79 J10 DS Brake Light STILL Not Working But Everything Else Does

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MNeug
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Joined: Sat Jun 24, 2023 10:06 am

79 J10 DS Brake Light STILL Not Working But Everything Else Does

Post by MNeug »

Hi all, sorry to bring this up again but I am stuck. I cannot figure out why my driver side 79 J10 brake light will not work. Also, the DS hazard light doesn't work. But the DS turn signal DOES work fine. They all use the brake light filament in the bulb so it seems clear to me that the signal does go through all the way to the filament. To my understanding the turn signal switch is what regulates these things.

I did swap out the steering column last year and everything worked great until last week. The PS brake light, hazard, turn signal all work fine. The PS side marker doesn't work but it hasn't worked since I've owned the truck. 20 years. I put in a new brake light switch. Might as well. They're cheap and take like 10 minutes to replace. I also put in a new ground wire at the DS assembly. It's mounted on the backside of the socket to the frame. The old one was very corroded. The lights do come on brighter now, for what that's worth. Is there another ground back there for the DS brake light that I don't know about?

What has me so confused is all three of these functions- DS brake light, DS hazard and DS turn signal all use the same path of wire to GET to the filament so how on Earth does only the turn signal light it up? Can it be the turn signal switch? The passenger side works fine so obviously the turn signal switch is sending the current to the back of the truck. I am so confused about this. Mainly because the DS turn signal works fine. Why does THAT work but the other 2 functions not work? Thanks again and sorry this is so long. -Mike
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tgreese
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Re: 79 J10 DS Brake Light STILL Not Working But Everything Else Does

Post by tgreese »

Switching between the brakes and indicators is handled by the turn signal switch.

You wrote a lot but it does not help. You need to measure and compare to the wiring diagram.

There are lots of diagrams online if you search. Jeep uses a GM column, same as GM cars. I can't recite how it works from memory, but you can look at the diagrams and the TSM and trace it through. Recall I suggested that you needed to use your multimeter and trace the circuits.

Sorry, there is no shortcut for this. Diagnosing electrical problems requires the car be in front of you where you can measure the circuits.

You don't need to understand what's inside of the switch. There are inputs and outputs that you can measure.

You could fire the parts cannon and replace the turn signal switch. That's the most common problem after grounds. No guarantee that will fix it though.
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
Dual Everything: '15 Chryco Jeep Cherokee KL Trailhawk, ECO Green
Blockchain the vote.

Topic author
MNeug
Posts: 33
Joined: Sat Jun 24, 2023 10:06 am

Re: 79 J10 DS Brake Light STILL Not Working But Everything Else Does

Post by MNeug »

I know, you're right. Hey, I don't mind putting in a new turn signal switch. I've already replaced the steering column and took out the original turn signal switch from the original column, which was working fine, so I know what it takes to work with the steering column and how everything comes apart so that doesn't scare me too much. The grounds... that's the part that scares me. But if I must multimeter the hell out of it I guess that's what I have to do. I'm sorry I wrote so much. I knew it was a lot to take in. I'll just buy a new TS switch, they don't cost all that much, and go from there. I will do some more wire-checking first of course but if it takes a new TS switch I'm OK with that. I do appreciate you taking the time to answer me. Thank you, very much.
tgreese wrote: Fri Jan 31, 2025 10:41 am Switching between the brakes and indicators is handled by the turn signal switch.

You wrote a lot but it does not help. You need to measure and compare to the wiring diagram.

There are lots of diagrams online if you search. Jeep uses a GM column, same as GM cars. I can't recite how it works from memory, but you can look at the diagrams and the TSM and trace it through. Recall I suggested that you needed to use your multimeter and trace the circuits.

Sorry, there is no shortcut for this. Diagnosing electrical problems requires the car be in front of you where you can measure the circuits.

You don't need to understand what's inside of the switch. There are inputs and outputs that you can measure.

