friggin taillights ughhhhhh!

[budget builder]

yall have prolly ran into this a lot but here goes. my tail lights are not working I replaced the flashers, fuses, lights, checked for ground, and the only time I can get them to work is to apply ground directly to the posts holding them to the truck or to the inside of the light to the backing plate. so I keep checking for more ground except they stay on when headlight switch is off. so I think that maybe the HL switch is bad so I replace it and still the same thing. then I notice that my switch has 7 prongs but I only have 6 wires. so I search and guess what someone has soldered a different wire to the courtesy lights that don't work. which is why the white wire on the headlight switch is hot (always). and everytime you shake that white wire going to the dome lights the tail lights blink so now I have to figure out why they something so dumb. any ideas or comments that will help please post.

[VileZambonie]

If only everything were as simple as the wiring in these trucks.... On that note, please post what we are working on. Pertinent info is needed or we can waste a lot of time and in turn you can get a lot of bad advice. In addition, future readers of these posts read bad advice and perpetually butcher these trucks up. Year, make model, engine, crew cab, regular cab, duallie, stepside, is it stock, etc etc

[Displaced_Txn]

if they are staying on when the lights are off also check your brake light switch and wiring.

[budget builder]

sorry about that. it's  a 77 gmc c15 shortbed stepside. with a 305-th350 to 454-th400 conversion. reg. cab square (bolt on) taillights.  I think I found the probs. 1. the headlight switch was jumped to the dome (always hot) lights. y? I have no idea.

2. the home made trailer light adapter was a complete fail in the back 3. the ground they made for the trailer lights was a disaster. 3. the spliced and soldered wires under the dash were not covered and were grounding out all the time. so far so good I have all the lights working as they should and my headlight switch is not catching fire and I am not blowing any fuses. I used heat shrink butt splices everywhere to keep moisture out and will do the same for all the cut and soldered wires under the dash that should last longer than the cheap black tape they used. even the dash lights and dimmer work now.

[budget builder]

Well I thought I was finished. They were predicting rain so I was putting the foam and covers on the taillights feeling.gud about thhe outcome. Well my son moved some things around to give me the foam and the taillights came on. No headlight switch on but taillights still on. Wiggle some of the wires under dash they go back off. Im at my witts end. I take dash apart and dont see any other wires cut. Except four large gauge red wires that are attached to two large gauge wires leading up to steering wheel. When I connect turn signal wires to their harness the taillights come on and when I detach them they go off.

77 gmc c15 305 to 454 conversion th350 to th400 conversion. Shortbed stepside.

Could it be the circuit board? and how to take apart?
 I will give more info when possible

[budget builder]

Oh and I  took  the fuse blok apart under the hood and I had power only on one wire a big red one. I guess it supplies every. I thought I would find something spliced into the grren and yellow wires in the harness at the back of the truck but nothing was. Continuity to the fuse block on all wires seem good. From the blok to the ts switch is intact. So im leaning.toward wiires touching in fuse blok or maybe the circuit board. Oh there are still time the headlight switch works just fine

[Captain Swampy]

When the tail lights are on with out being turned on, pull fuses to try to find where the power is coming from. Also start at the battery and clean both ends of the cables. Make sure there is a ground to the body.

[budget builder]

I have three fuses that have power running thru them with hl switch and ignition OFF. What does this tell me

A,b,I

Also I tightened my brake switch closer to the pedal where it hits and so far I think that made a difference. #fingerscrossed

[blazer74]

The brake light switch was mentioned in post 3

[budget builder]

Doh!


Thx to everyone.

[bd]

I have three fuses that have power running thru them with hl switch and ignition OFF. What does this tell me

A,b,I

Referring to your diagram, "A" and "B" are connected to B+ and should be hot all the time.  "I" is accessory power, indicating the ignition key may have been turned too far CCW past Off to the Accessory position.

[blazer74]

So it's fixed?  Good News!