Where to Find Brake Light Only Circuit on a 2008 GMC W4500

Question:

I have a 2008 gmc w4500 truck when not pushing the brake pedal there is power to the brake light switch ,when the pedal is pushed power goes away so in order to get the brake controller to work i went back to the brake light bulb wires and tied into them and brake controller works but the problem is it seems the brake controller is back feeding power into the lights because the headlight bulbs start to glow when brake is pressed dont know what to do to fix this please help

asked by: Tony

Expert Reply:

Does your 2008 GMC W4500 have a third brake light (or at least the circuit for a third brake light)? That would be a better option than the regular brake lights. You can check the fuse panel for the CHMSL (center high mount stop light) fuse, if applicable, and tap into it.

The issue with using another circuit though is that if anything else is on that same circuit then the brake controller could send voltage back to the trailer brakes when you don't want it to.

Another possible option is to locate the BCM (body control module though it may be called something different on your W4500) and locate the circuit for the brake light switch. It may or may not work in the opposite manner like your regular brake switch. But if it is hot only when the brake pedal is pressed then that is your best option.

If you cannot find a circuit that is going to work I recommend switching to the Curt Echo Mobile Wireless Brake Controller # C51180, since the only thing it needs to work is a functioning 7-Way blade-style trailer connector.

expert reply by: Michael H