I'll start with it's not original, but I don't think that has anything to do with it. My car is an '87 TA that originally had the light under the wing, and I swapped a '91/92 style hatch with the light at the top/inside of the glass. When I did it I had all the wiring from the '92 that I took it from and ran it like the factory wiring to the existing wiring. It worked fine for about a year, till I got pulled over for it not working a few weeks ago.

What it's doing- not working. If I pull the bulb and put a DVM across the socket terminals I get battery voltage when I step on the brake pedal.

I removed it, unplugged it at the plug that goes above the trim panel at the back of the headliner, made sure that the bulb socket was tight and wired it to a 12V power supply through that bit of harness and it lights up. While i have it out I was going to convert it to LED but didn't have any that were bright enough at the right angle (you need light at the right angle to come out though the faceted lens). Looking around I find 2 x 3 LED elements from some flashlights that fit into the grooves on either side of the bulb perfectly so I mount them there and wire them into the wiring for the bulb socket in parallel with appropriate current limiting resistors.

I put it back in the car and nothing. It doesn't work.

I try it on the bench again and it works fine. What the...?

I put it back in the car and it doesn't work. On a whim I pull the bulb and try it again and the LEDs line up. Huh???

I check the power side of the socket to a ground (the steering wheel hub was convinient) and I have battery voltage when I step on the brake pedal. I check continuity from the socket ground side to the ground and I have continuity with fairly minor resistance.

What the heck is going on??? What do I try next? It's almost like I have enough voltage at the socket but something is limiting current so I don't have enough current to light the bulb, but I have enough current to light 6 LEDs.

For anyone wondering about the LEDs, like I said, they're not quite bright enough on their own, but they do add that "instant on" look that LEDs give signal lights and they add significantly to the appearance of the brake light.


LEDs added to either side of the fixture



Reassembled



Lit up, it might not be obvious in this picture but it is more evenly lit than with just the bulb

 

Are the resistor(s) that I see needed for this ?

 

the resistors are wired into the back of the LEDs, they're current limiting resistors that prevent the LEDs from frying.

They're irrelevant to the problem, it did it without them or the LEDs, just the bulb and the socket: worked on the bend and didn't work on the car

 

Ok, so the LEDs aren't 12 volt leds, just wondered what was up with that.

Did the factory wiring for that light run through the headliner down the A pillar or B pillar? Its been awhile since I've messed with that

 

I didn't have factory wiring, my wiring went to a set of contacts by the lock and then to the end of the hatch. I put that together to the wiring that came with the hatch which I believe ran up the driver's B pillar. I didn't have the ground cable so I just ran it to one of the screws at the top of the sail panel on the driver's B pillar running parallel to the signal wire. I believe that I have a good ground, I have very little resistance between the body and the ground pin in the light bulb socket, and very little resistance between the signal pin and the original body wiring.

 

I take some of that back (I looked at it better last night trying to figure this out), the original wiring ran back around the driver's side of the hatch opening to the contact terminals in the back, I took that, pulled it out of the harness all the way back to the base of the sail panel and then connected it with the bit of harness I had with the hatch, I'm pretty sure that the signal side matches the factory wiring right now. The ground matches my previous description.

I also tried messing with it some more last night, I was getting somewhat wonky resistance readings across the ground (turned out it was the black oxide coating on the screw head I was using as a ground), so I took an allegator clip and a piece of wire, clipped one end onto a good ground (the center console ground) and back probed the ground at various points (the screw, then the inline plug and finally the back of the socket) while holding the brake down hoping that it was just a bad ground and that would turn on as soon as I bypassed the bad spot.... it didn't

 

Have you tried probing just the positive wire to the brake lights, i think its a blue wire.Is there 12v there when the pedal is depressed?
Is there 12v at the plug on the body side to the TBL when the pedal is depressed?Im thinking the issue could be in that area somewhere.

I hard wired my tbl from the blue wire at the b pillar point & ran it up to the hatch center, gr. is on one of the hinge bolts.Of course, my 84 never came w/ a tbl though.

 

My brake lights work fine in the taillights, just the TBL that's a problem

Mine had wiring for a tbl, but it ran to the back of the hatch, I pulled it out of that harness and sent it up the b-pillar from there and connected it to the wiring that came with the hatch that is on the car.

It's a yellow wire in that harness and that reads battery voltage when I step on the brake.

 

I was talking to my neighbor who's an EE and told him about what was happening.

His first reaction was:
"well you have a bad ground."

I told him that I tried measuring the resistance from the socket to the body ground and even tried connecting the body ground to a different screw.

"Huh, that's not it then, have you tried another fuse? I've seen fuses do all sorts of weird things when they're partially blown."

"No, but it's the same fuse as the rest of the taillights, if it was sort of borderline I'd imagine that they'd blow the fuse."

"Yea, that's not it, well can you just run a different ground and try that?"

"I did that already, the alligator clips are still here connected..."

We go and try that and test a few other things with no luck.

"Well if the LEDs work then you could check if they are bright enough to keep you from getting pulled over..." we look and he thinks they might be, I don't, "you know, you could just wire a few more LEDs in there"

"Yea, but you're an engineer, you have to have the same mentality as I do about this, would you just leave it that way not knowing what the problem is?"

"No, that would drive me crazy"

"yea"

We went around in circles for a little bit with no luck, had a discussion how I find the weirdest problems and I seem to have always checked everything before I ask, how it would be nice if I would just ask something that someone else had the answer to.

"tell me if you figure it out."

I end up thinking "I'll keep looking but for right now I have a brighter LED bulb that I could add to this thing and that will keep me from getting pulled over," and do it.

It works perfectly on the bench.

I plug it into the car and... WTF??? The 2 smaller LEDs light up, the bigger one doesn't... HUH???