7-pin and breakaway switch
The 7-pin brings brake, light and charge power from the truck. The breakaway switch applies the brakes from the trailer battery if it separates.
The short version
- Test each pin at the truck plug before blaming the trailer.
- Brake complaints: check the brake wire, the ground, and the controller in the truck.
- Pull the breakaway pin and the brakes should lock using the trailer battery.
- Corrosion in the plug causes most intermittent lighting problems.
On the job
- 7-way tester on the truck first. A dead pin there is the truck.
- Lights doing odd things: drop from the trailer frame to the truck battery negative with brakes on. Over 0.3 V is a bad ground.
- Breakaway: pull the pin, brakes lock (clamp the blue wire). Charged battery, clean switch. Put the pin back.
- Controller 'no trailer': ohm blue pin to ground at the trailer plug. Four 3.2 Ω magnets in parallel read about 0.8 Ω.
Try it on a service call
Put the lesson to work on a hands-on troubleshooting case.