Camper Shell Brake Light Wiring Help Needed.

[Zembonez]

OK

 

I've searched the world over and Googled until there are no longer any letters on my frikkin keyboard but I can't solve this problem.

 

I'm trying to hook up the CHMSL on my new shell. According to everything I read, the orange wire in the trailer brake wiring harness under the dash is for the CHMSL. I get zero power from that wire, key on, key off, running, not running, when stepping on the brake pedal. Brake lights are coming on so there should be 12V at the wire according to my schematics... Earlier model trucks had a CHMSL white wire near the back of the bed wiring harness but nothing like that here.

 

This has given me the opportunity to really look under the dash of my truck... it looks like a third grader wired it. Crappy wiring going all over. Ripped off pieces of black electrical tape everywhere. You'd think GM would have a better system after... say 100 years? Don't take me wrong here... I love my truck but it was pretty ugly under there.

 

Anybody have any input?

[asilverblazer]

Try looking at the trailer harness at the rear of the truck. Not trailer brakes.

[alaska511]

Is that a grounding circuit, Jim? I was "told" that the wiring for the trailer brake controller was a ground for the signal, not a positive circuit. I will try to confirm that with a meter, on the harness to my P3. If that is the case you could wire in a Bosch style relay.

[Zembonez]

Is that a grounding circuit, Jim? I was "told" that the wiring for the trailer brake controller was a ground for the signal, not a positive circuit. I will try to confirm that with a meter, on the harness to my P3. If that is the case you could wire in a Bosch style relay.

There is zero signal of any kind there. Tried with my VOM to see if it grounded or provided a 12V signal but zip...

 

American Technology makes a module that you hook both rear brake lights to and it provides a signal for the brake light but I can't find one anywhere. (module requires because brake and turn signal lights are shared bulbs) That would be simple enough. It is a AT-LC-1 part number but I don't findee!

[lmrizzo]

Is that a grounding circuit, Jim? I was "told" that the wiring for the trailer brake controller was a ground for the signal, not a positive circuit. I will try to confirm that with a meter, on the harness to my P3. If that is the case you could wire in a Bosch style relay.

There is zero signal of any kind there. Tried with my VOM to see if it grounded or provided a 12V signal but zip...

 

American Technology makes a module that you hook both rear brake lights to and it provides a signal for the brake light but I can't find one anywhere. (module requires because brake and turn signal lights are shared bulbs) That would be simple enough. It is a AT-LC-1 part number but I don't findee!

 

 

 

I had the same problem when I got my cap. I purchased a logic box from JC Whitney. Very simple to install, only 3 wires. 1 to each brake light and 1 to light on cap. Here's the link: http://www.jcwhitney.com/LOGIC-CIRCUIT/GP_...10067_10614.jcw

 

Hope this helps.

[Zembonez]

I had the same problem when I got my cap. I purchased a logic box from JC Whitney. Very simple to install, only 3 wires. 1 to each brake light and 1 to light on cap. Here's the link: http://www.jcwhitney.com/LOGIC-CIRCUIT/GP_...10067_10614.jcw

 

Hope this helps.

Cool! Looks like that will work. So far I've had three locations from three different dealers to look for the CHMSL wire and all three were incorrect. Everybody says it is the same as the classic 07 and down trucks but it isn't.

 

I'm ordering this part.

 

Thanks for the help!

[compulynx]

I think where you are getting mixed up is that the orange wire under the dash is NOT an input, but rather an output and goes to the trailer connector at the rear of the truck. There will be no voltage there because it is not hooked to anything but the connector at the rear.

 

You would need to connect the brake light switch output to this, then you would have voltage at the pin on the rear of the truck when the brakes are depressed.

 

 

I will test this today on mine, but I have looked at several pinouts, and the pin 7 (orange) is I assume used to work a CHMSL on the trailer is all I can figure...I guess there are some controllers that have this output built in.....

 

C

[Zembonez]

I think where you are getting mixed up is that the orange wire under the dash is NOT an input, but rather an output and goes to the trailer connector at the rear of the truck. There will be no voltage there because it is not hooked to anything but the connector at the rear.

 

You would need to connect the brake light switch output to this, then you would have voltage at the pin on the rear of the truck when the brakes are depressed.

 

 

I will test this today on mine, but I have looked at several pinouts, and the pin 7 (orange) is I assume used to work a CHMSL on the trailer is all I can figure...I guess there are some controllers that have this output built in.....

 

C

I appreciate all of the input.

 

Talked to a local camper sales guy and finally got the straight goop. He says they wire from the underdash harness. The wire for the CHMSL is located in the harness down near the E brake under the dash. I'm not going to run wires from the dash to the rear brake light so I ordered the module that separates the signal from the two rear brake/turn signal lights like lmr306 pointed out. (bid 13 bucks including shipping) It can be mounted right at the rear plug junction box on the rear crossmember and all of the wiring will be right there where I need it. Plus, there is already 12V there in my 7way trailer connector for my LED dome lights in the shell. Nice and neat.

 

Thanks again guys.

[builderpaul]

Don't know what year truck you have . My 04 came with the trailer package. The center brake wire hangs next to the connector plug. It is not part of the plug just a seperate wire that hangs next to it. Has some type of cap/connector end on it.

[asilverblazer]

/\ /\ /\ /\ Look there, should be light blue...

 

Goes to look at truck.

[asilverblazer]

Went and looked, maybe not, the harness had a tag with wires functions and colors, no light blue no CHMSL.

 

And for the record my 99 did have that wire.

[Zembonez]

Went and looked, maybe not, the harness had a tag with wires functions and colors, no light blue no CHMSL.

 

And for the record my 99 did have that wire.

From what I have found out the 99-07 classic GMT800 trucks had a CHMSL wire at the rear of the frame along the side rail that was tucked in the harness. I'm told it was a white wire. No such luck on mine though. Apparently it has been eliminated from the GMT900 wiring harness. What the heck. Maybe this will help somebody down the road that has a GMT800.