Trailer Brakes Lock Up When Connected to Truck Draining Trucks Battery

Updated 03/16/2017 | Published 03/15/2017

Question:

I hooked up my trailer and everything seemed to work fine, brakes worked, lights worked, etc. I parked it over night, and now the truck battery is dead and the trailer brake battery is dead. I unhooked the trailer battery and with the 7 pin connector plugged in, the trailer brakes engage and lock up the wheels even without a signal displayed on my brake box, what could cause this to happen

asked by: Joel C

Expert Reply:

Leaving your trailer connected to your truck overnight will definitely drain the truck's battery. The way to prevent this is to install a battery isolation solenoid # PK5231201 on the truck. The volt circuit on your trailer is a constant circuit so it will always provide power through the 7-way.

What do you mean you unhooked the trailer breakaway battery? Did you pull the pin? If you pulled the pin and provide power to the trailer the brakes will lock up. Normally it would charge the breakaway battery but if the pin is pulled power will run right through it engaging the brakes. I'm assuming you plugged the trailer into your truck after charging the truck's battery.

On the other hand, if your trailer brakes are always engaged when hooked up and the pin is not pulled then you either have the red wire of the brake controller connected to the wrong circuit or a short on the trailer. Either of these could cause the brakes to lock up, which could drain the trailer battery, which would drain the truck's battery.

Make sure the red wire of the brake controller is connected to the cold side of the truck's stop light switch. This will be the circuit that shows power only when the brake pedal is pressed. Also, inspect your trailer's 7-way connector and wiring and make sure they're clean and not damaged at all.

If I'm not understanding something correctly or none of this fixes your issue let me know and we'll continue to troubleshoot.

expert reply by: Charles S