7 way plug tester

Short reply:
Many different controllers, SOME provide a low-current source (Still battery voltage just current limited) all the time, or any time engine is running,

Some only when they feel the need (Braking while in motion)

All depends on the controller.

I have this 7-way TRAILER SAFETY LIGHT tester purchased from AMAZON.



I too cannot see the correct small LED light come ON by pushing the brake pedal on my truck.

It turns out my OEM FORD BRAKE CONTROLLER is disabled by the truck computer system when the truck is setting still. Pushing in on the BRAKE foot pedal does not send DC VOLTAGE to the trailer BRAKE MAGNETS unless the truck is going down the road at 10MPH.

The only way I can get my BRAKE test LED LIGHT to work when testing the BRAKE OPERATION is to use the BRAKE CONTROLLER SLIDE SWITCH instead of pushing in on the truck BRAKE PEDAL.

Also my Truck ignition key has to be ON to do any of the safety lights tests.

Maybe your model truck is doing the same thing to your tester????

Roy Ken

Your GMC truck needs a fuse for the brake controller to send power to the trailer brakes. You can go to your dealer to have them install a 40 or maybe a 50amp maxi fuse.

The 7 way connector has a ground connector that the tester would use.

I'd guess your controller won't push any current without the vehicle in motion.