03 Silverado truck cap wiring question

[Rowdyroscoe]

Hi, I bought a cap for my truck. I opened up the owners manual and it states the CHMSL wire is tied next to 7 pin trailer connector. However, using a test light, this wire runs the tail lights also. I can't seem to find this wire for the brake light only. The book states that this CHMSL wire is light blue. The light blue wire is live with both the brake lights and tail lights. I would like to figure this out because in my state it won't pass inspection and I am up for one. I don't mind running a wire back from the cab if someone could tell me where to splice into. Thanks

[stumpuller454]

you are tying into the CHMSL? or at the back near the plug? the blue should only be powered with brake only, there may be a short somewere...

[Rowdyroscoe]

I wanted to use chmsl wire which as described in the book as being light blue. The only light blue wire that is seperate back there is outside the main harness and has a removable cap on it. This is the one that works on both the stop and tail lights. I only want the stop light. I know they make a logic box for this but I thought that the light blue wire was it.

[stumpuller454]

do you have a brake controller?

[Rowdyroscoe]

There is a brake controller but I know nothing about it. Does the controller use this wire?

[stumpuller454]

the brake controller controls the power to the blue wire, it may be cranked up and sending juice all the time, there should be an adjuster wheel on it somwere, or it may have failled and sends power all the time. back it off and see if its till powered, if so disconnect it and see if that changes anything.

[crabtruck]

The 2003's had a mid-year wiring (part) change for the trailers.
Have you noticed the trailer brake controller backlight
only comes on when when using brakes ? Often times so.

There's a harness jumper plug that's goes into the relay block.
That's left side under the IP near kick panel with a cover usually.
Early year was brown/reddish color 6 pin (2x3 sorta like relay style)
Later part of year was supposed to be white plastic.
Problem is they both have the same label number on them: 5418.
Like stump suggested, try pulling the controller's plug
and see if that gets the blue to go out.

A 2001-2002 controller usually has to be "rewired" to go in 2003-2005.
It's only 2 wires that are swapped though, 4 are ok all 1999-2005.
OLD style was pin-A = LtBlue (stop lamps) and pin-D = Brown (cntrlr backlite).
NEW style is pin-A = Brown (backlite) and pin-D = LtBlue (stop lamps).
"A" and "D" are in opposite corners (kitty-corner) from each other.

Both styles have pin-B = Red (12v to cntrl), pin-C = DkBlue (brake to trailer),
pin-E = no connection, pin-F = Black (ground).

Depending on how you came to own the truck and whether the brake controller
has been modified, this may be a warranty claim. TSB #02-05-22-004C 4/2005
If you can still find and read a tag number on the wires:
1999 was pink/purple "1982" then "6255"
2000 was pink/purple "6255"
2001-2 was orange "6884"
2003-5 was orange "5418"

As you can see from the attached diagram, there are a LOT of LtBlue wires.
Mostly they are related but not the same thing. Easier to check relay plugin first.
Down off the par of wiring I posted, the CHMSL fuse goes straight to blunt cut trailer wires.
Drawing does not show the mods with controller installed.

Hope it's as easy as this, then you're only looking at 5 minutes work.
Good luck.

[gmctrucks]

Good info guys Rowdy you came to the right forum.
Good luck! GMC

[Rowdyroscoe]

I back off the adjustment wheel on the electric brake. Everything is still the same. If I unplug the unit at the box under the dash, will the light blue wire return to BRAKE light only? I guess that I am getting old. Wiring has never been this confusing to me on any vehicle. Wiring my house was much easier. All I want for xmas is a working truck cap brake light. Thanks for all the help. Chiltons repair book has no value when it comes to you guys.

[stumpuller454]

you are checking the blue wire at the rear of the vehicle? suppose you could unplug the controller....