You could fire the parts cannon and replace the turn signal switch. That's the most common problem after grounds. No guarantee that will fix it though.
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devildog80
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Re: 79 J10 DS Brake Light STILL Not Working But Everything Else Does

Post by devildog80 »

Suggest checking the rear light assembly, to be sure it is grounding like it should, then go from there.
'81 CJ5 Base, 258 I6, MC2100, T176 4 spd, 300 TC, D30 Front NT, 3.31, 2-Piece AMC 20 rear NT, 3.31, 4" high arc spring lift
FOR SALE - '84 Grand Wagoneer, 401 V8 (.030 over), Edelbrock clone 1406, 727 auto, Selec-trac NP229, AMC 20 REAR - D44 FRONT - WT 3.31, 4" high arc spring lift
Rather be driving, than waiting to be modified
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tgreese
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Re: 79 J10 DS Brake Light STILL Not Working But Everything Else Does

Post by tgreese »

If the indicators and the brake lights use the same filament in the same bulb, then the ground must be ok. If the indicators and brake lights are separate bulbs, then you could have a bad ground somewhere.
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
Dual Everything: '15 Chryco Jeep Cherokee KL Trailhawk, ECO Green
Blockchain the vote.
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tgreese
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Re: 79 J10 DS Brake Light STILL Not Working But Everything Else Does

Post by tgreese »

A simple test light can often be enough.

https://www.amazon.com/JASTIND-Automoti ... -Indicator

-or-

https://www.harborfreight.com/612v-circ ... 63603.html

Likely also for sale at your local parts store.

Just follow the signal. You'll still have to read the diagram to find which wires connect to the turn signal switch connector where.

Two things: trace the circuit through on the diagram, then measure. Measure, measure, measure.
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
Dual Everything: '15 Chryco Jeep Cherokee KL Trailhawk, ECO Green
Blockchain the vote.

Topic author
MNeug
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Joined: Sat Jun 24, 2023 10:06 am

Re: 79 J10 DS Brake Light STILL Not Working But Everything Else Does

Post by MNeug »

The ground I replaced is on the rear driver side assembly. The old one just looked like crap so I cut a new wire, threw on a connector and soldered the wire to the assembly where the old one was. I'm sure the ground is working because if I turn on the lights with the ground disconnected they come on but not very brightly and when I reconnect the ground the lights are nice and bright. Well, except for the DS brake and hazard flasher. TS works great. Is there another ground? Thanks!
devildog80 wrote: Fri Jan 31, 2025 1:23 pm Suggest checking the rear light assembly, to be sure it is grounding like it should, then go from there.

Topic author
MNeug
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Joined: Sat Jun 24, 2023 10:06 am

Re: 79 J10 DS Brake Light STILL Not Working But Everything Else Does

Post by MNeug »

Yeah, the brake and indicators use the same filament so I think the ground, THAT ground, is good. I've been staring at wiring diagrams for like 4 days and can't see where else a ground would be at the rear of the truck.
tgreese wrote: Fri Jan 31, 2025 1:34 pm If the indicators and the brake lights use the same filament in the same bulb, then the ground must be ok. If the indicators and brake lights are separate bulbs, then you could have a bad ground somewhere.

Topic author
MNeug
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Joined: Sat Jun 24, 2023 10:06 am

Re: 79 J10 DS Brake Light STILL Not Working But Everything Else Does

Post by MNeug »

Thanks! I'll check those out. I appreciate it. I have a few open tabs with the wiring diagrams on them as I type. The part that confuses me about the diagrams is it 'll say "Tail, Stop & Back Up Lamp" then underneath say something like "16,17, 18". I have no idea what those numbers mean. Like another assembly style on the same page will say "15". Do you know what those numbers mean? Thanks again.
tgreese wrote: Fri Jan 31, 2025 4:52 pm A simple test light can often be enough.

https://www.amazon.com/JASTIND-Automoti ... -Indicator

-or-

https://www.harborfreight.com/612v-circ ... 63603.html

Likely also for sale at your local parts store.

Just follow the signal. You'll still have to read the diagram to find which wires connect to the turn signal switch connector where.

