Brake Lights Stopped Working

[BP_MJ_978]

Hi, I’m having some issues with my brake lights, both stopped working simultaneously. I’ve only owned the a Jeep for a about 2 months now, so I don’t know a ton about it, but I know both brake lights worked for inspection a month ago. 
 

I know this topic has been discussed before, but I’ve done some searching on here and haven’t found anything that has fixed the problem yet. 

 

I check the electrical connection behind the drivers side rear wheel, the prongs were clean and appear to be working.

 

I checked all  the bulbs and all are working. Headlights, taillights, running lights, both directionals/flashers and reverse lights all work. 

I checked all fuses, including the haz/stop fuse, all appear to be in good, working order. 

 

After doing some research, it appears to have an upgraded master cylinder and brake booster, shown in the pictures below. From other pictures I’ve seen, it looks like the MC and brake booster are out of an XJ (would make sense, Jeep has other XJ parts swapped in by previous owner). 

 

However, there is no brake light switch on the pedal, as shown circled in the pictures. All the write ups and pictures I’ve seen show the brake light switch attached to the brake pedal. Where else could the switch be? Has anyone seen a set up like this before? 

I’m thinking the switch is likely the cause of my issues, but I can’t be positive without finding. I’ve gone through the obvious culprits, but if anyone knows of  any other potential causes that I haven’t checked yet, let me know. Any help or guidance would be greatly appreciated! 

[jdog]

Sounds like the switch to me, if you can find it you can jump the two wires and if the lights come on then it's the switch, if not it's in the turn signal switch

[Eagle]

It does sound like a bad switch.

 

Looking at your photos, it appears the modified system has an adjustable proportioning valve just below the master cylinder. There's a "thingie" in that that looks suspiciously like an old-school, pressure-actuated brake light switch next to the dial for adjusting the proportioning. I haven't seen one of those pressure switches for decades, but that's what was used back in the 1950s and 60s when I first started driving and wrenching. I would begin by checking out the adjustable proportioning valve. See if you can determine the make and model, then maybe you can figure out if that thingie is a switch.

 

 

 

 

{Edit to add} Looks like NAPA still carries hydraulic brake light switches.

 

https://www.napaonline.com/en/p/GRO822159

 

https://www.napaonline.com/en/p/TWR8629BX

 

[Eagle]

I think I found it: https://ssbc-usa.com/products/a0730

 

That thingie IS a pressure-actuated brake switch. It should have wires connected to it. The item description says that switch is for brake system warning, though. I'm not sure just how that works. I would suggest contacting the company that makes that unit to request an instruction sheet, so you'll know what you've got. Maybe you can talk to someone in their technical department (if they have one) and ask for guidance with your problem.

[BP_MJ_978]

I figured it out! Thank you for both very much for your replies. Eagle, thanks for pointing me in the right direction. Being born in 97, I don’t know much about switches from the 50s and 60s. Turns out the electrical connection had come completely undone. All I had to do was slide it back onto the two small posts on the switch and I’m back in business!

[AZJeff]

Cool!!   Yeah, Eagle and I are both "seasoned veterans" when it comes to working on automotive stuff.   Other persons might use terms like "fossil" or "old codger"   In any case, I remember when (as a YOUNG teenager) there were brake light switches like that.

 

I am glad you were able to fix it with one old codger's advice.

[Eagle]

1 hour ago, BP_MJ_978 said:

I figured it out! Thank you for both very much for your replies. Eagle, thanks for pointing me in the right direction. Being born in 97, I don’t know much about switches from the 50s and 60s. Turns out the electrical connection had come completely undone. All I had to do was slide it back onto the two small posts on the switch and I’m back in business!

 

 

Excellent. I'm happy that some of my long out-of-date recollections were helpful. I can't remember when was the last time I saw a pressure-actuated brake light switch. I wasn't familiar with that brand of adjustable proportioning valve, so we both learned something.

 

Did you even know that you had an adjustable proportioning valve? Even though you got the lights working, I would still recommend that you contact the manufacturer to get whatever documentation they have on it so you can file it with your FSM (you DO have a FSM, right?) for future reference. Depending on how you use your MJ, you may want to learn what setting to use for running empty and running loaded.

