Wtf is going on with my brake lights??

[Bagged Dually]

I'm working on my daily drive right now not my square but you guys are some of the most knowledgable I know so I figured I run this by you all.
The truck is a 1995 GMC 1500 2wd SCSB 350 and a nv3500.
I was driving to school a few days ago when someone pulled up next to me and told me my brake light weren't working. I thought ok no problem probably just a switch on the pedal needing replaced. I pulled it into the shop at school and noticed my third brake was working but not the main 2. So I know the switch is still good. I checked my flashers and they still work fine so I know it's not a broken wire to the bulb and I know the bulbs are still good. I checked the relay and it's working fine. I changed it out just to be sure and still nothing. I thought maybe it was a ground so I redid the grounds at the firewall,chassis, and engine block. I also redid the grounds at the rear of the frame for the lights them selves although since the hazards are working I don't think that was an issue. I ended up checking it the next day and messing around with it for a while. I accidentally left my headlights on for about 20 minutes while I was doing this. I pulled the dash apart and pulled the headlight switch out after someone told me it could be in there. After the light being left on for only 20 minutes the back of the switch was so hot it almost burned me when I grabbed it. Since I have aftermarket headlights and high output bulbs I thought they were just over loading the switch. All the wiring looks great so I replaced the entire headlight switch assembly with a brand new one. It's working great but nothing has changed with the brake lights. Still nothing.
I'm not sure what else to do short of running a wire from the third brake light through a relay to the main lights. I really don't want to goober it up like that. I'd like to fix it the right way and be done with it. I just can't figure it out and don't have the extra cash to be blowing on parts I don't need.
Any suggestions??

Sorry for the long post just trying to give everyone the whole picture.

 

[foamypirate]

Honestly, I would still just replace the bulbs to start with. I'm not sure, but if they are dual filament, the blinkers/flashers can work, but the brakes might be a different filament. While the bulbs are out, you can also check for voltage at the socket with a multimeter.

 

[Bagged Dually]

Honestly, I would still just replace the bulbs to start with. I'm not sure, but if they are dual filament, the blinkers/flashers can work, but the brakes might be a different filament. While the bulbs are out, you can also check for voltage at the socket with a multimeter.

My bad. I forgot to mention that I've already replaced all the bulbs, the circuit boards inside the tail lights, and all the fuses and relays. I even tried jumping the 12 volts across the relay and I still can't get them to light up. I tested the connection at the light and I'm not getting any power with the brake lights but I get the flashing 12 volt signal from the same wire with the flashers on.

This damn thing has me stumped. I've never seen an issue like this.

 

[89Suburban]

Does this truck have the orange turn signals in the back or the red?

 

[Jims86]

IIRC, The GMt 400 tail lights are each on a circut board which is part of the tail light housing, and contain electronics to keep the 3rd brake light from blinking when the blinkers are activated.

 

[RetroC10Sport]

Turn signal switch.

Jiggle the turn signal stalk with your foot on the pedal and see if the lights come on.

 

[Bagged Dually]

The turn signals on the back are red. It's a completely red housing except the back up light.
The way I understand the third brake light on my truck is that it has it's own wire from the steering column. The line splits off into 4 seperate 12 volt brake wires. Third brake, rear lights, cruise control, and one that goes no where on my truck. I'm guessing it was for some kind of trailer wiring since it was capped from gm.
I tried wiggling the turn signal lever and smacking the hazards button and it didn't do anything. Is there a way to test if the internal mechanism for the hazards and blinkers is bad without buying a new one?? I forget the name of that part but their about 200 bucks at my local part store. I think I can get it for about half of that at rock auto. I just don't want to buy it if it's not bad.

 

[89Suburban]

Trying to Google up some help for ya bro...

Next is to check the wiring at the signal light switch,check were the connecter from the switch plugs into the main harness this is located at the lower part of the steering column, its common for the brake light hot wire to heat up there causing a bad connection, ive repaired a few of these,

 

[Bagged Dually]

Well I think I've about got it narrowed down to the multi switch. I've checked all of the harnesses and wiring now and all looks well and ohms readings are fine. I'm just gonna have to buy that high priced switch and hope it works. Someone on another forum said you can get a cheap one on fleabay for about 50 bucks. I'm sure it's not the best of quality but at least if I buy it and it doesn't fix the issue I'm not out $200.

 

[bucket]

Turn signal switch.

Jiggle the turn signal stalk with your foot on the pedal and see if the lights come on.

X2. Ran into this countless times at work. Usually the flasher switch part of it craps out and causes only the third brake light to work. IIRC, the third brake light has a relay and gets it's power directly from the brake light switch and does not run through the turn signal switch.

 

[Bagged Dually]

I tried it with no luck but I think your onto something with the hazard button. I pushed it in today and it didn't feel right. I don't really know how to describe it but it almost felt really gritty or something. I read that a lot of the blazers actual got recalled for the multi switch. I'm going to try and order one tomorrow off of eBay and see what happens. I'm just dreading putting it in. From the looks of it it's going to be about as much fun to put in as a clock spring. Makes me nervous anytime I have to mess with taking an air bag out. I'm usually so paranoid about those little bombs they call a safety device that I lay them under the car being worked on once I get them pulled until its time to put it back in. I've heard that static electricity could even set one off while working on it.

 

[89Suburban]

BD, I feel your pain and hope you figure it out.