Renix stoplight switch not working
 

Hey guys! I've got an issue that recently came up after my mechanic swapped in a booster/master cylinder from a newer xj into my 1990 renix. I traced the issue down to my stoplight switch but I'm at a loss trying to figure out what exactly is wrong and even how the switch is meant to work. I attached a video here:

I was able to work the switch by hand to turn on the lights, but it only works with the brake pedal when situated in a very specific position. Eventually it falls out of place without much effort. Something has to be out of adjustment or not fitting correctly since the new booster and plunger was installed. My mechanic hasn't responded to any of my calls or texts so I can't use him as a resource to diagnose this. Let me know if I can paint a clearer picture so we can find a solution.

If there's a thread already dedicated to this, please light the way.

I put a 97+ booster in my renix, but that was like 15 years ago... iirc i had to grind a flat spot onto the push rod to get the brake lights to work.
Hopefully someone else will have more info.

What year did the new booster come from?

The rod from the booster should look like this.

https://cimg6.ibsrv.net/gimg/www.che...d33bd668f0.jpg

Check the alignment of the stoplight switch with the brake pedal. Adjust it if needed so it activates when the pedal is pressed. Make sure it's securely mounted; if it’s loose or still not working, consider replacing it. Let me know if you need more help!

Thanks for the responses everyone. The booster I have installed now is from a 97-01. Your image is correct Saudade, that is what I had installed prior. From what I've read online and from your comment 89Laredo, I need to modify this new booster plunger in some way. Before I go hacking, would any of you guys be able to explain what I need to change and why? Looking at the original, I can see that flat spot and can imagine as the pedal arm goes through it's travel, it will keep the stoplight switch from rotating too low around the pivot point, turning off the brake lights while still allowing it to rotate upward. If you guys have any experience with this modification, I'd be happy to hear it. I'll have to go another day without brake lights, but hopefully tomorrow I can fix it!

I might be able to go look at my old jeep some time but it's out in a muddy field.
iirc the switch works kinda opposite of what you would think. In resting position the switch is depressed, contacts are open, and the lights are off. When you push the pedal the rod moves away from the switch and the contacts close. So pretty much you need to grind enough off of the back of the push rod so that the lights come on when you step on the brakes, but not too much or the lights will be on all the time.
iirc I cut too much on mine and had to shim it somehow.

I did this quite recently on my 1990 xj.
i found this 3 part series helpful for me, could only find no 2 now but there are 3 of them on this guys channel :


main learnings for me:
-you have to make a new hole in your brake pedal rod to get the right angle, otherwise the plunger won’t go straight into the booster and I guess your brakes will be ****e, maybe also affects brake lights?

-I also grinded too much off the flat surface and had to put a shim in the brake light switch assembly (ok, it’s a piece of zip tie) to get the perfect distance and angle.

-I got the wrong year booster one time, tried to install it and the brake pedal was very far from the floor. Did not work, so make sure the rod length is correct, measured from the face of the booster.

-there was a big square metal shim between the booster and the firewall that is supposed to be there (also some white soft material that I guess works as a gasket). I tried stacking 2 of them during desperate attempts to get the wrong part to fit- bad idea.

Sounds like you’re close!

Here's the one from BleepinJeep. You can advance to about 6:15 min to see the mods to the brake rod end.

Well this took me wayy too long to get to but I'm happy to say I found the issue and a solution. My mechanic who installed the booster did modify it to work with the old switch. However, after reading Skuggan's post, I looked into the gap between the switch and plunger and it was definitely too large. I opted to follow in your footsteps and temp install a zip tie to shim that gap out. I'm hopeful it was stay for a bit until I think of a more permanent solution, but at least I should have brake lights for my commute to work this week. Thanks for all the help everyone!