Thanks for any help in advance. 1995 GMC Suburban K1500, 5.7L 4x4. Tried a new brake pedal switch but no go, exchanged for a another in case it was a bad one but haven’t put the new one in yet. Brake lights used to flicker on and off only when I had a trailer connected and the truck had to be physically running for all lights to work on the trailer and truck, truck brake light operation returned to normal when we disconnected the trailer and has been fine for years this way. The other day the brake lights would come on the first time you pressed the pedal but the second time just a flicker quickly on and right off again. If you try it again right after that nothing happens. I waited a few seconds testing different scenarios with lights on, running lights on and ignition on or truck running and after a few seconds it would usually work again once. Second set of testing using no variables, just brakes and same thing, but now nothing for brakes at all. Chances of the new switch being bad is slim but I will try the other before jumping to conclusions, any thoughts? Could it have anything to do with trailer connectors/wiring? I have 4-pole connector on the truck temp until I install a 7-pole (been on there for 3 years with no problems though so nothing new done to trailer wiring at all). Thanks again

 

Check your grounds in the rear and also pull you tail lights out and inspect the circuit boards in the lights. They are known to corrode and do weird things before they completely go out. The trailer test makes me think it is a ground issue, so I would start by double checking all the wires in the rear going to the lights. Also the brake light swithes have to be adjusted when installed - possibly check that as well. Good luck.

 

Thanks very much for the input. What kind of adjustments need to be done to the switches? As an update, last night the new switch was locking every time I shifted into park, I had to grab and push and then back down into place to get it to shift out of park, when I parked again same thing all over again. All the wires that I could see under the bumper and around the rear end were tightly grounded but I will pull and clean rust up around them to be sure. I also pulled one lens cover off last night and one bulb looked rough although it lit fine as well as reverse bulbs and running lights worked. Third brake light above the tailgate glass on the roof also does not work of course

 

I just went through this with my suburban, I bought 2 circut boards for 30.00 from autozone and it fixed the problem. check for power at the plug if you have it there then change the boards.

 

Thanks. The brake lights are now working (from time to time) but most times when I park and shutoff I have to wiggle the switch up and down or side to side to get the truck to shift out of park and the brake lights to work again. Where are the circuit boards. Is it possible a wire or two on the harness that connects to the switch are bad. I would think so as it all works great when I grab the body of the switch and move slightly. Could any other part be worn that the switch sits on and possibly not be holding in place properly?

 

The circut boards are at the tail lights, it is the thing you un screw and remove to change bulbs, the bulbs are plugged into the board. It sounds like you just have a bad switch though (a pain to get to from what I see) the problem with it not shifting out of gear untill you wiggle the switch is because of the brake interlock switch issue... directly related to your brake light issue.........change the brake light switch and all should be fixed

 

This is the third switch including the original we took out, chances of two new bad switches are slim no? they are going to think I am crazy returning a second one. Is there a relay related to the switch setup that could be gone? Last night we got it to shift but lights weren't working. After adjusting the switch and with the truck parked the brake lights worked perfectly for about 10-12 pushes on the pedal. We shifted into reverse and the lights weren't working. Could the "pin" high up on the pedal assembly that the switch sits on/in (in the round hole), possibly be worn or screwed up in some way? It must have a metal contact or such for the metal piece that sits
inside the hole on the switch but even zip tying the switch firmly in place on this pin didn't do it last night. Also, we could physically push the button (the one you hear make the click sound when the brake pedal is depressed) and the lights would come on manually, but when pushing the pedal normally, nothing.

 

its possible that the "pin" is messed up...but you confirmed that the switch is good by testing it like you did.....is it adjustable at all (the switch)?

 

There isn't much of anything adjustable on it, nothing seems to move at all on the switch itself, I will try to get a picture up but there is nothing on it that I can see that would make any part of it adjustable. One thing I did forget to include in my replies/updates, which someone else brought up/reminded me of on another forum, is that a week or two before replacing the switch I had a rear line break that leaked quite a bit of fluid and caused the pedal to travel quite a bit, when we repaired and were bleeding the line it's quite possible I damaged/bent that assembly by applying the pedal too hard and too far too the ground? Thanks for the continued input. I am getting it looked at today by a local garage I have been going to for years. Usually a buddy aof mine can figure most repairs out so we have avoided this route for a long time. The shifting problem seems to have gone away for the time being since we tried zip tying the switch up 2 days ago but the zip ties broke quickly. Still shifts good though

 

Apparently there was a small clip missing that holds the switch onto the pin/pedal assembly tight and straight, rather than go to a wreckers we put a standard R-clip pin into the groove at the end of the pin where the factory clip used to be and it holds nicely now. Brake lights on truck and shifting working perfectly now. Thanks for all the suggestions, I appreciate it.