Vehicle is a 2004 suburban k1500

During a recent drive (that including towing my trailer for part of it) someone told me I had no brake lights. It seemed to be an intermittent problem where sometimes pressing the brake pedal would light up the brake lights and sometimes it would not. The 3rd brake light would never light. I started testing there and found that I had 12V getting to the 3rd brake light so I replaced it and it solved that problem. I continued to have intermittent periods with no brake lights and there was a burning smell coming from under the dash. I investigated and found the stop light switch had a badly burned contact and so I replaced that switch. Now I have no brake lights at all. Turn signals, hazards, DRL, etc all seem to be working but non of the 3 brake lights will light.

I have confirmed both the VEH STOP 15A and the 10A VEH CHMSL fuses are good, but I replaced them anyway but still have the problem. Next I pulled out the flasher relay unit because it seemed like it had been at fault in other peoples weird lighting issues. I replaced it just as a precaution, with no change in symptoms. I also disconnected my trailer brake controller...again no luck.

I have pulled the new stop light switch out and confirmed that it seems to be opening and closing the right connections when you activate the switch. Even though the old stop light switch was clearly bad and would have explained all the symptoms, clearly there is some other fault hiding in there.

Any thoughts on what could be common to all 3 brake lights, not be the stop light switch or either of those 2 fuses, or the flasher relay??

Thanks!

 

This is a Longshot but on my 01 s10 there are some ground wires that bolt to the inside of the frame at the tail end of the vehicle. If those grounds don't have a good connection you lose break lights kind of like what you are saying. I'd imagine your truck has something like that to .

You prob already checked that stuff but I figured I'd mention it.

 

Make sure that the new brake switch is exactly the same as the old brake switch. They may look the same but be a bit different.

 

Not a longshot at all and thanks for the suggestion. I did a visual inspection of both the left and right side ground attachment points and they both 'look' ok. I did not put a meter on them to confirm. The other reason I think they are good is that the rear tail lights light up just fine and I believe they share a common ground path since they are both just different elements in the same bulb.

 

Hmm...this has promise as it would explain all the symptoms. I will check to confirm when I get a work break later this morning. Thanks for the suggestion!

 

So I've confirmed the parts are identical so although a great lead, not the answer unfortunately. The internals on these switches are so simple I just dont think the switch is the culprit any more. Anyone have any other thoughts?

 

Check to see if the 2 fuses you mentioned - STOP and CHMSL are getting 12 volts when you apply the brake pedal. If they aren't, test at your brake pedal switch to see if you have 12 volts there, should be hot all the time at one leg. This will greatly narrow down where your problem is.

 

It has been a long strange trip and thanks to some helpful suggestions here along the way.

FYI: The 3rd brake light just took this opportunity to die to confuse matters. I've replaced it and it works now along with the others and I dont think it had anything to do with anything other than don't make hasty assumptions about symptoms when troubleshooting something like this.

So here is the final answer:
Based on dtcassist comments I went back to square one. Checking for power with the brake pedal depressed, there was no power to STOP fuse. Grounds looked good at the fuse panel (ie near zero resistance between ground wires and chassis ground, and in the back by the bulbs as well. Next stop was the under hood 25 amp fuse (i forget its name at the moment) but holy cow, it was blown! Now, earlier in my troubleshooting when I replaced the stop light switch with the new one the the brake lights started working aging for a short period of time, that fuse was decidedly NOT blown.

So then I replaced the fuse and assumed it would just blow again, but it did not. Brake lights were working and I started putting tools away, and then the brake lights stopped working again. Checked the 25 amp fuse, NOT blown. WTF. I pulled the stop light switch out again and found I had somehow swapped back in the original flakey one during all of the troubleshooting work. I reinstalled the new part and I'm back to all lights working again.

My final assessment of this whole thing is that I think the first thing to fail was the stop light switch. When I replaced it, I knocked something loose with the trailer brake controller wiring and during my troubleshooting I shorted something and took out the 25 am fuse. As I cleaned up all that wiring and put it all back together I was actually fixing the issue but the 25 amp fuse was already blown at that point and it did not occur to me to check it again. Everything would have been fine if I hadnt swapped in the flakey switch again and that was the last thing I had to swap out. Fingers crossed i've put this one behind me.

Thanks again to all who take the time to offer suggestions! Even little details can be helpful to someone in the end.

--Jeff