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Old 11-14-2019, 09:23 AM
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1994 Brake light switch trouble shooting

Hello! I am hoping someone can help me troubleshoot my brake light issue.

No brake lights, but tail lights work, all blinkers work and the hazard button works.

I unhooked the brake light switch and use my power probe to identify the function of the four wires leading INTO the switch. I have found 1 ground, one hot 12V in that is hot only with the key on, one tab has two white wires coming out of it and when I gave that 12V both of my brake lights come on and the final wire (red or pink with a blue or black stripe) doesn't do anything that i can see BUT a PO had spliced into this one for some reason? I have been assuming that this red or pink wire has something to do with cruise control?

My first thought is that since I can fire the brake lights up by giving the harness at the switch 12V on one of the tabs that the switch is at fault. Instead of testing the old switch I decided to just order a new one since they are reasonable in price and this one, even if working, is pretty old. What a nightmare to swap out! Especially since the PO added an alarm and some other stuff to this jeep adding a mess of wires infront of the switch mount Replaced switch, still nothing.

It doesn't seem like I would have two bad switches, but maybe the old one did fail and the new one is defective? I was trying to use my power probe plus a test light to test the switch but am having trouble staying on all of the tabs and pressing the peddle enough to let the switch button out.....

Am I correct in assuming that if I can give it 12V at one of the tabs in the harness leading into the switch that everything from that point to the tail lights is good? It looks like the path routes through the hazard button at the multifunction switch and I was thinking I could have a problem there....but now I'm thinking that is fine since I can provide power on the other side of that and get the brakes to light up.

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Old 11-14-2019, 11:35 AM   #2
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Find a post by Got it at last. In his signature there's a free link to several Factory Service manuals and a couple Factory Parts Manuals.
Open that link and click on the manual that you need. There should be a brake light schematic in the electrical troubleshooting section.

Without looking at the schematic, I'm wondering why there would be a ground wire at the switch. That might be a short that's preventing your brake lights from working.

Good Luck, L.M.

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Old 11-14-2019, 01:05 PM   #3
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Open up the dropbox link below in the '94 FSM. Scroll down to pg 1415 for the wiring diagram.
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Old 11-14-2019, 01:27 PM
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I had downloaded the FSM out of Got It At Last's sig last year when I first got this 94' Extremely helpful, thank you for that link!

I printed that diagram off earlier this week and have studied it, but I don't see where the wires go. I see the switch with the four 18 gauge wires (and the wire colors appear to be what I have) but I don't see where they go when they leave the switch? I am way overwhelmed at work and over tired and may have lost the ability to analyze.

I have also searched the entire electrical section 8 for trouble shooting the brake lamps and can't find a write up on that....I found trouble shooting blinkers and hazard switch but unfortunately for me those things are working
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Old 11-14-2019, 01:46 PM   #5
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Quote:
Originally Posted by Luckymac View Post
Find a post by Got it at last. In his signature there's a free link to several Factory Service manuals and a couple Factory Parts Manuals.
Open that link and click on the manual that you need. There should be a brake light schematic in the electrical troubleshooting section.

Without looking at the schematic, I'm wondering why there would be a ground wire at the switch. That might be a short that's preventing your brake lights from working.

Good Luck, L.M.
Looks like that ground runs to the ABS system, Which since we are not fortunate to have, leaves us with a different type of switch at the pedal.
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FSM Manuals https://www.dropbox.com/sh/0vfmk6jc5...v7lceZk7a?dl=0
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Old 11-14-2019, 03:21 PM
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Ok, I will have to expose my low IQ to the world here and admit that I didn't connect the obvious dots.....that the harness shown above the stop lamp switch on diagram #34 is what plugs into the switch.

Now I can see that the pink wire that I described in the original post is what runs to the fuse block. This was my dead mystery wire that I wrongly assumed was to do with the cruise control. That wire is dead, key on key off, I get zero there. So this must be my problem! I have checked that fuse and it is good...but I did notice that a PO has spliced into this wire so I will check that splice out...it must be junk because I have found some sloppy splices all over the place.

This will be a fun splice to repair.....two inches of wire to work with, upside down on my head and can only fit one hand in there....but we will get it!

Thanks for the help!
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Old 11-14-2019, 03:53 PM   #7
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I am looking at a '95, don't have the '94 in front of me. Looks like the brake switch goes to 4 spots:

a) power coming from fuse 3
b) center high stop lamp (Also goes to the turn signals through the turn signal switch)
c) Ground
d) PCM sense of stop light

From the diagram it looks like when brake switch is depressed A connects to B and C connects to D.

So it looks like the short answer to the question, the 4th lead is the PCM sense circuit. I am guessing the PCM likes to know brakes are applied.
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Old 11-14-2019, 04:08 PM   #8
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PCM only needs to see a break signal on an automatic for the torque converter.
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Old 11-14-2019, 04:15 PM   #9
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PCM only needs to see a break signal on an automatic for the torque converter.
I am not sure what the PCM strategy is for knowing the brake light, but what I quickly saw was it didn't reference it being only for an automatic. So I assume that both auto and manual trans require that circuit.
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Old 11-14-2019, 05:49 PM   #10
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Let's not forget that these harnesses tend to be wired for everything possible so they didn't have to make specific units, it may be a useless wire and we are barking up the wrong switch and the question is What did that splice to? and is it no longer necessary or connected to anything re the brake lighting.

