Help needed sorting an electrical issue

[E.D.75GMC]

Alright guys, I need help. I just put a rust free box on my truck the other day after having spent the last few weeks grinding and painting the frame and bottom of said box. Just got around to installing the tail lights and harness and noticed a small problem. The drivers side tail light does not work when you hit the brakes or flash when you put on the signal light or hazards. (Note that the drivers side front signal works fine) Now, you're probably thinking "well that's easy!", but here's the thing; since I first noticed this issue I have replaced not only the bulb, but the flasher unit, as well as swapped out the whole harness for the extra one I had in the garage and none of them had an effect on it. Do any of you guys here have an idea? I'm pretty stumped.

[E.D.75GMC]

*but it lights up fine when you turn the headlights on as seen in the picture*

[SkinnyG]

Does the "park light" that lights up, shut off when you touch the brakes?

I'd suspect a grounding issue. You could confirm that my seeing if you are actually getting voltage to the bumps in the tail. If voltage is getting there, it's probably not finding its way out through a good ground.

[Man With A Gun]

Check that you put the correct bulbs in the rear light assembly. I don't recall the numbers but if you put the bulb in that has only one filament in the rear stop/tail light socket (which is supposed to have a bulb with two filaments) it causes problems.

There are three bulbs- the small one (194 I think) for the side marker, and two larger ones: a single filament (1156 I think??) for the backup or reverse bulb and a dual filament (1157 maybe??) for the stop/tail combination.

[Man With A Gun]

Check the front turn signal lamps for the same thing. Look for an 1156 bulb where there should be an 1157 bulb.

[E.D.75GMC]

Yeah all the bulbs are correct. And okay, I'll get my multimeter out when I'm home from work. Do the bolts that hold the box to the frame act as a ground? Because I only have four of the eight in and they aren't bolted in, just sitting in the holes to hold the box on.

[hatzie]

The bed is your ground for the tails. If it's not bolted down that could be the problem.
I ran the grounds to the frame rather than the sheet metal tab off the bedside behind the taillamp housing.

[TheWhiteElephant]

I almost guarantee it is a grounding issue. Make sure your tail lamps are EACH grounded to the body at a bolt, and then run a ground wire attaching from the same bolt to a spot on the frame. This should take care of the problem.

Anytime the electrical issue is weird and unexplainable, it is in all likelihood a ground issue.

[TheWhiteElephant]

Also make sure you use a star washer (to cut through the paint into the sheetmetal).

[TheWhiteElephant]

And the truck looks great.

[E.D.75GMC]

Thanks a lot guys! And thanks Mando, I went though your build thread a few months ago and I love the look of your truck!

[TheWhiteElephant]

Originally Posted by E.D.75GMC

Thanks a lot guys! And thanks Mando, I went though your build thread a few months ago and I love the look of your truck!

Right on, Eric! Thanks for the kind words. See if that fixes the issue and remember to report your results for the benefit of others!

[E.D.75GMC]

Which wire should I be attaching my new grounds to?

[E.D.75GMC]

I made two new new grounds, one coming from the original ground on the sheet metal tab behind the drivers side tail light that now connects to the frame. And another from the pig tail where all the grounds come together in the tail light harness. Still no luck though. Not sure where to go from here?

[hatzie]

The LH bright filament (Brake/Turn) is powered by a single wire... The color varies by year...
You have round lamps so it's pre 1980... In 1979 LH bright filament (Turn/Brake) will be the YELLOW wire. Other years? Download the wiring diagram and look. It's Circuit 18.

If it was working before you took the bed off it may be right at the molded harness connection on the frame.
Unplug and test for volts on the LH bright filament wire with a meter or test lamp...
If it is a bad molded connector you will have to replace it. The molds are not reproduced. You can update to the 1985-1991 4 position 2x2 square Weatherpack for not much money. Download the 1989 wiring diagrams for the GM pinout.

Mouser electronics ships in/to Canada just pick the maple leaf flag in the upper right corner of their web page or open ca.mouser.com. I usually pad the terminals and seals I order JIC I drop some or foul up a crimp. Need 8 order 12... Need 4 order 10... There are usually price breaks from Mouser at 10 pieces...

Part # Desc.
12089188-L Automotive Connectors TERM FEMALE TIN PLATED CABLE RANGE .80-.50MM² (Need 4)
12089040-L Automotive Connectors TERM MALE TIN PLATED CABLE RANGE .80-.50MM² (Need 4)
15324982 Automotive Connectors LOOSE CABLE SEAL GREEN (Need 8)
12015798 Automotive Connectors 4P BLK 20 AMP WEATHER PACK TOWER
12015024 Automotive Connectors 4P BLK 20 AMP WEATHER PACK SHROUD

[E.D.75GMC]

Thanks for the information! It will help a lot.