Can someone explain this wiring diagram to me please?

[Captain Skip]

See if this helps the discussion......

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[chevymitchell]

I know the difference between a CAN bus and a potentiometer thank you very much. What I'm getting at is that that ABS module IS on the CAN bus and that switch is probably monitored by the CAN bus. It's probably not even a 0-12 volt signal going across that potentiometer, impossible to know without seeing what is coming out of the ABS module. I think hacking that relay in there is going to cause problems. Once again without seeing what happens in the module it's a guessing game, I hope it does work for the OP.

That's great you know the difference. Your first reply was very vague.

The purpose is to just disconnect the brake lights. When the brake line is disconnected with the relay, the vehicle won't even be on.

The wiper voltage won't matter if it's 12vdc or 5vdc.

You really could put the disconnect anywhere between the pedal and the BCM.

B137 or B15 ... either one will accomplish what you need OP.

 

[Captain Skip]

That's great you know the difference. Your first reply was very vague.

The purpose is to just disconnect the brake lights. When the brake line is disconnected with the relay, the vehicle won't even be on.

The wiper voltage won't matter if it's 12vdc or 5vdc.

You really could put the disconnect anywhere between the pedal and the BCM.

B137 or B15 ... either one will accomplish what you need OP.

Thanks Shawn, I appreciate your input.

 

[JimLee]

That's great you know the difference. Your first reply was very vague.

The purpose is to just disconnect the brake lights. When the brake line is disconnected with the relay, the vehicle won't even be on.

The wiper voltage won't matter if it's 12vdc or 5vdc.

You really could put the disconnect anywhere between the pedal and the BCM.

B137 or B15 ... either one will accomplish what you need OP.

I see what you are saying but I think there are still a couple of potential issues. Bus monitoring often relies on resistance readings (or a range of) of the circuit, IF (unknown) the brake pedal travel sensor is monitored adding a relay will add resistance to the circuit, which could trigger faults, and as another poster stated could also effect the reading coming off the potentiometer. Depending on the voltage across the potentiometer it could be no big deal at all. But, if the signal across the potentiometer is in millivolts (again we are not sure since we can't see what comes out of the module) adding any resistance could cause a lot of issues with everything that gets a variable brake signal though the ABS module. Now if it's a digital signal coming through that line (potentiometers and digi-pots use the same basic symbol), all bets are off, adding anything to that line is going to cause problems. I think this scenario is unlikely though, simply for the reason that FCA seems to do things cheaply if possible. Honestly I'd be tapping into the furthest thing down the line myself, the signal that goes to the tail light itself, but even that can cause "bulb out" warnings. Some people have hacked 4 pin harness' into the brake light wiring and not gotten errors, it just depends on how much resistance is added to the circuit. OP, wherever you tie in I would do it temporarily and test everything very well before finalizing your install, that way if you trip any codes or things aren't working properly you can move on down the line to another point.

 

[Captain Skip]

I see what you are saying but I think there are still a couple of potential issues. Bus monitoring often relies on resistance readings (or a range of) of the circuit, IF (unknown) the brake pedal travel sensor is monitored adding a relay will add resistance to the circuit, which could trigger faults, and as another poster stated could also effect the reading coming off the potentiometer. Depending on the voltage across the potentiometer it could be no big deal at all. But, if the signal across the potentiometer is in millivolts (again we are not sure since we can't see what comes out of the module) adding any resistance could cause a lot of issues with everything that gets a variable brake signal though the ABS module. Now if it's a digital signal coming through that line (potentiometers and digi-pots use the same basic symbol), all bets are off, adding anything to that line is going to cause problems. I think this scenario is unlikely though, simply for the reason that FCA seems to do things cheaply if possible. Honestly I'd be tapping into the furthest thing down the line myself, the signal that goes to the tail light itself, but even that can cause "bulb out" warnings. Some people have hacked 4 pin harness' into the brake light wiring and not gotten errors, it just depends on how much resistance is added to the circuit. OP, wherever you tie in I would do it temporarily and test everything very well before finalizing your install, that way if you trip any codes or things aren't working properly you can move on down the line to another point.

Thanks Jim

 

[JimLee]

That's great you know the difference. Your first reply was very vague.

The purpose is to just disconnect the brake lights. When the brake line is disconnected with the relay, the vehicle won't even be on.

The wiper voltage won't matter if it's 12vdc or 5vdc.

You really could put the disconnect anywhere between the pedal and the BCM.

B137 or B15 ... either one will accomplish what you need OP.

And, i'm absolutely not trying to internet brawl with you here, it's obvious you know electronics. I've seen so many problems caused by people cutting into vehicle wiring on newer vehicles. Buses are tricky beasts, as I'm sure you know, I personally try to avoid breaking the integrity of factory wiring wherever possible for that simple reason. Not having detailed breakouts of the modules and data flow on the bus makes troubleshooting them even more difficult. I have a 1553 bus monitor, I wish I could get one for CAN.

 

[JimLee]

Thanks Jim

Let us know how it goes please. Every success is knowledge we can pass on to others.