Not being in my 87-90 my era of wiring, I'm lost without something other than the wiring diagrams to follow, but that switch is tied to the common ground in the harness, I would check that for continuity to ground and if it is, I would look at the power going into the turn signal switch and then back out, OP mentioned he couldn't hold the pedal down and handle the leads on his meter. Try wedging a short piece of wood against the pedal and the front of the seat.
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Old 11-14-2019, 06:16 PM   #11
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I'm an old buzzard and don't bend like I used to.
When I work under the dash, I remove the drivers seat and lay a blanket on the Jeep floor. Then I can lay on my back with my feet where the back seat belongs without damaging my ribs on the sill.
A somewhat common fault is the terminals in the fuse panel that grab onto the fuse can become weak and not make a proper connection, or the wire can actually break off from the terminal. If that happens, the fuse can show as good, but no current is leaving the fuse.
The only way I know to check is to disconnect the under hood plug from the firewall near the brake booster. Then remove the tachometer and use a long philips screwdriver to disconnect the fuse panel from inside the firewall. The fuse panel can then be opened up and you can look for broken wires.
Be sure to disconnect the battery first.

If you find broken wires in there post again. There's an easy fix.

Good Luck, L.M.
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.................................................. ........................
1987 YJ-4.2L-Standard Shift- Re-manufactured Carter Carb-2" body Lift-31X10.5X15 BFG KOs-190K Miles No back seat.
HEI distributor with computer and all related relays and wiring removed.
12K Badlands winch with dual batteries.
Warn front bumper. Matching imitation Warn bumper on the rear.
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Old 11-14-2019, 07:29 PM   #12
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Luckymac brings up a good point about the circuits carrying load. For now I would forget about the wires that I called C & D.

Test by bridging A&B without the switch. See if the brake lights will light up. You mentioned that you had 12v on one of the wires (A). But if you are measuring that with a DVOM, you might think it's good but that can be misleading cause the circuit might not hold enough current to light all the brake lights.

Just a note, try not to feed 12v directly into the wire I labeled "D". Unless you know what you are doing you have to be careful about putting power into pins of the PCM that aren't designed to take it.
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Old 11-15-2019, 08:24 AM
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Good morning everyone!
We have success! My real issue was that I didn't realize that the schematic was showing the other harness right above the stop lamp switch! So dumb of me, but Got It At Last's comment about the ground leading to the ABS is what made me go back and look at that diagram again..Thank You!

When you turn the key on the PCM wire becomes hot...which I had been mistaking for the power in wire the whole time while falsely assuming that my pink dead wire might be for an auto tranny or something that my Jeep doesn't have/need? Of course I was wrong....again.

My power in wire (pink) was not becoming energized with key on. Fuse good. Someone had spliced into it with one of those damn cut in type connectors. I haven't found the purpose of that spliced in wire and I don't really care because a PO added a lot of stuff to this Jeep that I am removing. I removed that wire and soldered/heat shrinked my pink power in wire back together...now it has power with the key on. A this point I am smiling because I know....It's happening! Plugged back into my stop lamp switch..brakes work!

I'm getting ready to put the Jeep in storage for the winter and have been getting pretty upset with myself that the reason I'm not having fun cruising around in the old Jeep for the last few weeks of having it out is because of damn brake lights! What?! This 94 doesn't have a flake of rust on it...the body and frame are really exceptional so I will never let it see a Maine winter.

Thanks for all of your replies and help!
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Old 11-15-2019, 09:09 AM   #14
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Quote:
Originally Posted by I_Like_Grease View Post
Good morning everyone!
We have success! My real issue was that I didn't realize that the schematic was showing the other harness right above the stop lamp switch! So dumb of me, but Got It At Last's comment about the ground leading to the ABS is what made me go back and look at that diagram again..Thank You!

When you turn the key on the PCM wire becomes hot...which I had been mistaking for the power in wire the whole time while falsely assuming that my pink dead wire might be for an auto tranny or something that my Jeep doesn't have/need? Of course I was wrong....again.

My power in wire (pink) was not becoming energized with key on. Fuse good. Someone had spliced into it with one of those damn cut in type connectors. I haven't found the purpose of that spliced in wire and I don't really care because a PO added a lot of stuff to this Jeep that I am removing. I removed that wire and soldered/heat shrinked my pink power in wire back together...now it has power with the key on. A this point I am smiling because I know....It's happening! Plugged back into my stop lamp switch..brakes work!

I'm getting ready to put the Jeep in storage for the winter and have been getting pretty upset with myself that the reason I'm not having fun cruising around in the old Jeep for the last few weeks of having it out is because of damn brake lights! What?! This 94 doesn't have a flake of rust on it...the body and frame are really exceptional so I will never let it see a Maine winter.

Thanks for all of your replies and help!
Awesome, that's why we are all on here. Somebody at some point had a same or similar issue, so when we get together and collaborate the combined experience eventually roots out the problem.

Glad you got it figured out, and let this be a lesson on why we Don't "Bubba Hack" stuff when it comes to wiring. Soldering tools, good connectors, and heat shrink is just as important as PB Blaster when it comes to these old heaps....JMHO
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FSM Manuals https://www.dropbox.com/sh/0vfmk6jc5...v7lceZk7a?dl=0
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