Two things: trace the circuit through on the diagram, then measure. Measure, measure, measure.
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tgreese
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Re: 79 J10 DS Brake Light STILL Not Working But Everything Else Does

Post by tgreese »

You need to be more specific with your question. 15, 16, 17 could be the model number or could be the circuit number. Model numbers are shown in section 1. The circuits are numbered in a table with the diagrams. On the diagram you;ll see something like "16 WT 18" for circuit 16, white wire, 18 ga.
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
Dual Everything: '15 Chryco Jeep Cherokee KL Trailhawk, ECO Green
Blockchain the vote.

Topic author
MNeug
Posts: 33
Joined: Sat Jun 24, 2023 10:06 am

Re: 79 J10 DS Brake Light STILL Not Working But Everything Else Does

Post by MNeug »

Hi. I'm not talking about the wires, their gauges and colors, I'm talking about when you look on the wiring diagram and you get to the tail light assemblies there are numbers underneath the description of the part it will say 16, 17, 18. The diagram is of course for a few different models so there's usually a different assembly above or below that will say 15. Those are the numbers I don't understand.
tgreese wrote: Sat Feb 01, 2025 5:59 am You need to be more specific with your question. 15, 16, 17 could be the model number or could be the circuit number. Model numbers are shown in section 1. The circuits are numbered in a table with the diagrams. On the diagram you;ll see something like "16 WT 18" for circuit 16, white wire, 18 ga.
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tgreese
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Re: 79 J10 DS Brake Light STILL Not Working But Everything Else Does

Post by tgreese »

A '79 J10 is a model 25 or 45. The teens are wagons. Page A-5 in the TSM. Read Section A.
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
Dual Everything: '15 Chryco Jeep Cherokee KL Trailhawk, ECO Green
Blockchain the vote.

Topic author
MNeug
Posts: 33
Joined: Sat Jun 24, 2023 10:06 am

Re: 79 J10 DS Brake Light STILL Not Working But Everything Else Does

Post by MNeug »

Much appreciated. Hey, any idea why some folks indicate swapping out the turn signal switch is such a pain? I've swapped out the steering column before so I know how to do that. Swapped out the lock cylinder in the past so I know how to get the TS switch under the steering wheel. No big deal there. I've removed the old switch from my old steering column (just for practice). It was a snap. It seems like a mildly time-consuming job but not at all difficult. Am I missing something if I go that route? Since I put this column in something always bothered me about the TS switch in it. I mean hey, the switch in it now is likely 47 years old. Thanks again. I hope you're having a good weekend.
tgreese wrote: Sat Feb 01, 2025 9:16 am A '79 J10 is a model 25 or 45. The teens are wagons. Page A-5 in the TSM. Read Section A.
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devildog80
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Re: 79 J10 DS Brake Light STILL Not Working But Everything Else Does

Post by devildog80 »

On these old rigs, many times what was built at the factory, requires grounding through body pieces/parts bolted/welded together. Over time, the bolted pieces can oxidize between the parts, and lose grounding.
So when chasing grounds, feel free to add any you think are needed between body parts, frame, engine, battery, and give the electricals a fighting chance to work as intended. Hurts nothing, and if nothing changes nothing lost for your efforts, as they will be helpful with grounding issues you do not even know you have yet with other lighting on the truck. Found this out when working on my own rigs.
And I do agree, if the RR tail light is working correctly, the LR should too, but not always. Could be in the steering column, or not. Just need to keep at it, and you will find the issue :/
'81 CJ5 Base, 258 I6, MC2100, T176 4 spd, 300 TC, D30 Front NT, 3.31, 2-Piece AMC 20 rear NT, 3.31, 4" high arc spring lift
FOR SALE - '84 Grand Wagoneer, 401 V8 (.030 over), Edelbrock clone 1406, 727 auto, Selec-trac NP229, AMC 20 REAR - D44 FRONT - WT 3.31, 4" high arc spring lift
Rather be driving, than waiting to be modified

Topic author
MNeug
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Joined: Sat Jun 24, 2023 10:06 am

Re: 79 J10 DS Brake Light STILL Not Working But Everything Else Does

Post by MNeug »

