Triggering LED Light Bar with High Beam switch

[MrScottly]

2025 HD ZR2 Duramax... In the past people would use fuse taps to trigger the light bar from the high beam circuit in the fuse box. The new LED lighting doesn't allow you to do that, as each headlight is it's own module and those modules are controlled by the LCM (lighting control module), located under the dash next to the BCM (body control module). It is the smaller of the two modules.

To solve this problem, I ordered a trigger circuit from Amazon

https://www.amazon.com/dp/B00LW15D1M?ref=nb_sb_ss_w_as-reorder_k0_1_35&amp=&crid=XLY36GPO9YTK&amp=&sprefix=hiletgo+12v+1+channel+relay+module+

This circuit requires 12v power and only 3mA to trigger it. It engages a relay which can switch on or off. The trigger voltage can be Positive or ground...Interchangable by plugging the jumper into the proper holes on the board. I used positive.

I mounted the board in a small box

Amazon.com

I used a three wire connector on one end of the box and a two wire connector on the other.

The board has a 10amp relay, not enough to power most light bars reliably. But, the relay on the circuit board is used to power a 30amp relay that runs the light bar. If using a wiring harness supplied by the manufacturer, you are effectively replacing the switch with the circuit board relay.

The high beam circuit in the truck is activated by the turn signal lever, which sends a momentary ground signal to the BCM, which in turn sends a signal to the LCM, which then sends PWM (Pulse width modulated) voltage to the headlight module. This PWM is constant, and is the signal you will use to trigger the circuit board.

This signal is located in the white wire, coming from the green plug (pin 11) from the LCM. (see pics)

Run a wire into the cab and tap into this wire...it is the signal wire for the circuit board.

While the light bar relay gets power straight from the battery, the circuit board should get power from a keyed power source...I chose Fuse #40 in the fuse box located on the driver side, next to the battery. I used a fuse tap to pull the power from it. The circuit board uses a very small amount of power, and using this fuse has not triggered any CELs.

See pics below. Any questions, ask.

 

[GATORB8]

@MrScottly Any chance you bought the FSM software? If so, do you mind sharing where? I normally use AllData but they aren’t showing the last couple years yet.

 

[MrScottly]

@MrScottly Any chance you bought the FSM software? If so, do you mind sharing where? I normally use AllData but they aren’t showing the last couple years yet.

No, I didn't. This was dealer accessed.

 

[GATORB8]

No, I didn't. This was dealer accessed.

Damn, thanks for sharing, I need to pull the high signal for my upcoming install so this helps!

 

[amateurhour]

This is timely as I ordered a light bar and would like it to be triggered by the high beams. Thank you for sharing

 

[GATORB8]

Been a long time since I've owned a GM, so I forgot about the body builder manuals from GMUpfitters. Tons of wiring diagrams and info. Downloadable here:

Light Duty 2025 - GM Upfitter

www.gmupfitter.com

 

[8ball_99]

Been a long time since I've owned a GM, so I forgot about the body builder manuals from GMUpfitters. Tons of wiring diagrams and info. Downloadable here:

Light Duty 2025 - GM Upfitter

www.gmupfitter.com

Yep, that's the best place online to find all the plug pinouts and wiring. Can take a min to sort through, but you can find most anything.

 

[DBAG]

I have my light bar and ditch lights triggered by the high beams. I unplugged the passenger headlight and found the high beam pin with a voltage meter spliced in with a 2A fuse to the control side of the relays. Took like 15min.

 

[MrScottly]

I have my light bar and ditch lights triggered by the high beams. I unplugged the passenger headlight and found the high beam pin with a voltage meter spliced in with a 2A fuse to the control side of the relays. Took like

Well...Good luck with that. The wire going to the headlamp module is 24ga. Check back with us next year and let us know if you still have a functioning LCM.

 

[DBAG]

Well...Good luck with that. The wire going to the headlamp module is 24ga. Check back with us next year and let us know if you still have a functioning LCM.

The wire on my truck was 14 gauge. I know because I spliced it with 18. It takes less than .25A to trigger a relay. I ran the same set up on my 2020 for 4 years and it was still working when I traded it in. But ok

 

[AXE]

The wire on my truck was 14 gauge. I know because I spliced it with 18. It takes less than .25A to trigger a relay. I ran the same set up on my 2020 for 4 years and it was still working when I traded it in. But ok

Yeah, I guess he missed the relay part and assumed you direct wired it to the lightbar. LOL. I've run a lot of relays off light guage wire and fuses.

 

[MrScottly]

Yeah, I guess he missed the relay part and assumed you direct wired it to the lightbar. LOL. I've run a lot of relays off light guage wire and fuses.

I didn't miss anything and I didn't assume shit. As I said...check back next year.

 

[AXE]

I didn't miss anything and I didn't assume shit. As I said...check back next year.

That second statement was the second time you've made an assumption. Your 2-0 now, 3-0 is just a post away. Get 'er done!

 

[DBAG]

I didn't miss anything and I didn't assume shit. As I said...check back next year.

You assumed my truck is an HD! The 24 ZR2 1500 headlight wiring must be different than the HD. I was giving my experience with my truck so maybe it could help someone in the future. Sorry I may have triggered you by posting in the wrong section, I'm new here.

 

[JaredW]

This is a genuine question, what would be wrong with using the bright trigger right off the headlight to trigger the relay?

 

[MrScottly]

This is a genuine question, what would be wrong with using the bright trigger right off the headlight to trigger the relay?

In the case of my truck, the signal coming from the LCM to the headlight module is a PWM signal. The high beam circuit in the truck is activated by the turn signal lever, which sends a momentary ground signal to the BCM, which in turn sends a signal to the LCM, which then sends PWM (Pulse width modulated) voltage to the headlight module.

Not sure what you mean by "the bright trigger right off the headlight", but the PWM signal will not properly power a relay on it's own and could possibly cause damage to the LCM.

 

[JaredW]

In the case of my truck, the signal coming from the LCM to the headlight module is a PWM signal. The high beam circuit in the truck is activated by the turn signal lever, which sends a momentary ground signal to the BCM, which in turn sends a signal to the LCM, which then sends PWM (Pulse width modulated) voltage to the headlight module.

Not sure what you mean by "the bright trigger right off the headlight", but the PWM signal will not properly power a relay on it's own and could possibly cause damage to the LCM.

Okay that makes sense, I appreciate the explanation!

What I meant by that was using a wire tap on the bright beam power wire to trigger on the relay.