2001 GMC 2500HD trailer wiring

[Dmace]

Electric brakes work fine. Everything works on truck, all but brake LIGHTS on trailer

So on the truck's plug receiver, when someone holds the brakes on the truck are the truck's brake lights coming on? If so then what is the voltage at pins 2 and 5 when the brakes are ON?
This is when it's nice to have another trailer or a friends trailer to hook up and see if the problem is the trailer or the truck.

 

[Soundguy]

you can also get a plug that goins into your 7 pin and tells you which pins are powered / when.

I'd still check the ground issue for the lamps.

soundguy

 

[Diamondpilot]

I just had this very issue come to me last night. The rear lights on the trailer would not work. It was on a 2004 GMC Yukon XL. As SoundGuy suggested on this trailer it was a bad ground. Fount that the wire was simply stripped and put under a bolt on the trailer. Not even a ring terminal.

I started my trouble shooting by first checking the Yukon with my 7 pin test box. It was ok so on to the trailer. I again have a text box and applied 12 volts to the circuit with no lights so started checking things over.

Chris

 

[KB9UDE]

OK guys. I found out that both my trailers have no brake lights, but all other lights work. On my HOPPY 4 pin test light plugged into a 7 pin adapter it shows brakes on both LT and RT...SO do I have a bad ground or whatcha think??

 

[Diamondpilot]

Did both work before? If so its possible you have had a bad day and both have a issue. Again start at the front of the trailer and work toward the rear. Look at the grounds.

Chris

 

[KB9UDE]

Yes both worked and they still do on other trucks

 

[Diamondpilot]

It had to be in your trailer. Check the grounds. Start at the tongue and make sure the white ground wire has a clean/no paint/connection with a good ring terminal. Same thing on the lights themselves. I just fixed a GM truck trailer combo last night that came back to being bad grounds at the lights themselves.

Chris

 

[Soundguy]

I use a set of jumper cales to go from fram to frame.. metal to metal.. if lamps suddenly work.. you know it is ground.

soundguy

 

[Soundguy]

that change is usually the power and brake power line..

soundguy

 

[Diamondpilot]

One other thought is to actually check the voltage at the rear of the truck. It may be low but yet enough to light up you little tester. It will only take a few minutes, a helper, and a volt meter. Make sure you use the ground pin in the plug and not another ground on the chassis so you can get the real story, not just part of it.

Chris