In this guide I will show you how I added a shock sensor to my 2021 Silverado.

VERY IMPORTANT: I TAKE NO RESPONSIBILITY FOR ANYTHING THAT HAPPENS IF YOU DO THIS. I RECOMMEND THAT YOU DO NOT DO THIS TO YOUR VEHICLE! THIS IS FOR INFORMATIONAL PURPOSES ONLY!

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The hood switch uses a pull-down resistor to drop the resistance from 2300 ohms to 730 ohms in order to tell the truck that the hood is opened. If the hood is opened while the alarm is armed then it will trigger the alarm.

I used a Directed Electronics 504D shock sensor which uses a negative ground signal to trigger the factory alarm.

I got the 504D off of eBay for 7 bucks. Then t-tapped the hood signal wire on the BCM with 1k & 68 resistors to trick the truck into thinking the hood was opened. I diode isolated the sensor wires to protect the circuit. I could have kept the 504D green wire looped to the blue wire but I cut it and ran the green wire to a secondary horn circuit for the warn-away feature. I connected constant 12v and accessory 12v both with 16ga wire & a 5A fuse on the circuit as close to the battery as possible. I used 16ga wire for power & ground and 22ga wire for sensor wires. The BCM was located in the driver side dash above my left foot. It is a black box with several 20+ wire plugs.

The 8616 relay gives the sensor trigger a little extra boost and the 8617 relay turns the sensor off when the truck is running.

I tapped into the hood signal with a t-tap on the hood signal wire:

504D Shock Sensor:

Add 1N4001 diode to blue wire grey stripe towards sensor
Add 1N4001 diode to green wire grey stripe towards sensor or leave it looped to blue wire
Red to 8617 Yellow
Black to GND
Use 504D looped harness or cut & Green to aftermarket horn GND

8616 Mini Relay:

Yellow to GND
Red to Constant 12v
Black to 504D Blue
Brown to 1/2W Resistors (1000 & 68) to Hood trigger (BCM BK plug pin 24 Brown/Green)

8617 Relay:

Yellow to 504D Red
Red to Accessory 12v
Orange to Constant 12v
Black to Ground

**Soon I am going to design a circuit to utilize the “warn away” feature of the 504D so the horn honks when a lighter impact is detected. I will post the writeup here when complete.