondary switch.

Metal.Dave · February 1

Ok I checked it today and confirmed- it is in fact a momentary signal / circuit. It works even with all light switches off, vehicle off.

asilverblazer · February 2

On 1/31/2026 at 10:33 PM, Metal.Dave said:

Ok I checked it today and confirmed- it is in fact a momentary signal / circuit. It works even with all light switches off, vehicle off.

This might negate the need of a secondary switch, if they ever got out of whack, turn the truck off and cycle the switch to turn off the light bar.

I'm imagining a situation where you flip the switch and the latching relay doesn't catch for whatever reason, so that the high beams are on without the light bar. Then you flip the switch again and high beams go off but the relay DOES LATCH- ON, now high beams are off and the light bar is on, flipping the switch then just alternates them... until the relay doesn't catch again.

I don't know how likely that scenario is.