Awesome advice. And yes, I will succeed. Can't stop until I do. Help from folks like yourself and the people in the forum is invaluable. Now I just have to start googling how to add grounds. But that's OK. The weather is crappy and I don't have anything else to do.
devildog80 wrote: Sat Feb 01, 2025 3:00 pm On these old rigs, many times what was built at the factory, requires grounding through body pieces/parts bolted/welded together. Over time, the bolted pieces can oxidize between the parts, and lose grounding.
So when chasing grounds, feel free to add any you think are needed between body parts, frame, engine, battery, and give the electricals a fighting chance to work as intended. Hurts nothing, and if nothing changes nothing lost for your efforts, as they will be helpful with grounding issues you do not even know you have yet with other lighting on the truck. Found this out when working on my own rigs.
And I do agree, if the RR tail light is working correctly, the LR should too, but not always. Could be in the steering column, or not. Just need to keep at it, and you will find the issue :/
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devildog80
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Re: 79 J10 DS Brake Light STILL Not Working But Everything Else Does

Post by devildog80 »

Can be simple as a 4-6" 10-12 gauge wire, eyelet on each end, star washer screw on each side. Or existing screw/bolt on each side, make the wire with eyelets to fit and connect, making sure metal & hardware are super clean where eyelets contact.
Concept is to have the wire/cable connected from one piece of the rig to another. Lets use front grill, fenders, frame, battery, as an example.
Example
Mounting screw for headlight mounted in grill....
grill to negative battery post
OR...grill to fender to negative battery post
OR..grill, fender, frame to negative battery post..
OR..grill,fender, firewall, negative battery post
You are creating a pathway, like the little line puzzles in kids books, enter here and find path to center of puzzle. You are doing this with your grounds. Start at battery negative terminal, take your ground wire path connected between them out to all metal pieces on the truck with the short wires, and they will connect back to your negative battery terminal.
You can use buss bars for connection points, to allow multiple wires more easily, mostly around the engine bay and interior of the rig.
You get the idea.

Here is one on mine, next to battery in engine bay. This connects from battery negative, then I have a wire into my cab, and another buss bar in there, for anything needing grounding inside. Gauges, switches, sound, etc.

Image20221127_151219 by Scott Weckerly, on Flickr

Image20221127_142915 by Scott Weckerly, on Flickr
'81 CJ5 Base, 258 I6, MC2100, T176 4 spd, 300 TC, D30 Front NT, 3.31, 2-Piece AMC 20 rear NT, 3.31, 4" high arc spring lift
FOR SALE - '84 Grand Wagoneer, 401 V8 (.030 over), Edelbrock clone 1406, 727 auto, Selec-trac NP229, AMC 20 REAR - D44 FRONT - WT 3.31, 4" high arc spring lift
Rather be driving, than waiting to be modified

Topic author
MNeug
Posts: 33
Joined: Sat Jun 24, 2023 10:06 am

Re: 79 J10 DS Brake Light STILL Not Working But Everything Else Does

Post by MNeug »

Wow, that is a very cool idea. So you're basically just increasing the ground capacity/reach of the whole truck.
devildog80 wrote: Sat Feb 01, 2025 5:00 pm Can be simple as a 4-6" 10-12 gauge wire, eyelet on each end, star washer screw on each side. Or existing screw/bolt on each side, make the wire with eyelets to fit and connect, making sure metal & hardware are super clean where eyelets contact.
Concept is to have the wire/cable connected from one piece of the rig to another. Lets use front grill, fenders, frame, battery, as an example.
Example
Mounting screw for headlight mounted in grill....
grill to negative battery post
OR...grill to fender to negative battery post
OR..grill, fender, frame to negative battery post..
OR..grill,fender, firewall, negative battery post
You are creating a pathway, like the little line puzzles in kids books, enter here and find path to center of puzzle. You are doing this with your grounds. Start at battery negative terminal, take your ground wire path connected between them out to all metal pieces on the truck with the short wires, and they will connect back to your negative battery terminal.
You can use buss bars for connection points, to allow multiple wires more easily, mostly around the engine bay and interior of the rig.
You get the idea.

Here is one on mine, next to battery in engine bay. This connects from battery negative, then I have a wire into my cab, and another buss bar in there, for anything needing grounding inside. Gauges, switches, sound, etc.

