Brake light problems

[SierraIndiaTango]

Greetings,

I have recently purchased a 94 Z71, got it for $800 with some problems, most of the problems are being worked out, but one has stumped me completely

the truck had a trailer brake installed under the steering column, tied into a yellow wire under the dash and had 3 wires entering the engine bay, black-blue-white, they were already cut when we got the truck so I have no idea where they went, somewhere on the passenger side of the engine bay, i think to the far left empty stud on the junction box, at first the brake lights would not work at all but everything else did, we discovered the yellow wire that had a "quick splice" type connector was broken inside, so we rewired the yellow wire back together and had functioning brake lights, however once we turned the ignition on, no brake lights

we checked all the fuses, and then got confused a bit since the "HTR A/C" fuse lights up with the light tester, which of course makes the A/C display come on I guess, the 4WD fuse also lights up when you press the brake pedal.. does not seem like these fuses should be doing anything with the ignition off

when you press the brake, the relay also does something weird , with the ignition off the brake relay has no hot wires, when you press the brake the 2 yellow wires light up hot and the relay clicks, I believe this is how its suppose to work? the other 2 wires do nothing (cant recall what color they are at the moment), however when you turn the ignition on, the yellow wires are hot but very dim(led test light) when you press the brake pedal they light up brighter but the relay does not click and the brake lights will not come on, there is also a third wire on the relay that is hot all the time with the ignition on, if you take the cover off the relay and physically push the relay, the tail lights come on like normal

I tried a known working relay from my other truck and it still does the same thing, ignition off, no hot wires on the relay until you press the brake pedal(2 yellow wires light up) ignition ON and the 2 yellow wires are dim but get bright if you press the brake pedal (brake lights do not work) and a third wire is hot at all times as far as I know, fourth wire is never hot on the relay as far as I know

another possibly unrelated oddity is the PCM or ECU whatever, its loose behind the glovebox and I assumed it should be mounted securely ?, it also has the wire soldered with a small block of stuff, taped up with tons of electrical tape, which is grounded with a wire to the top of the PCM, its soldered into the number 10 wire on the blue plug I believe, pink with a black stripe, no idea if this could be related?

thanks in advance for any help, I can attempt to provide more information if need be, I just have no idea where to go from this point

 

[badazzbulldog]

http://www.justanswer.com/uploads/M...5_225332_brake_control_wiring_diagram_500.jpg heres a diagram of how it should be wired

 

[SierraIndiaTango]

http://www.justanswer.com/uploads/M...5_225332_brake_control_wiring_diagram_500.jpg heres a diagram of how it should be wired

is that a wiring diagram for a trailer brake? that is not exactly what I am looking for, the trailer brake was removed before I bought the truck, I don't even have the unit any longer

but that diagram is somewhat helpful, I have been worried that the trailer brake could have been the problem, but it looks like the only place it interferes with the oem wiring is the red wire, tapping into the yellow

I have been checking around more, could it possibly be the ignition switch causing the problem? I am not sure if it interacts with the brake lights, but it does make sense, because the brake lights function ALMOST properly until you turn the ignition, the A/C lighting up with the brake pedal is still confusing however

 

[SierraIndiaTango]

Tried a different ignition switch and it still does the same thing(used not new)

another new or rather, previously unnoticed symptom and just a bit of clarification, with the ignition on, everything functions properly except the windshield wipers(possibly unrelated?)

when to turn the key off, the A/C display will remain lit as though the A/C were off, press the brake pedal and the brake lights will not light up, press the blue A/C button and the display goes off, now the brake lights will work.. however

pressing the brake pedal will turn the A/C on, not just light up the display like I thought, its actually turning the A/C on, it will also turn the radio and you can listen to the radio, as long as you have the brake pedal pressed

pressing the brake pedal with the ignition off will send power across the A/C, 4WD, Radio and Brake fuses, with the ignition off should these fuses have any power going through them at all?

 

[badazzbulldog]

check your hazard/flasher switch mines bad on my 97 and if its in the on position when i step on the brakes it lights up front marker lights also check all plugs on steering column

 

[SierraIndiaTango]

Problem solved!

finally, 3 straight days of working on just the tail lights, we replaced nearly everything relating to lights, flashers included, rechecked all the grounds multiple times and even made new grounds, but it still revealed itself to be a grounding issue

the brake light relay appears to be grounded to the back of the intake with a strap, it was still bolted in position but we decided to check it further and attempted to peel back the split loom on the harness, the ground strap pulled out of the harness, broken in two inside the harness, soldered the wire back together and covered it in heat shrink, everything works perfectly even the windshield wipers

so much headache over a ground

 

[TruckinAin4Sissies]

Problem solved!

finally, 3 straight days of working on just the tail lights, we replaced nearly everything relating to lights, flashers included, rechecked all the grounds multiple times and even made new grounds, but it still revealed itself to be a grounding issue

the brake light relay appears to be grounded to the back of the intake with a strap, it was still bolted in position but we decided to check it further and attempted to peel back the split loom on the harness, the ground strap pulled out of the harness, broken in two inside the harness, soldered the wire back together and covered it in heat shrink, everything works perfectly even the windshield wipers

so much headache over a ground

I'm glad you figured it out!
Could you post some pictures?



-Truckin-

 

[thohill]

another new or rather, previously unnoticed symptom and just a bit of clarification, with the ignition on, everything functions properly except the windshield wipers(possibly unrelated?)

The windshield wiper control board goes out on these trucks with some regularity. I would look into that.