What is the actual chassis it's built on? Perhaps a wiring diagram for the truck might help find more fuses, or tell you if there's a stoplight relay, or just give you an idea how those circuits are wired.
1994 Class A chassis electrical issue
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highdesertranger
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yeah it sounds like a ground issue. I never like to piggy back anything on factory wiring. Check the grounds starting at the third brake light that you tried to use. while you are at it check the positive side.
BTW have you checked the fuse links at the starter?
Highdesertranger
BTW have you checked the fuse links at the starter?
Highdesertranger
highdesertranger
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Well if you have a meter you check the ohm reading from the frame to the ground at the light. You do have a meter, right? You will need long leads which you can buy or make. Highdesertranger
highdesertranger
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You should have 2 or 3 fuse links coming of the starter. How many wires are connect to the large stud? Highdesertranger
If you haven't already disconnect the connections you made to power and ground the camera. Check for 12 volts at the license plate bulb socket with the running lights off while someone activates the brake petal with your multimeter. There should be no power as the positive side of the brake light circuit should not be connected to the running light circuit. If there is no power turn on the running lights and re check for power which there should be. So if there is no power to the license plate bulb socket and none of the marker lights or rear tail lights come on when the running lights are turned on but the front running lights do light turn off the switch and go ahead and check the ground from the license plate bulb socket to the ground on the frame and see if you have continuity (little or no resistance) with your multimeter then do the same with the positive contact in the license plate bulb socket ( red lead on the contact and black on the frame ground). You should read infinite resistance or no continuity this time. Let us know what you get. Sorry I was gone awhile internet went out when our power went off! LOL!!!
Test the ground at the 3rd brake light to a known ground, metal frame or a nearby working light assembly, somewhere in the vicinity. Use the continuity/resistance test function on the multimeter, the ohm symbol for resistance. You are looking for zero ohms, no resistance. Anything else means the 3rd BL is not grounded properly.
Ground faults can give off flaky happenings. Three states of ON-OFF-FLICKER. I've seen the flashing when supposed to be steady and steady instead of flashing type of magic tricks. Corrosion of a ground connection is a primary cause.
The factory lighting wiring harness is intended only to run the lights. They save money on each build by using the smallest wires that they can. That adds up over 100K vehicles.
As HDR said, any equipment that is added on later needs a separate fused positive wire and a solid ground connection.
Ground faults can give off flaky happenings. Three states of ON-OFF-FLICKER. I've seen the flashing when supposed to be steady and steady instead of flashing type of magic tricks. Corrosion of a ground connection is a primary cause.
The factory lighting wiring harness is intended only to run the lights. They save money on each build by using the smallest wires that they can. That adds up over 100K vehicles.
As HDR said, any equipment that is added on later needs a separate fused positive wire and a solid ground connection.
It is not uncommon for several grounds that are connected to a bolt that goes through a rusty hole in a frame to back feed ( when the power is not able to connect to the frame it tries to go through the other wires to find ground and the reason for checking for power in the license plate light when the brake petal is pushed) so make sure all wires bolted to the frame are electrically connected by checking continuity between the terminals and close by shinny metal frame. you may need to spend some time wire brushing the frame, terminals and replacing rusty bolts. One of the problems with a fiberglass body is everything must have a separate ground wire that in the rear normally goes to a shared bolt on the frame.
Back-feeding is my suspicion, especially since the 3rd brake light flashes when the hazard flashers are on. Who remembers when Chevy had that problem with bad tail light sockets? As Bullfrog said, when a circuit can't find the shared ground, the current goes down any shared ground wires looking for a good ground somewhere, and that causes some strange things to happen.
There may be a connection to what you did but it most likely caused a problem you already had to get worst and it most likely isn't a switch because according to you activating the switches seems to make a change which is what they are suppose to do, no blown fuses found, eventually you will probably have to follow the wire from the head light switch to the rear lights to find the bad connection or bad fuse or burnt wire if there is one, you may have two problems but the main one that affects your brake lights almost has to be a circuit problem and most likely a ground circuit which affects all the circuits. It is concerning the rear running lights don't work but the switch is because the front lights use the same switch. If the brake switch wasn't working it wouldn't affect the turn signals when you apply the brake, so it is working. The turn signals wouldn't come on at all if the switch was bad and they do so it isn't bad. The hazards work so that switch is good. What switch to you think is not working cause there are not anymore I can think of?
Well normally fuse boxes are mounted inside driver's side inside fire wall close to where the main wiring harness enters thru the firewall. Most have the turn signal flasher mounted in them or close by. My motor home actually uses the factory GM fuse block mounted in that location. I would lay on the driver's side floor and look up under the dash with a light and follow the main wiring harness to it. Maybe the sound of the turn signal or hazard flasher will help guide you to it if they are mounted to it or nearby. Nothing you have found other than the running lights indicates a possible blown fuse as a blown fuse interrupts the circuit and there would be no power to the switches and therefore they would not affect any of the rear lights.
Quick little update...have meter on ohms.. several different settings to be on .. 200.. 2k. 20k.. etc.. I have it on 20k... when I put the leads on the pos and neg wires of the license plate light I get 0... they should not be correct?
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