BCM safety with wiring/control mods

[Mounce]

So I recently done away with the DRL's on my truck as some know. I'm looking to put some light bars on that circuit to replace the loss of DRL's, so, two small LED bars as DRL's mounted in the bumper.

My concern isn't the current capability of the circuit necessarily.

My concern/question is... Could I tap in to the DRL trigger coming out of the BCM that heads to the relay to trigger that circut with +12v or ground (whatever it needs) for night use of the bars? The concern is that applying power or ground to a control of the BCM without it saying so or knowing will damage the board. In my mind it should do nothing, the circuit is off and only runs the DRL's as far as I know. I just don't wanna fry my BCM

Basically I want to run a switch to said control wire. Switch off = normal DRL-like function. Switch on = DRL control override ; always on.

 

[Blckshdw]

I could see you making this work with a pair of relays. One obviously provides switched power from a 12V source to your light bars.

Basically I want to run a switch to said control wire. Switch off = normal DRL-like function. Switch on = DRL control override ; always on.

The other one provides this logic. Feed the DRL signal into the normally closed pin. 12V to the normally open pin. The common pin is your output to the power relay. Connect one side of the relay coil to power or ground, the other side to your switch (and the opposite pole)

May be a smart idea to source those 12V signals to a switched source, so you don't risk killing your battery if you leave the switch on by accident, or bump it somehow.

 

[Mounce]

Yeah my plan was to use an add-a-circuit on a switched source with a 3 amp fuse to feed the switch and activate the stock DRL relay in the underhood fuse block via the BCM trigger wire. That way there's minimal wiring and it's reusing that stock circuit.

I was just worried back feeding 12v or ground (again whatever it needs, I've gotta lookup the control circuit) to that trigger line in this fashion would damage the BCM.

 

[Blckshdw]

Going through the relay, only 1 input would be connected to the output at a time, and never to each other, so no chance the back feed into it. If you are still concerned, you could splice in a diode for peace of mind.

 

[Mounce]

Going through the relay, only 1 input would be connected to the output at a time, and never to each other, so no chance the back feed into it. If you are still concerned, you could splice in a diode for peace of mind.

That's the reassurance I was hoping for

I'm very comfortable wiring and all, but where the wires go into the modules is where I lose confidence not knowing exactly how the insides work and interact. A BCM isn't in the budget so I wanted confirmation before I tried it. Thanks, Carlton!

 

[Mounce]

Just wanted to update that I went a different route. 5 pin relay, NC (off) allows stock DRL and perimeter lighting control. NO (on) allows control with switch. Switch relay with ignition and boom. Perfection.

Initially just cut the wire and ran a switch which gave me an unforseen issue since the DRL circuit is ground switched and I didn't have an ign-on switched ground to control my feed source. So, with power feed off and the light off on the switch like it was off, it'll still pass the ground and leave the lights on. Ended up leaving them on 3 times, each time for a couple hours it had no effect on starting. I got lucky. Another issue was I lost perimeter lighting which is the whole reason I added these lights anyways, I've just added these controls because I don't want to run them for DRL all the time.

 

[Blckshdw]

Ahh, the joys of modding. Glad it worked out.

 

[Mounce]

Yeah, I'm scared of inputting 12 volts into a spot it shouldn't be and frying something. However I don't mind just unplugging whatever connector I need access to honestly. Don't know that I'd treat the pcm the same though lol.

 

[littleblazer]

I've probed the bcm on the TB with a test light many times, if that helps any. Still works fine too.