Taillight Issues - 1968 C10

[Graham57a]

I am working through a list of issues on a factory 1968 c10 for a buddy and I am striking out trying to figure out a tail light issue that arose. When I got the truck the rear taillight harness was non existent so I pieced one together and used new tail light and reverse light housings along with new bulbs, lenses, etc. Once I put everything together it worked perfectly, about 3 weeks ago. Working through the rest of my list I noticed the other night that the tail lights were no longer burning when I turn the headlights on. I did replace the neutral safety switch and my reverse lights work and so do my brake lights. So that tells me that the issue is not a ground or a burned out bulb. My initial thought was a fuse but I have replaced all of the fuses in the fuse box and I specifically checked the tail light fuse with my multimeter and I have 12v on both sides of that fuse. I also thought maybe the headlight switch so I swapped it out for a known good one, still no change. My last guess was to check all of my grounds and double check the hot junction block near the battery on the inside of the pass side fender and everything checked out, still no change.

I did some searching through the older threads on here and I didnt see anyone with my specific problem so hopefully I am not the only one. Any help would be greatly appreciated.

Thanks guys!

[Graham57a]

Pics of the truck I’m working on for your enjoyment

[MS66]

Nice truck. I ran into a similar problem when I was getting my '68 road ready a few weeks back. It had me going through some of the same hoops as you. I started checking voltage on the brown tail lamp lead at the firewall junction after I wasn't getting fire to the lamp socket at the rear. It ended up being the tail light feed wire was chafed through in several spots in the frame clips further back near the rear end. Ended up cutting it out and splicing in a new 6ft section of wire. Not sure what you're using for a light harness, but one of your solder joints may be bad causing this. Good fortune.

[RustyPile]

Andrew, First, let me explain something.. Backup lights are "stand alone" bulbs, meaning they are on a totally separate circuit and don't share any components with the other lights or bulbs.. Brake/turn/tail lights are the same bulbs but each bulb has a filament for the brake/turn and a totally different filament for the tail lights.. It is possible you have bulbs with working brake lights, but both the tail light filaments could be burned out..

You've already checked the fuse for tail lights (there's only one fuse), so assuming your test was accurate, the next test is to determine if you have voltage at the tail light "post" inside both sockets.. If voltage is there, you have burned out bulbs.. If no voltage is in either socket, start "back tracking" and determine along the brown wire just where the voltage gets "lost".. Don't rule out the light switch as faulty..

[68bowtie]

Wow nice truck

[Graham57a]

Thanks for the help guys. I believe a new taillight harness is the remedy needed here. I was using a test light to determine the presence of voltage at the tail light bulb sockets and it would light up so I assumed I was good. After reading the comment above I traced out a handful of bad areas in the existing harness where the insulation was cracked and some of the wire strands had broken. I then went back with my multimeter to check voltage at the tail light bulb sockets and I had about 1v give or take but I have a full 12v at the connection point on the crossmember under the back of the bed floor. New harness is on the way and should have us fixed up.

Thanks again for the help!