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I posted a thread a while back asking if anyone had installed an aftermarket brake controller in these trucks and it seems no one had done so, so I went about figuring it out myself (tagging @sambor, @JimmyD, @blythkd1, @Melonhead1102, @viper8315, and @Hoser since you all expressed some interest). To set the stage, I received the "service trailer brake system" error message and found that by simply clearing DTCs with my BlueDriver the message went away and all was well. Upon reading the factory service manual, it turns out that a litany of BCM DTCs will cause the system to throw that message, and to make things worse, depending on the DTC the trailer brakes may or may not function at all once it's displayed. In my case the brakes were clearly non-functional and I find it completely unacceptable that some stupid piece of software can disable my trailer brakes while I am barreling down the road at speed. This is incompetent engineering and I have since banished it from my truck; read on for details.
Here's what you'll need:
Brake controller and truck-specific wiring harness (brand of choice; I went with the Tekonsha P3)
GM female terminal lead part number 13580829 (Wire 13580829 | GMPartsDirect.com)
Hopkins 40985 (https://www.amazon.com/gp/product/B005Q87T6E/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1)
Hopkins 35508 (https://www.amazon.com/gp/product/B0002RNS8C/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1)
10 AWG wire and loom (50 feet, just to be safe)
Miscellaneous wire splices and associated hardware/tools
To understand this process it helps to know a little about how the OEM system is laid out. Below is a typical aftermarket brake controller wiring diagram; most folks who have had older trucks with these systems will find this familiar.
![Rectangle Font Parallel Slope Handwriting]()
And here is a version I hacked up that reflects what's in our trucks.
![Rectangle Font Parallel Screenshot Slope]()
The important thing to understand here is that on an aftermarket controller, the actuator (the thing that contains the user interface, i.e. adjustment buttons and a slider for manual trailer braking) and the controller (the thing that completes the circuit and applies voltage to the brake circuit) are inside the same physical unit, whereas in our trucks they are separated into an in-cab actuator and a controller module located by the BCM above the spare tire. In an aftermarket setup the current for the brake circuit has to run into the cab and through the controller before being passed to the 7-way connector, while in our trucks this circuit never actually enters the cab. Instead, the in-cab actuator sends a very low current signal (thin red line) to the K133 controller module telling how much braking force to apply, and the K133 uses its direct 12V connection (thick black "12 Volt Hot Lead") to apply voltage to the brake circuit (thick blue line). The main takeaway here is that there is no blue wire inside running from the cab to the receptacle, so you will have to run new wire.
On the other hand, the pin header on the X61A junction block (under the dashboard, left side of driver's footwell) does contain a pin for the brake circuit. Below you can see it circled in yellow with my new brake controller harness plugged in. And yes, that pin is connected to a wire. But where does it go?
![Green Electrical wiring Circuit component Electronic engineering Computer hardware]()
It goes here; pin 32 on the X138 male (truck) side wiring harness (highlighted in yellow; immediately behind the driver-side relay/fuse panel under the hood):
![Motor vehicle Light Gas Auto part Electrical wiring]()
Well isn't that convenient? Not so fast, there's a little problem...
![Hand Communication Device Finger Eyewear Portable communications device]()
There's no terminal lead for that pin on the female-side harness, so there's no "clean" way to tie into that wire from the junction block.
To remedy this you'll need to install a new lead. Cut the small zip tie securing the wires exiting the female side harness and then use a small flathead screwdriver to pop the cover off.
![Motor vehicle Automotive design Vehicle Automotive exterior Electrical wiring]()
Next, use your screwdriver to pull/pry the purple "thing" out.
![Hand Finger Thumb Audio equipment Wrist]()
Now you can insert your new lead (thick white wire toward the bottom of the image below). Make sure it is fully seated and aligned in the same direction as the pins on the male side of the harness.
![Finger Electrical wiring Gadget Sports equipment Technology]()
Reassemble the harness and plug it back into the male side.
Now there's the matter of tying into the existing 7-way receptacle. GM uses Hopkins hardware for this, which is junk as far as I'm concerned but it's at least modular so it's easy to connect to without cutting anything. Lower or drop the spare and take a gander at what's in front of the rear bumper.
![Hood Vehicle Motor vehicle Electrical wiring Automotive exterior]()
The gray plug (red oval) is the truck-side connector, and the black receptacle (blue oval) goes to the 7-way receptacle on the bumper (green oval) as well as the "blunt cut" wires that are intended for GN/FW 7-way prep. The gray plug and black receptacle some apart so it's easy to make an inline (series) splice into this wiring that's completely modular.
Time to slice and dice the Hopkins hardware. What you want is to construct a splice with a male plug on one end and female receptacle on the other with the shortest length of wire possible (to minimize voltage drop). You will splice together all of the conductors except the blue brake wire. In my case, I ran an additional ground from the battery so I tied that together with the existing ground from the truck-side harness (hence the Wago lever nut). The truck-side brake wire (free blue wire below) should be terminated and sealed with electrical tape or heat shrink.
![Electrical wiring Tool Composite material Electrical supply Wire]()
As you can see, I used MC4 solar panel connectors for the tie-in with the wiring I ran along the frame. You could simply crimp this splice onto those wires permanently or use some other style of connector, but I like the modularity and MC4s are totally waterproof.
![]()
Now install it in-line with the existing wiring and secure it all to the bodywork with zip ties. In the image below my splice is already connected to the new wiring from the under-hood harness.
![]()
Next up is mounting the brake controller and routing the wire to the under-dash junction block. I used a Tekonsha P3 and the truck-specific wiring harness (https://www.amazon.com/gp/product/B00GJX1NYE/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1). I used a combination of two-sided tape (1" VHB tape works for this) and the screws provided with the controller. If you route the wire just right (I used a spare piece of romex I had lying around to fish it through) it will reach the junction block without an extension.
