Another NO brake lights truck

[stutaeng]

Hi guys, I need some help with this K3500 5.7 crew cab. I noticed that the brake lights don't work. I initially thought it was the light bulbs and replaced them, but din't do anything. I checked all of the fuses and relays.

Next, I blamed the brake light switch and bought a new one. Last night I replaced the hydroost valve because it was leaking and put in the new brake light switch, and still nothing!

The lights (not the brake lights) come on on the first setting on the lights switch (can't remember what it is called.) Also, the hazards work, along with turn signals, just no the brakes.

The truck did have a trailer wiring and it seems the previous owner cut it off. I was going to start looking at a wiring diagram. Where can I find one? I've got a Haynes manual, but it is for the GMT800 trucks.

I just saw another recent thread about a circuit board? Is this in the actual taillight? When I realized it was not the bulbs I had checked with a voltmeter...

I need to figure this out...

 

[Raulfelix24]

I know that when I bought my truck I had a similar issue. It ended up being a ground wire that was not connected. It was on the driver side under the bed close to the back bumper. Your issue may be different but it’s still worth looking at.

 

[stutaeng]

I just happened to be watching a video on a K1500 that they did a trailer brake controller. Maybe when the previous owner took off the braker controller he disconnected a wire. I'll see if I can figure it out.

Here's the video in case anybody has a similar problem down the road:

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[stutaeng]

Did a little more digging. The white wire from the brake light switch DOES have 12V when the brake pedal is tapped. Looks from the wiring diagrams this goes into the multi function switch (turn signal lever.)

The wire that goes under the frame that power the brake light is green and is also connected to the multi function switch...

 

[stutaeng]

Did a little more digging. The white wire from the brake light switch DOES have 12V when the brake pedal is tapped. Looks from the wiring diagrams this goes into the multi function switch (turn signal lever.)

The wire that goes under the frame that powers the brake light is green and is also connected to the multi function switch...

 

[stutaeng]

Well, I finally figure it out! Turned out to be a head scratcher!

To recap: I changed the brake light switch and multi-function switch and still no brake lights. I found the connector going from the engine bay under the frame back to the taillights and they turned on when I hot-wired it, so I knew the wiring was still good. Cab brake light had burnt out bulbs, so I replaced those and it those did work.

The same connector going into the main harness had the wires stripped and all bundled together. I undid the corrugated pipe covering the wires all the way back to the fuseblock, then realized the actual relay didn't have power. I could hear the relay working at this point...eventually traced the wire back to a large 30A fuse that was missing!

I think the underlying problem was brake light switch and/or multi function switch, but previous owner must have stripped those wires, in an attempt to fix it. My windshield wiper washer motor now works too!

The bad news is I accidentally cut a purple and a green wire when stripping the corrugated plastic. I resoldered and tapped them up, but now my fuel gauge is not working, LOL.