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C/K Chassis Cab Brake Lights Not Working

GM Guy

GM Guy

Manual Trans. 2WD Enthusiast
Messages
4,838
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Location
NW Kansas and SC Idaho
On dad's 1998 GMC in sig, we have had brake light problems since day one. When we first owned it, at night, all of a sudden, all 3 would light up. Ever since day one, the brake lights will not work. After our vo-tech adventure, where they replaced the switch, and cut off the nice factory lights, just to put cheap ass wal-mart ones on :sad:, the brake lights worked for a short while, then quit.

This whole time, the clearance function has worked. Basically, every single light function works except the brake lights. We have occasionally blown a hazard fuse, the one in the cab. I switched brake light relays between the underhood centers on the 98 and my 95. The brake lights still worked on my 95, so its not the relays. ALL fuses have been visually checked, all good.

Not a single code, not a single idiot light, (except the one in my head telling me i'm one ):h ) All functions work, ABS, Hydroboost, the whole works.

This vehicle has a brake controller, one that we put on (Tekonsha Voyager), that replaced the broken controller it came with. we partially used the existing controller wiring. It also of course has a trailer light plug, it has a t-harness that is all sealed up with electrical tape. (PO doings)

The controller's wired as follows:
PO's wiring: the blue wire headed to the plug, the red 3M Scotchlocked to the brake wire,
My wiring: the white grounded to the tube under dash, and the 12v coming from the accessory point on the underhood relay center, routed to a 20amp circuit breaker to the controller.

There is no signs of life from the controller.

This problem is annoying and dangerous, and the dealer is a last resort, but if needed, The truck will go there. I would like to get this fixed.

ANY help GREATLY appreciated, thank you!!!
 
Has this truck been in a wreck and had a panel replaced and tack welded? Sounds weird but my dads truck was hit on the ext. cab panel. When the dealers spot welded the drivers side ext. cab panel the shielding for the wires running to the back melted and the wires welded together from the heat. Every time my dad pushed the brakes a fuse would blow out causing him to lose his brake lights.

I don't even remember what possessed me to check the wires inside that panel but I did and that was it.
 
it sounds like a broken wire. I'd check where the PO spliced on the brake controller first. sounds like you've got some tracking to do.
 
No signs of a wreck, so i doubt that is it. I will probably change the brake signal tap to the appropriate pin on the under dash relay center, and then take out the scotchlock connector and splice the wiring back together. I guess i need to trace some wires!:banghead:

Also, how hard is it to change the brake switch? I can hardly even see it!
 
Check the trailer plug socket. On my 95, wierd things used to happen with the brake lights. Replacing the socket did the trick. If it's an aftermarket socket, bybass it for a test.
 
jrkrace said:
Check the trailer plug socket. On my 95, wierd things used to happen with the brake lights. Replacing the socket did the trick. If it's an aftermarket socket, bybass it for a test.
Click to expand...

what do you mean by bypassing it? (and they say nobody can ask a stupid question!:D) when wiring up a socket, is it best to use a t-harness?
 
what I think jrkrace means is if it has the tee plug for wiring in the trailer socket, take out the tee section and just hook the original wiring back together. the tee harnesses are alot cleaner than having to splice and tape each wire.
 
Thanks for the answers guys.:thumbsup:

Also, would cheapo wal-mart tail lights cause any problems?

(on another note, the flatbed clearance lights i think are wired into the wires coming off the t-harness headed to the plug, if that makes any difference!)

Thanks again for the help.:)
 
all the rest of the light work including the flashers and signals. I got to thinking that the flashers/signal and brake light are all the same bulb so if only one doesn't work it should be where the all join together. Does this make any sense?
 
just a number said:
all the rest of the light work including the flashers and signals. I got to thinking that the flashers/signal and brake light are all the same bulb so if only one doesn't work it should be where the all join together. Does this make any sense?
Click to expand...

Kinda, as i don't trust the wiring the vo-tech did (as well as PO) I'm starting to wonder if it would be worth it to buy a new harness and lights from GM. (or just see if it matches my HD, then i'll just buy new for mine and give dad my old (and cheaper) stuff.):D

I would also wonder if something is going on with flashers/relays, etc.

i will look under it sometime soon, kinda hard, as it is always busy, gas being cheaper means its getting a lot more work.
 
i was looking online and found that the regular c3500 used a stop lamp relay with it's own fuse. it's a MAXI 30A and was in the under hood fuse/relay center. Might be worth a look
 
just a number said:
i was looking online and found that the regular c3500 used a stop lamp relay with it's own fuse. it's a MAXI 30A and was in the under hood fuse/relay center. Might be worth a look
Click to expand...


Thanks for bringing that up... i now remember that when we got it it had a 60 in that slot, i put it back to 30. if it fried something, would it be the relay that got fried? the underhood/underdash wiring looks intact. I'm still tempted to switch to new lights/ harness, but the price is delaying that.
 
you should be able to test the relay itself and save yourself some cash. just jam the brake pedal down and see if the wire going to the light is live.
 
the wire colours are red is fused live, white is from the brake switch,light blue is to the lights, black is ground.
 
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yes and no. you can pull the relay and check for live power, ground and the signal from the brake switch. the last resort is to jump the live the the wire going to the lights and get them to work that way. if any of the testing before fails, start from there.
 
I fought with my 88 wireing for the brake light forever the brake switch was bad, one of the hardest things I have ever had to replace on a C/K series it is way up there at the top of the pedal, I ended up just haveing my entire bed rewired by my Mecanic he did a great job, but the turn signals are always fast without a trailer on her, but seein as she never gets unhooked anyway I can deal with it.
 
It prolly depends on the year mine being an 88 C3500 as aposed to the R3500 my truck is kinda a bastard child for being the newer body in a year that was still kinda the old body till 89-90, but mine was a bolt at the top of the switch through the fire wall.
 
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