3rd brake light goofiness

I've added a 3rd brakelight in the cargo light location on my MJ, however it lights up like the tail lights if the headlamps or parking lights are turned on. It will go to full brightness if the brakes are pressed, just like the tail lights. The 3rd light has 2 wires, one directly from the brake switch (to the blue wire if I remember right), the other goes to the dash ground. It's not an issue of a shared ground as I ran a test lead from the ground to an engine bay ground and had the same problem.

Anybody have some suggestions on fixing it?

Anybody?

Got to use a 3rd Brake Light module, see here:

http://www.comancheclub.com/forums/viewtopic.php?f=2&t=14822&hilit=3rd+brake+light

I you need a wiring diagram, let me know.

If you have a tow hitch with the trailer connector, you could do what I did, and just get one of those 3rd brake lights that fits into the hitch and is wired to the connector. That way, you can still have use of the cargo bed light. I don't have a cargo light, or a third brake light up above the rear window, so this trailer light is a good substitute.

The JCWhitney link isn't valid anymore. I did find this though: http://www.jcwhitney.com/jcwhitney/product.jcw?nval=0&statenval=0+200001169+1988&productId=2018847&shopid=100001&pageid=12&_requestid=4185736

Could you pm me the wiring diagram anyways Hornbrod? Thanks

I managed to find the difference in the factory wiring. The MJ has a wire going directly from the brake switch to the hazard/turn signal switch, while the XJ has the wire leading back to a different connector instead of into the hazard/turn signal switch. I'm pretty sure this is my problem, but why is there a difference?

but why is there a difference?

:doh:

It's a Jeep thing, and you Should understand that.

:teehee:

And, I think you found the part to make it work :thumbsup:

(When do I get my ride????)

This is the one you need here: http://www.jcwhitney.com/LOGIC_CIRCUIT?ID=12;0;0;0;100007;ProductName;0;0;0;0;2003583;0;0

Shoot me a PM with your email address and I'll end the wiring diagram as an attachment. :D

... but why is there a difference?

Because the XJ uses dedicated amber turn signals in the rear, independent of the brake lights, whereas the MJ has both bulbs on each side doing double duty as brake & turn signal lights. When the same bulbs are doing both jobs, you need a logic module to make a third brake light function properly.

Exacerly. The logic module gets a separate feed from the left and right stop lamp wires. The left and right stop/turn lamps are dual filament bulbs with a common ground. W/o the logic module to block it, the 3rd brake lamp would receive the turn signal backfeeding thru it's filament as well as the stop signal, thus would blink when you applied a turn signal. I think an inline diode on the left/right turn signal wires might work; never tried it though......... :D

Nothing like a wiring problem to tickle your brain until you have to go out at night in the cold to figure it out. :D

So, you say you ran a wire from the third brake light to the brake switch on the pedal, hooking to the light blue wire, right? And then grounded the other wire to the dash ground.

Well reading that, it should have worked as you were bypassing the turn/hazard switch. Since it would have bugged me to death, I went out to the 86 and did just what you did and the third brake light work the way it is suppose to without any problems. Either the small differences in the wiring between 86 and 88 are causing this(Don't think so) or you may have hooked to the wrong wire(Doesn't sound like you did) or you have another problem.

The rear lights are like a three way bulb in your house. They're dual filaments that are powered separately depending on the switch setting. When you have the parking lights on, only one filament is lit and that power is coming from the headlight switch(This would be the low setting on a three way bulb). When you hit the brakes, the second filament lights up getting it's power from the brake switch which passes through the turn/hazard switch.(This would be high on a three way bulb). Hitting the brakes without the parking lights on gets power the same way(This would be medium on a three way bulb as the brake filament is bigger than the parking filament). Now when you have the turn signal on and the brake applied, the filament alternates on and off caused by the flash module interupting the power. The turn signal has it's own power source as well.

With all that said, your turn/hazard switch maybe causing it(Don't think so since you just claim that the only problem is it comes on with the parking/head lights, not that it's blinking with the turn signals) or, your headlight switch is bad and is trickling power to ground when turned on(this would explain why it comes on dim with the parking lights and when you apply the brakes it goes bright.) If you have a spare headlight switch handy, swap it out. If it doesn't correct the problem, then check the wiring harness and fuse panel for shorts.

Did you have any problems prior to hooking up the third brake light?

:cheers:

I'll double check that it's not blinking with the signals, but I'm 99% sure it isn't. That is how I wired mine, the blue wire from the brake switch and the dash ground.

Although I just had a brain fart and realized that I do have led bulbs in the tail lights. I'll see if I have enough standard bulbs and if that is the cause... well. :doh: and :wall: among others. Since they didn't have the same symptoms as led bulbs in the front it only just occurred to me. If that isn't the cause, anybody know a way to test the headlight switch while it's in the truck?

mfpdm, mucho thanks for testing on your own truck. It's been so long since this truck has been in one piece (roughly 3 years) I don't even remember if the original cargo light worked.

No problem. That's why we are all here, to help each other. :thumbsup:

Did you change the flasher module for one suited for LED's? Not sure but that may make a difference but it doesn't seem likely. There is a slim possibility that a bad contact in the signal switch is back feeding juice to the brake switch but, as said before, that should make the light blink. Not unless the combination of LED's and incondesant bulb doesn't allow it to blink. :dunno:

Let us know what you find.

:cheers:

Turns out it was led tail light bulbs. Very strange since it would have been working correctly if it wasn't for the 3rd brake light.

Turns out it was led tail light bulbs. Very strange since it would have been working correctly if it wasn't for the 3rd brake light.

Not entirely strange...I think if you load up the circuit with LEDs, the end result is they won't work right. I bought some LED bulbs for the turn signals and brake light on my motorcycle, and when I had all 5 bulbs in, they didn't work at all. Replaced the two fronts with "regular" bulbs, and the three LEDs in the back worked, just blinked a little faster than usual. Apparently, the only way I could get all 5 to work correctly was to put some kind of modulator gizmo in the circuit, which to me wasn't worth it. I'm all about plug and play, so if takes anything more than swapping bulbs, I ain't gonna do it...

Glad you found out what was causing your problem. In all actuality, it probably would be fine to put regular bulbs in the tail lights and an LED in the 3rd brake light, or vice versa, just not LEDs in all of them. Sounds like a simple fix...

Actually, the 3rd already is an led strip.