 

[Captain Skip]

Let us know how it goes please. Every success is knowledge we can pass on to others.

Will do

 

[chevymitchell]

And, i'm absolutely not trying to internet brawl with you here, it's obvious you know electronics. I've seen so many problems caused by people cutting into vehicle wiring on newer vehicles. Buses are tricky beasts, as I'm sure you know, I personally try to avoid breaking the integrity of factory wiring wherever possible for that simple reason. Not having detailed breakouts of the modules and data flow on the bus makes troubleshooting them even more difficult. I have a 1553 bus monitor, I wish I could get one for CAN.

Oh, it's all good.

I 100% agree with testing/scoping everything before jumping in. I mentioned that in one of my first posts.

OP will get it figured out.

 

[Captain Skip]

OK, after a whole bunch more research I have determined that I can’t use the relay. On the JK it was needed but I just learned that on the JL the lights on it won’t come on when it’s being towed so #1 the relay won’t do anything and #2 there’s no 12v wire coming out of the brake switch. People are having the same problem with trying to install brake controllers for towing a trailer with electric brakes behind the JL - no where to get a 12v signal other than the center mounted brake light in the tailgate. Apparently that one is a simple switch and not computer controlled like the regular brake lights. Turns out I don’t need to cut into any wiring after all. I am going to get a separate brake light switch that tells me if the brake pedal is depressed but that doesn’t hook into anything electrical on the JL other than the battery. Thanks for all your input, it made me go slower and research the whole thing further.

 

[chevymitchell]

OK, after a whole bunch more research I have determined that I can’t use the relay. On the JK it was needed but I just learned that on the JL the lights on it won’t come on when it’s being towed so #1 the relay won’t do anything and #2 there’s no 12v wire coming out of the brake switch. People are having the same problem with trying to install brake controllers for towing a trailer with electric brakes behind the JL - no where to get a 12v signal other than the center mounted brake light in the tailgate. Apparently that one is a simple switch and not computer controlled like the regular brake lights. Turns out I don’t need to cut into any wiring after all. I am going to get a separate brake light switch that tells me if the brake pedal is depressed but that doesn’t hook into anything electrical on the JL other than the battery. Thanks for all your input, it made me go slower and research the whole thing further.

If you need help, PM me.

 

[Captain Skip]

If you need help, PM me.

Thanks Shawn. Actually it’s fairly easy. Road master makes a kit for the JL that has a bracket, switch and wiring. The whole thing gets added with no need to cut into any wiring. It just needs power off the battery but I’m thinking I may use the power lead on the motor home that comes through the 7 way plug to power it. That way it will be completely isolated from the JL’s electronics. You guys have made me extra cautious because of all the CAN bus and computer controls. This is the kit: https://www.etrailer.com/p-RM-751490.html

 

[BobaLong]

Another thought on this is that the potentiometer is part of the ESS system. I have noticed that the engine restarts as I begin to remove pressure off the brake pedal. A pot would be the likely way to sense this. Just my .02.

 

[FreedomFur]

Reviving an old thread here to see if @chevymitchell can chime in.

If a switch/relay was wired into the wire that controlled output to the brake lights (lights only), and the vehicle was running, would that potentially cause issues with the ABS or computer systems in the jeep?

A friend of mine with an older vehicle (electrician/fire fighter) did this on his vehicle. We offroad with night vision and getting blinded by someone’s brake lights is dangerous. We typically use some type of fabric over them, but it is a pain to deal with. They can be prone to coming off which could create an issue for the next driver behind the vehicle if they are navigating difficult terrain.

Short story is that if there is a simple toggle switch that could disconnect brake lights through a similar fashion without messing with a cann bus or the computer, it would really help us out.

 

[chevymitchell]

Reviving an old thread here to see if @chevymitchell can chime in.

If a switch/relay was wired into the wire that controlled output to the brake lights (lights only), and the vehicle was running, would that potentially cause issues with the ABS or computer systems in the jeep?

A friend of mine with an older vehicle (electrician/fire fighter) did this on his vehicle. We offroad with night vision and getting blinded by someone’s brake lights is dangerous. We typically use some type of fabric over them, but it is a pain to deal with. They can be prone to coming off which could create an issue for the next driver behind the vehicle if they are navigating difficult terrain.

Short story is that if there is a simple toggle switch that could disconnect brake lights through a similar fashion without messing with a cann bus or the computer, it would really help us out.

The module would detect a failure since it's looking for a specific amperage draw value to detect whether or not the lamp driver is working or if the light assembly is working. As far as wiring in the relay and switch, you wouldn't have an issue with that. It would just be as if you removed the bulbs or disconnected the light assembly. No issue with the CAN bus like this. Just an annoying tell-tale saying your lights are inop.

Just as another thought: you could have the relay switch over to an IR LED assembly. The IR would show up under goggles and you can place them in a tinted housing or buy tinted IR LED's. We did this with our helicopters in Iraq. Just zip-tied IR LED strobes to the stingers and the skids.

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