Advice on troubleshooting tail lights/rear marker lights?

[chewie37]

Did some searching but didn't find anything.  So...

New electrical gremlin to chase that's going to, yet again, detour me from going after the front seats.  But I digress...

Friend of mine pointed out yesterday that I have no tail lights/rear side marker lights.  However, everything else at the back of the car works as expected.  Turn signals are good, hazards good, brake lights good, reverse lights good.  No blown fuses, brake switch at the pedal seems to be adjusted ok. 

Kind of weird - I see topics here and elsewhere about no brake lights, but I've got the opposite.  Anyone ever run in to something like this before?  Any pointers? 

Thanks!

[b_hill_86]

When you say everything else works, how about plate lights? I assume not. Someone else may chime in with more help but I would thing to look either at the headlight switch and/or plug at the switch  or one of the two harness plugs between the switch and tail lights.

[b_hill_86]

The more I think about it too, you could probably pull the headlight switch and apply 12v to the appropriate terminal on the plug (brown wire for my 77) and see if you get lights. If so, switch would probably be the culprit. If not, gotta move down the line to those other plugs.

[chewie37]

Huh,  didn't even think about checking the plate light.  Will definitely look, as well as scoping out the headlight switch/connections there.  Thanks for the advice.

I'll try to actually circle back and update what I dig out, but it may the weekend before I get a chance to go after anything.

[Jack]

The same switch also controls the front marker lights so if those are working I would not think its the switch. But if the license plate is also off I would think you have an open circuit somewhere in the trunk. I would unplug the main harness in the trunk and check with a meter if you are receiving voltage with the switch closed.

[b_hill_86]

The same switch also controls the front marker lights so if those are working I would not think its the switch. But if the license plate is also off I would think you have an open circuit somewhere in the trunk. I would unplug the main harness in the trunk and check with a meter if you are receiving voltage with the switch closed.

Duh, after looking at my diagram again you’re right. My mistake.

[chewie37]

I had a brain cell fire during work today.  I actually have a 12v car battery on a rolling cart that I use to field power a high powered compressor for my pre-charged pneumatic toys.  Adult pellet guns basically. 

After work broke out the trusty multimeter and also whipped up a functional, if bulky, 12v power supply.

Disconnected the trunk harness from the intermediate harness that goes from the trunk to the main body connector.  Used the multimeter directly on the isolated trunk harness and got good tone between the brown wire (tail lights) and the black wires (ground.)  Then supplied 12v from my clunky mobile battery on said wires.  Tail lights, plate lights and side marker lights fired right up.  So the trunk harness is  good to go.

Next step it is to check the intermediate run from the trunk to the body connector.  Little bit more involved since getting to the body connector side is a bit of a PITA.  I get to take the driver side front wheel and inner fender liner out.  Again...yay!  That's definitely a wait for the weekend event.

[chewie37]

(Half)witted joke about pulling out the inner fender liner; know that's not necessary. 

In any case, found it.  At some point in its life, both the brown and orange wires were cut almost right at the body connector.  And then put back together with butt crimps.   Which have of course failed.

[b_hill_86]

Hey at least you found it though!

[5th T/A]

Chewie,

Great find and good troubleshooting. Whoever attempted the repair certainly could have done better. At least solder and cover with heat shrink tubing. When I installed the EFI system in my car I shortened some of the wiring harness excessive length. I purchased a weather pack connector kit from Eastwood that not only provides high quality crimp’s and connections but also keeps,the moisture out.

[chewie37]

Not a lot of room to solder in there - tight space and snausage fingers are less than ideal.  For now, used push-in spliceline connectors. 

Lighting's not great because the sun is behind me, but I have lights...you can see the plate lamps pretty well at least.

[firebirdparts]

That's some quality diagnosis.

[rkellerjr]

Nice job!