Poor Braking

[busy dad]

It sounds odd that you have -7.5 volts...I assume controller is wired properly..Red=stop light switch White=ground Blue=trailer plug Black=12V power. Possibly some one wired the power to ground in the tow vehicle.. I pulled this information from etrailer.com. My controller is an antique and still uses brake fluid to trigger it .

 

[Diamondpilot]

Put a known 12v source at the trailer plug then measure at the first magnet connection.

Chris

 

[wmonroe]

Put a known 12v source at the trailer plug then measure at the first magnet connection.

Chris

This is what I would do also.