1997 GMC K2500 Suburban - no brakes in tow harness

Yellow wire should be energized when the brake lights are on and toggle on and off when the left turn signal is on. Green wire should be energized with the brakes are on and toggle on and off when the right turn signal is on. No other wires provide power to brake lights or turn signals.

Thanks DavisK,

I have the service manual and I showed in my original post how the manual says they're supposed to work, but there are no brake signals in those two wires on my vehicle. This is the problem.

My service manual is on CD and I scanned the CD looking for some special relay that might be in question but the only thing could find was the TCC/StopSwitch for the Suburban on the brake pedal and it seems to work fine since brake lights are ok and cruise disconnect is ok.

Could it be that since you have the towing PKG on the Suburban, that there are separate fuses some where for the brake Circuit, that wire diagram does not show. If all else fails, you could try wiring into the taillight/turn signal wires on the Suburban at the back of the vehicle tail lights.

Thanks Dr Quick,

The GM service manual says that with the HD tow pkg there are 7-wires in the tow harness and there are indeed 7 wires, with exact colors as in the manual, with the only difference between vehicle and spec being that brake signal is not in the two blink/brake wires.

I almost went ahead and wired the brake into the blink wires at the rear yesterday but I wanted to explore further to see where the brake signal was being dropped and if I could correct it properly. I couldn't find in the drawings where the turn signal and brake circuits are ganged or OR'd together and I wonder about possible faulty relay(s) or fuse(s) and I wonder about ganging them at the rear if I might be putting more amps in those small wires than they were designed for (were amps stepped down when the brake+turnsignal circuits were ganged together?).

Betcha they used 'taps' and are notoriously intermittent later on

KISS it, as making changes to the OEM design will have it potentially almost impossible to diag later on

Here is a post of mine trying to help those with GMT400's (yours is one)

1996 Suburban tow harness

Thanks BenK.

I've been looking thru the service manual and I had hit the same pages you uploaded in your thread. I still haven't found exactly where the brakes combine with the blinkers for the tow harness, but I've come across a few things that let me think I'm getting close.

On page 8A-110-6 (box at top) "WIRE LEADS ARE TO BE SPLICED TO TRAILER WIRING HARNESS OR ATTACHED TO TRAILER WIRE PLUG". I think this backs up what you mentioned - that they are (or were) spliced (somewhere).

On page 8A-202-9 there is a description of a 48-pin plug that is said to join the steering column harness, and that 48-pin plug has the white wire (pin A1) for TCC/StopLamp Switch for Suburban and it also has the dark green wire (pin E6) and yellow wire (pin E7) that are the correct colors that go to the tow harness.

So, would it be safe to splice that white brake switch output wire into the blink wires for the tow harness ??

Many thanks!
Marty

Actually where your stop lights turns into blinker lights is in the "signal light switch". When a lamp goes to blink it is disconnected from the brake circuit for that side of the vehicle. You should not have a problem with overloading a wire, as draw is very low. Also before they had a special sets of wires for a trailer, you always wired into the circuit at the tail light.

Thanks Dr Quick,

I'm going through drawings in the manual. I didn't find anything labeled "signal light switch" but I did find a "turn/hazard switch" and outputs A & B from that switch are the Yellow & Dk Green wires that go to the tail lights and on their way to the tail lights they get spliced to feed X & Z wires (same colors) of the tow harness. I haven't found from the drawing exactly where the turn/hazard switch is, maybe under the dash?

I'd sure like to figure this out to do the correct way if possible. If I wire the brake in at the rear then as I understand the brake light will be hot while a blinker is also blinking and I'm guessing we would see the blinker as an intensified pulsing light since the brake light is already on (not lifted)?

MartyW wrote:
Thanks Dr Quick,


I'd sure like to figure this out to do the correct way if possible. If I wire the brake in at the rear then as I understand the brake light will be hot while a blinker is also blinking and I'm guessing we would see the blinker as an intensified pulsing light since the brake light is already on (not lifted)?

If all of your lights are now working properly without the 7 pin connector, and the brake lights are not intensifying I don't see how hooking the green and yellow wires at the bulb will cause the brake lights to intensify. By adding those wires you are only adding an extension of what is there already.

As an afterthought have you checked all of the pins on the connector for proper operation. was helping a friend with his 5er a few weeks ago and the pins were rotated and not in the proper positions. Just a thought.

Thanks BTPO1, My first thought was to add a brake feed (white wire from brake switch) to the blink feeds in the harness. But this would have the tail lights taking input from two sources and that's what I meant by having them on with brake and then pulsing in addition with the blinker and more current pull.

I hadn't thought of wiring the '97 burb at the tail lights. Maybe this was what Dr Quick was also suggesting. I've wired vehicles at the tail lights in the past but that was when a single 2-element bulb did full duty for lights + brake + blink. I even remember we liked the special bulbs that U-Haul carried that was a regular 2-element bulb but with a soldered pigtail to use for the trailer light on that side. But with the '97 burb there are multiple bulbs for each rear tail light. I'll have to check to see what the multiple bulbs do. One is for backup lights and I think (not positive) two are used for stop/blink. If one is for brake and one is for blink then the wires are separated there as well and not combined. I'll do some more studying when I can get to it next, probably this weekend. Thanks!

Edit: I did have a few minutes today to pull the tail light assys, and each has a 4-wire harness and sure enough - left/Yellow & right/DkGrn are wires in the left/right taillight harnesses and these wires have the correct brake+blink. So, if we can't figure a correct way to get brake feed into the tow harness, I do have the option of pulling those feeds from the left & right tail light harnesses. Many thanks Dr Quick and BTPO1!