Third Brake Light stays on

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120mm

120mm

10 Year
Joined
Jan 12, 2015
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92
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4,859
Location
Cunningham, Tennessee
Website
hotmilkforbreakfast.wordpress.com
I have a 1996 LC that has a third brake light that comes on with the headlights and stays on constantly.

Normal brake lights activate when brake pedal is depressed.

I also have a rear windshield wiper that runs constantly, and I unplugged it.

I've "heard" a bad ground might be a culprit. Any clues to where the most lightly grounding parts are back there? Not an easy place to look around in.

Any alternate theories about why it would stay on? I've read various threads about how the zener diode can make it stay "off" but nothing about why it would stay "on".
 
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I don't have one of those fancy, newer 80s song don't have a third tail light. My guess would be that it has its own ground otherwise wouldn't the other brake lights do the same thing?
 
Bad ground.
Start with the body to rear hatch grommet wiring.
If your body is rusted, drivers side rear quarter panel below jack wiring plugs. Check these for green oxidation, clean and coat with dielectric grease.
 
Update: It's definitely not in the tailgate area. I've determined that the issue has to reside somewhere between the headlight and taillight circuit. My "help" that said my taillights lit when the headlights were on wasn't very accurate; the taillights glow slightly brighter, but they are definitely coming on with the headlights.

My driver's side kick panel is a mess. windshield washer fluid has dripped down there and gotten into everything. Starting tomorrow, I am going to start pulling junctions apart, cleaning them up and putting them back together. I'm also going to look for places where the headlights can actuate the brake lights. Any suggestions?
 
Fix *ALL* the ground issues *FIRST*.

LEDs draw little current, they can draw enough to 'glow' through various false grounds (e.g. the filament of an incandescent bulb that isn't even lit).

So...

Fix *ALL* the ground issues *FIRST*.

cheers,
george.
 
george_tlc said:
Fix *ALL* the ground issues *FIRST*.

LEDs draw little current, they can draw enough to 'glow' through various false grounds (e.g. the filament of an incandescent bulb that isn't even lit).

So...

Fix *ALL* the ground issues *FIRST*.

cheers,
george.
Click to expand...

Would a ground issue cause the dash lights to dim as if the lights were turned on when the brake is applied with the lights off?

Ground issues. I have not a single clue where the wiring is grounded. I've been all over this vehicle and haven't seen a single ground wire, other than the big ones up on the engine. Don't even know where to start.

Yet people say "fix the ground issues" as if that is supposed to make sense to me.
 
I did some further experimentation. with the ignition off, no lights on, if I step on the brake pedal the front and rear marker lights, and the dash lights turn on, as if I had my lights turned on.

Spent some more time with a flashlight and an inspection mirror. STILL haven't seen anything that looks like a ground. Is it a wire on a screw to the body?

I'd seen on another thread to "examine" the wire bundle coming from the body to the lift gate. Yep. There is one. Can't see in there, but the bit you can see looks unmelted and not abraded or cut.

I hate wrenching on cars. Cars suck.
 
Surfdc has his headliner out ( unless he's got it back in now) and has been taking requests for pics. He can probably show you where the grounds are up there. His thread is at the bottom of the page right now.

I'd definitely check the main ones up from first though. A weak main ground up front will only get weaker further back as resistance goes up. One of the first things I like to do to older vehicles is run a new, and usually much bigger ground wire to the engine block, frame, and body.
 
Yep, download the free Birfmark produced EWD. I will SHOW you (amongst everything else) where all the ground splices and grounding points are - there are quite a few...

cheers,
george.
 
^

949 post so far and you haven't learned to used search yet - dearie me.... :)

1996 FSM and EWD

cheers,
george.
 
george_tlc said:
^

949 post so far and you haven't learned to used search yet - dearie me.... :)

1996 FSM and EWD

cheers,
george.
Click to expand...

A - didn't know what EWD meant. didn't know what "Birfmark" meant.
B - It's a Mud "thing" for people to refer you to "just use search" and when you do, you get hundreds of pages of threads that tell you to "just use search". You could make a lifetime hobby of searching and then opening up thousands of threads without getting anywhere. I've learned search here to the extent that I've learned not to waste time using it unless I have to.

C - Thank you.
 
The EWD doesn't answer the question of what the grounds LOOK LIKE.

You know; are they a flat woven strap? A wire? Are items self-grounded (sweet Jesus, let that not be the case)?

What I am seeing so far is spaghetti, thrown up by the spaghetti monster, into a sea of spaghetti. And it doesn't help I'm using tri-focals, so mostly what I see is a blur. Of spaghetti.

And yes, I've done a search.
 
Yes, the EWD is tricky, you have to read it :)

You aren't going to get a pretty picture, an arrow etc to 'see' where things go.

Looks like you're figuring it out...

cheers,
george.
 
george_tlc said:
Yes, the EWD is tricky, you have to read it :)

You aren't going to get a pretty picture, an arrow etc to 'see' where things go.

Looks like you're figuring it out...

cheers,
george.
Click to expand...

Tell you what, it's heads and shoulders above other EWDs (still feels weird calling it that) as for actually showing where the grounds might be.

Not something I wanted or had time to be doing right now, but oh well....
 
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