 

Also: Note what I posted above. The description of that proportioning valve says that switch is the brake system warning switch. If that's correct, it's not supposed to operate the rear brake lights, it's supposed to operate the warning light on the dashboard if either the front or the rear brakes fail. The description also says that unit is a distribution block as well as an adjustable proportioning valve. If that's correct, that means the lines to the front brakes should also be run through that unit and, from your photo, it doesn't appear that was done.

 

This raises a couple of concerns:

1. You may not have a functioning brake system warning light. In practice, you'll most likely know immediately if your front or rear brake circuit fails, but the warning light should be functional as a confirmation.
2. If only the rear brakes are run through that valve, and the pressure switch is what controls the brake lights, then if the rear brakes fail you will not have brake lights. The front brakes do the majority of the work on an MJ, especially when there's no load, so you can drive it on just the front brakes. (Not recommended nor legal, but possible.) If the brake light switch is a pressure switch in the rear circuit, you won't have brake lights if the rear circuit springs a leak.

[BP_MJ_978]

Eagle, sorry for the late reply, I haven’t had much time to get out and do any looking or wrenching around with it. 
 

No, I had no idea I had an adjustable proportioning valve. After looking at it some more it’s actually from Summit Racing (You can’t see in the pictures, but it says Summit right on the side), so I was able to find some info/literature about it online starting from Summit. I do have an FSM, so I’m going to print out what I found and keep it in there. 

 

I could be wrong, but from what I’ve read the valve can be used to operate the rear brake lights, but it is not for the brake warning system. It’s tough to see in the photos I posted before, but the front brake lines are connected as they should be, according to the manufacturers website. Here’s what I found:

 

https://www.classicperform.com/Instructions/PDF/APV-DB.pdf

I really appreciate you taking the time to respond and research all of this. I started this thread off by saying I don’t know much about the Jeep, but I’ve certainly learned a lot already! Thank you very much!

[Eagle]

I'm very happy that your problem was easily solved, and now we all know a little more about another adjustable proportioning valve option. My comment about that switch being for the brake system warning light was based on what I read on the web site I found and provided a link to. So either that unit functions differently from the Summit unit, or (more likely) whoever wrote the copy for the web page didn't know what he/she was talking about. The instruction sheet you downloaded clearly says it can be used for the brake lights, and that it's NOT for the brake system warning light.

 

This probably means that you don't have a functional brake system warning light. The light is still in the dash cluster and probably lights up when you turn on the ignition (that's set up that way to confirm that the bulb works), but it won't light up if you have a front or rear brake circuit failure. You'll know it anyway -- once you have a few miles behind you in the vehicle, if either circuit fails your foot will know something's wrong immediately.

 

[Eagle]

This gets more and more curious. The instruction sheet for the same unit under the Summit Racing brand says that switch is for the brake warning light on the dashboard:

https://static.summitracing.com/global/images/instructions/sum-760250.pdf

 

Summit sells what appears to be the same unit under the Leed name, and they also say the switch on that one is for the warning light. Jegs sells the same Leed unit and Jegs says the switch is for the brake lights. Jegs sells what appears to be the same unit under their own name, and the specs for that one also say the switch is for the brake lights. Lastly, Jegs sells another version of what appears to be the same thing under the RPC name. When I looked that one up on the RPC web site, they say the switch is for the brake lights.

 

It appears that some vendors don't understand the products they're selling.

[Eagle]

Another one for the data banks: I contacted SSBC (Stainless Steel Brakes Corporation), another company that sells what appears to be the same adjustable proportioning valve. They sent me a copy of their instruction sheet.

 

Quote

The switch supplied with the Prop Block is a pressure switch designed to activate the brake lights
on the vehicle (and not the brake warning light on your dash). If your vehicle is equipped with a
brake light switch on the brake pedal, the switch in the Prop Block will not be used. If your vehicle
has no other brake light switch, then this switch must be properly connected for the brake lights
to function.