This 1994 Chevrolet K1500 pickup came in with the very unusual complaint that the stoplights would not work with the ignition on. I checked the brake light operation and sure enough the stoplights worked with the key off but as soon as the key was turned on they stopped. I did notice that the CHMSL (center high mounted stop light) did not work at all. I installed new CHMSL bulbs and found that it worked properly with the ignition on or off. Since I have worked on so many of these trucks and have the wiring schematics tattooed in my brain (I thought), I went under the dash and disconnected the harness connector from the stoplight switch.
The first thing that struck me as unusual was there was not a white wire present. Normally on a GM vehicle of this era the orange wire would be power into the stoplight switch from the stoplight fuse. When the pedal is depressed, power is sent out on the white wire to the turn signal switch assembly. There it splits and goes to the right and left rear turn signal // stoplight bulb elements.
I confirmed that the orange wire had power on it with the key on or off. I then installed a fused jumper wire and rechecked the lights. Same problem.
I checked the wiring at the turn signal switch harness connector and found that the wire was white as it should have been. I checked the signal at this wire and it reacted the same as the signal for the lower stoplights. Experience tells me that there must be something wired in between the stoplight switch and the turn signal switch.
I did some research of the vehicle wiring diagrams and found that GM placed a relay in the system between the two switches. It is located under the hood above the power brake booster.
Looking at the diagram I realized it would impossible for the relay to cause the problem if it was getting the proper signals. I saw that the ground for the relay was located at the rear of the passenger side cylinder head. I also remembered that the customer stated the windshield wipers do not work (customer was not interested in fixing them though). From experience I also knew that the ground for the wiper system terminates at the same location. It is hard to see but under the heater hose pipe, back in the shadow is the ring terminal that is supposed to be attached to the rear of the cylinder head. Only the edge can be seen and it is positioned in a line from 12:30 to 6:30.
In the next picture you can see the ring terminal in the center of the picture. There is a stud just below it that one might think it is supposed to attach to but it is not the correct location.
In the next picture you will see the grounds properly attached to the ground stud at the rear of the passenger cylinder head assembly.
So many times the lack of a good ground is the cause of really strange vehicle electrical problems. This truck was no exception. Some checking around and it was evident that the engine and or the transmission had recently been changed.
I find it strange that as long as I have been doing this and as many of these trucks that I have worked on, I have never seen this exact problem before. Any how this one is fixed.
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94 silverado has no power to the orange wire at the brake switch. But I can jump the yellow wire with power and my brake lights work. Where does the power come from for the orange wire to power the switch? Any help would be appreciated very much. All other lights work but emergency blinkers.
The orange wire at the stoplight switch is powered by the Stop fuse in the underdash fuse box. This fuse is easily overlooked and most people look at the Brake fuse which is for the ABS system. If I remember correctly the Stop fuse is at the very top row of the fuse box and you have to really turn your head to see it.
My 1994 k1500 with the 350 tbi just had an engine swap. I can’t recall if the stop lights worked before she got the swap or not. But ever since then the third brake light works but the tailgate ones never do. The windshield wipers don’t work either and I’m not sure if they’re connected or not. The school control panel is also always on and I had to pull the fuse for it. Thermostat ground and the second one in the harness are both connected. I’m at a loss on what it could be:/
Have you physically checked the grounds at the rear of the passenger cylinder head? Guess it was a typo but I have no idea what you meant by “the school control panel”?
Is there a way for me to send you a picture of my wiring under my steering column because I have many questions on why my brake lights are not working. By the way I also recently had a steering column replacement
Yes, You can upload pictures in the Ask a Question section. click on the My Account button and then scroll down to the My Questions section. Click on the green Open A Ticket button. From there you will be able to Ask a question and upload an image. File sizes are limited to I believe 2mb. Also if you need to load more than one picture you will have to make additional replies.
What would cause my brake lights to come on when my running lights are on. The only light that works with the brake pedal is my 3rd brake light. Brake lights wont work when pressed other than 3rd with lights on or off. But with the key off and when I press the brake pedal my ac controls light up and when I release the pedal it goes off.
Assuming you have a 1994ish GM full sized truck the culprits would be the elements in one of the bulbs touching each other, wires touching each other (especially around trailer wiring) or a bad ground that is letting the circuits bleed back through each other.
Yes it’s a 1994 k1500 the brown wire for tail lights is getting continuity on ground even with the lights off could it be a bad headlight switch? Or some were else
What color wire runs from the brake switch to the relay under the hood. The relay also runs the ac control unit when the pedal is pressed the relay isnt clicking replaced relay but same thing. But when pedal is pressed its sending power to the ac control unit.
The yellow wire sends the signal from the stoplight switch to the relay.
The brown wire should have continuity to ground unless you remove all of the bulbs so this is not a good test. The headlight switch would not be the problem.