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2003 Silverado Cruise Fail, Stop Light Issues

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I recently purchased a 2003 Silverado. The cruise control would disengage whenever the head and park lights were on, so the first thing I checked were the brake lights and all fuses. None of the brake lights worked- so we checked bulbs: low mount were fine, high mount center the bulbs had been pulled. (Previous owner had a topper on it, I assume that the reflection in the back window was a pain) Put bulbs in the high mount center and it works fine now.


Took it back to the independent local dealer and his tech worked on it for a while. He replaced the stop lamp switch, multi-function switch, and finally settled in on the Rear Lamp Junction Block... replaced it as well. But, while he was back there, it appears that he cut the light blue wire for the stop lights and used a red one coming off of the trailer lights harness to connect into. Now the brake lights work great, but no cruise at all. Hooked up a scanner and the TAC sees the "Brake applied" all of the time, even without brake pedal applied. Cruise won't ever work that way. I am not sure what signal he is drawing from- trailer brake signal maybe? Also not sure why TAC sees it all the time, but the brake lights work only when applied. I would obviously prefer not to feed my brake lights from another circuit... you just never know what kind of nasty stuff there is out there to catch.


Went back to the wiring, reconnected the light blue wire to the other end of the light blue wire, and no low brake lights. High center still works fine. No brake applied signal at the TAC. Could have cruise, but no brake lights low.


The signal comes off of the stop light switch and splits and goes through the fuses to low and to high. It would seem the TAC gets it's juice from the low light side, as the high center doesn't seem to care one way or the other what we do on the butt end of the truck. He always works now that I put bulbs in him.


So... I am down to my last nerve and would appreciate any thoughts. This seems like an open between low brake fuse and rear lamp harness on the light blue feed wire, except for the fact that when we are (I am speculating here) back feeding with the red wire as our signal, the TAC sees the brake applied signal always. An open on the feed side sure couldn't see the signal coming back upstream from another source. So, could TAC be seeing something all the time, but why only when the red wire is connected to the light blue wire to feed the brake lights. Even if I was cool with that, which I am not, if the red wire is feeding the lights "normally" when the brake is applied, where the hell is TAC getting his signal from?


Reading glasses and blurry wiring diagrams... ready for someone with better eyes and more patience to share some insight.


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  • 2 weeks later...

Try a continuity check on the light blue wire and body/frame you shouldn't have a reading. If you do it is grounded out somewhere then using a test wire (for length) check the light blue to the hot side of the fuse block. Also check the all the fuses as I have seen where a fuse is blown that has nothing to do with another fuse mess with its circuit. Let me know what you get.

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