Image20221127_151219 by Scott Weckerly, on Flickr

Image20221127_142915 by Scott Weckerly, on Flickr
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devildog80
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Re: 79 J10 DS Brake Light STILL Not Working But Everything Else Does

Post by devildog80 »

Yep.....that is what your doing.
But this is NOT the magic smoke fix......for everything electrical, just helps eliminate grounding as the issue. Make sure to investigate further, as needed and stated in other posts above, for ultimate fix of the issue.
Good luck :)
'81 CJ5 Base, 258 I6, MC2100, T176 4 spd, 300 TC, D30 Front NT, 3.31, 2-Piece AMC 20 rear NT, 3.31, 4" high arc spring lift
FOR SALE - '84 Grand Wagoneer, 401 V8 (.030 over), Edelbrock clone 1406, 727 auto, Selec-trac NP229, AMC 20 REAR - D44 FRONT - WT 3.31, 4" high arc spring lift
Rather be driving, than waiting to be modified
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tgreese
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Re: 79 J10 DS Brake Light STILL Not Working But Everything Else Does

Post by tgreese »

Ya know, adding extra grounds won't hurt. If you want to add a dedicated ground bus where you can tie in your ground wires, go ahead.

However, both the body and the chassis are large conductors that will abley carry ground current back to the battery. A big wire has much less resistance than a small wire, and the body and chassis are very large, much larger than any of the wires used in a car. What they lack in conductivity compared to copper is likely made up for to large degree by shear size. More certainly a ground problem comes from corrosion or looseness in the ground connections, rather than some deficiency in design.

I'd suggest refreshing all the factory ground connections first, rather than add a lot of redundant wire. If the taillight housing depends on metal-to-metal contact, it should be clean and firm. You can add local ground jumpers to the chassis or body if you wish. New connectors, proper crimps and solder (if so inclined), new hardware and new shakeproof washer. But a solid local connection should be enough IMO.
Tim Reese
Maine beekeeper's truck: '77 J10 LWB, 258/T15/D20/3.54 bone stock, low options (delete radio), PS/PDB, hubcaps.
Browless and proud: '82 J20 360/T18/NP208/3.73, Destination A/Ts, 7600 GVWR
Copper Polly: '75 CJ-6, 304/T15, PS, BFG KM2s, soft top
GTI without the badges: '95 VW Golf Sport 2000cc 2D
Dual Everything: '15 Chryco Jeep Cherokee KL Trailhawk, ECO Green
Blockchain the vote.

Topic author
MNeug
Posts: 33
Joined: Sat Jun 24, 2023 10:06 am

Re: 79 J10 DS Brake Light STILL Not Working But Everything Else Does

Post by MNeug »

I definitely like your idea of going around and refreshing the factory grounds. They worked from day 1 so they seem to be in the right places. And the ground at the tail light assembly should be great. I took off the old one, cleaned it up, added a new coated copper wire, soldered it to the assembly and attached it to the housing/steel mounting hole of the assembly, all cleaned down to bare metal. It made the actual working lights much brighter. I have a feeling my issue is the hazard light in the turn signal switch. But I'll keep looking. I have a new TS switch coming and I know how to install it. The one in the column now is surely 40+ years old.
tgreese wrote: Sun Feb 02, 2025 12:39 pm Ya know, adding extra grounds won't hurt. If you want to add a dedicated ground bus where you can tie in your ground wires, go ahead.

However, both the body and the chassis are large conductors that will abley carry ground current back to the battery. A big wire has much less resistance than a small wire, and the body and chassis are very large, much larger than any of the wires used in a car. What they lack in conductivity compared to copper is likely made up for to large degree by shear size. More certainly a ground problem comes from corrosion or looseness in the ground connections, rather than some deficiency in design.

I'd suggest refreshing all the factory ground connections first, rather than add a lot of redundant wire. If the taillight housing depends on metal-to-metal contact, it should be clean and firm. You can add local ground jumpers to the chassis or body if you wish. New connectors, proper crimps and solder (if so inclined), new hardware and new shakeproof washer. But a solid local connection should be enough IMO.
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