![]()
![]()
![]()
The wiring from the under-hood harness to the splice is pretty straightforward. I used marine-grade 10 AWG which had two conductors inside a jacket (same stuff I used for my RV solar panel install), so I used the second conductor to double up the grounds by running one from the battery. I enclosed the entire run in wire loom. I don't think any details are necessary but I'll show the pics; if anyone has questions about this part just let me know.
![]()
![]()
![]()
![]()
![]()
And that's about it. The controller works great so far and you can easily revert to the ITBC by simply removing the homemade splice connector and reconnecting the OEM wiring. Comments and questions welcome.
Here's what you'll need:
Brake controller and truck-specific wiring harness (brand of choice; I went with the Tekonsha P3)
GM female terminal lead part number 13580829 (Wire 13580829 | GMPartsDirect.com)
Hopkins 40985 (https://www.amazon.com/gp/product/B005Q87T6E/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1)
Hopkins 35508 (https://www.amazon.com/gp/product/B0002RNS8C/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1)
10 AWG wire and loom (50 feet, just to be safe)
Miscellaneous wire splices and associated hardware/tools
To understand this process it helps to know a little about how the OEM system is laid out. Below is a typical aftermarket brake controller wiring diagram; most folks who have had older trucks with these systems will find this familiar.
And here is a version I hacked up that reflects what's in our trucks.
The important thing to understand here is that on an aftermarket controller, the actuator (the thing that contains the user interface, i.e. adjustment buttons and a slider for manual trailer braking) and the controller (the thing that completes the circuit and applies voltage to the brake circuit) are inside the same physical unit, whereas in our trucks they are separated into an in-cab actuator and a controller module located by the BCM above the spare tire. In an aftermarket setup the current for the brake circuit has to run into the cab and through the controller before being passed to the 7-way connector, while in our trucks this circuit never actually enters the cab. Instead, the in-cab actuator sends a very low current signal (thin red line) to the K133 controller module telling how much braking force to apply, and the K133 uses its direct 12V connection (thick black "12 Volt Hot Lead") to apply voltage to the brake circuit (thick blue line). The main takeaway here is that there is no blue wire inside running from the cab to the receptacle, so you will have to run new wire.
On the other hand, the pin header on the X61A junction block (under the dashboard, left side of driver's footwell) does contain a pin for the brake circuit. Below you can see it circled in yellow with my new brake controller harness plugged in. And yes, that pin is connected to a wire. But where does it go?
It goes here; pin 32 on the X138 male (truck) side wiring harness (highlighted in yellow; immediately behind the driver-side relay/fuse panel under the hood):
Well isn't that convenient? Not so fast, there's a little problem...
There's no terminal lead for that pin on the female-side harness, so there's no "clean" way to tie into that wire from the junction block.
To remedy this you'll need to install a new lead. Cut the small zip tie securing the wires exiting the female side harness and then use a small flathead screwdriver to pop the cover off.
Next, use your screwdriver to pull/pry the purple "thing" out.
Now you can insert your new lead (thick white wire toward the bottom of the image below). Make sure it is fully seated and aligned in the same direction as the pins on the male side of the harness.
Reassemble the harness and plug it back into the male side.
Now there's the matter of tying into the existing 7-way receptacle. GM uses Hopkins hardware for this, which is junk as far as I'm concerned but it's at least modular so it's easy to connect to without cutting anything. Lower or drop the spare and take a gander at what's in front of the rear bumper.
The gray plug (red oval) is the truck-side connector, and the black receptacle (blue oval) goes to the 7-way receptacle on the bumper (green oval) as well as the "blunt cut" wires that are intended for GN/FW 7-way prep. The gray plug and black receptacle some apart so it's easy to make an inline (series) splice into this wiring that's completely modular.
Time to slice and dice the Hopkins hardware. What you want is to construct a splice with a male plug on one end and female receptacle on the other with the shortest length of wire possible (to minimize voltage drop). You will splice together all of the conductors except the blue brake wire. In my case, I ran an additional ground from the battery so I tied that together with the existing ground from the truck-side harness (hence the Wago lever nut). The truck-side brake wire (free blue wire below) should be terminated and sealed with electrical tape or heat shrink.
As you can see, I used MC4 solar panel connectors for the tie-in with the wiring I ran along the frame. You could simply crimp this splice onto those wires permanently or use some other style of connector, but I like the modularity and MC4s are totally waterproof.
Now install it in-line with the existing wiring and secure it all to the bodywork with zip ties. In the image below my splice is already connected to the new wiring from the under-hood harness.
Next up is mounting the brake controller and routing the wire to the under-dash junction block. I used a Tekonsha P3 and the truck-specific wiring harness (https://www.amazon.com/gp/product/B00GJX1NYE/ref=ppx_yo_dt_b_search_asin_title?ie=UTF8&psc=1). I used a combination of two-sided tape (1" VHB tape works for this) and the screws provided with the controller. If you route the wire just right (I used a spare piece of romex I had lying around to fish it through) it will reach the junction block without an extension.
The wiring from the under-hood harness to the splice is pretty straightforward. I used marine-grade 10 AWG which had two conductors inside a jacket (same stuff I used for my RV solar panel install), so I used the second conductor to double up the grounds by running one from the battery. I enclosed the entire run in wire loom. I don't think any details are necessary but I'll show the pics; if anyone has questions about this part just let me know.
And that's about it. The controller works great so far and you can easily revert to the ITBC by simply removing the homemade splice connector and reconnecting the OEM wiring. Comments and questions welcome.
