BMW M5 Forum and M6 Forums banner

Brake light crazy problem

3K views 19 replies 5 participants last post by  jross036 
#1 ·
Hello!

I'm getting this anoying crazy problem with my euro 1995 M5. Once I hit my brake pedal, the brake lights would come on and stay on, even after I shut down the car. Got the brake circuit msg and went for the brake light pedal switch fix.

Now, yesterday I changed the switch (no bended brackets) and now I get this very odd syntom: when I apply the brake pedal, light go on; when I release it, they will stay om for like 10 seconds, when they will be shutting down. I really don't think this is a switch issue. I'm not getting the brake circuit check msg anymore, but it's clearly crazy. I don't know where to start from. Searched the forums and found little info on that. Heard of a brake circuit relay, but actually couldn't find its location.

Any help on that matter would be much appreciated. Have you had this issue or something similar?


Regards
 
#2 ·
http://wedophones.com/Manuals/BMW/1... - 540i Electrical Troubleshooting Manual.pdf

Have a look at pages 356 / 357 of the Adobe document.

You'll see that there are only two possibilities here:
Either the new brake switch is sticking or faulty, OR
The LKM (Lighting Control Module) is faulty.

There will be internal relays in the LKM, and LKMs are known to go bad, in almost every way possible. That said, I would absolutely make sure that the brake switch is working correctly, and not sticking, or not in a slightly bent bracket, before I threw an LKM at it.

Here's how:
Use a test light, with clip on a good ground, to backprobe or stab the Blue/Red wire at the brake switch. (Key on, engine off). Push the brake pedal. Light should illuminate. Release pedal...light should go out.

If the light goes out immediately when you release pedal, the LKM is bad. If the light stays lit for any time at all, the brake switch is sticking.
 
  • Like
Reactions: Nilo and mottati
#5 ·
Hello,

Sorry for the late replay to this thread. My brake lights are still faulty. I got a new original switch and the problem persists. Not the LKM, since I tested it and used a second one, and the problem persists.

What happens is something is in short: when I plug the new switch in and press the brake pedal, it lights up and stays on. I can unplug the switch and it'll be on. Have to disconnect the batery to make it go off. Sometimes, if I just plug the switch, It'll turn the brake lights on, and it'll be on until I disconnect the battery. Some short in the wires that go to the switch? Ant help?


Regards
 
#6 ·
It might be worth a good look at your wiring bundle up under the left side in the trunk. I've found pinched and broken wires in there a few times. Some very mysterious symptoms when a hot wire would rub the ground (brown) side of the license light, never blew a fuse because that brown wire was clean severed and power was feeding back the light when the trunk lid was closed. Anyway, that's just an example of one issue. All the rear wires are in that bundle on the hinge point. Good luck.
 
#7 ·
Thanks for your suggestion, I'll try it tough I have my doubts about that. The car is only 45.000km and it was always well kept, in almost like new condition. At first the wires don't seem so bad, by looking. But I'll look in detail and get back here.
I noticed I have a green wire, coming from the plug where the brake switch is supposed to connect to. That green wire seems to have been soldered, and there are two other wires welded to it. I'll take some pictures and post later today.

Regards!!
 
#8 ·
Ugh. Custom wiring on BMWs.....usually leads to problems.

In later (E38 and E39) cars, the ignition switch would be the likely culprit for weird electrical issues, including brake light issues. But since you've got "custom" wiring at that switch, this is absolutely the primary suspect.

Where do the extra wires go?
 
#9 ·
Hello, Chris and thanks for your time to answer. I managed to take a photo of the wire I´m talking about. Actually, the wire is blue and red, welded to a blue only wire. The blue and red wire comes from the brake switch plug and it is welded to this blue wire. This blue only wire goes with a grey and red wire to the left side, hiding on the left foot plastic molding. Don´t know what they´re for, really. Actually, if anyone knows what those wires are for, it would be a great help, so I can diagnose the prblem and trackdown the short.

Again, if I plug the brake switch on the brake switch plug, the brake lights are just going to come on and stay that way, not withstanding a correct switch operation (new original unit). Also, after the brake light is on, even if I disconnect the brake switch it´ll kepp on. It´ll only turn off if I disconnect the battery.


Best regards
 
#12 ·
Photos from the pedal cluster. Note the brake switch and blue and red wire coming from it. Goes into a blue only wire. This blue only wire is taped to a grey and red wire. They both go to the left of the footrest, closest to the plastic molding to the left of the car
 

Attachments

#15 ·
Hello, guys! Good news! I have cut the strange blue wire and the brake lights are working properly again. Still have no idea about what that was good for, but since the OBC and check control didn´t point anything wrong, I assume it´s going to be no problem!

Thank you very much for all your help. This is indeed a great place to exchange info and knowledge concerning our cars!


Best regards
 
#16 ·
Ha! Success! Great to see you fixed it.

Now, you need to solve that mystery: What would someone want to energize ONLY when the brake pedal is pressed (brake light switch turned on)????

Can you think of anything useful? Extra brake lights? (why?)

Rear-firing arresting net? Air cannon? Ejection seat? No, I think you would want to be moving when that went off...
 
#17 ·
If I had to guess, someone rigged some sort of cruise control kluge, to shut it down, because the stock wiring had been compromised. OR, someone was trying to rig a trailer wiring harness....BMW's Lighting Module would have hated finding extra bulbs wired in. Maybe a U-Haul "technician" did it..... except, well, the U-Haul guy wouldn't know what solder was; he'd have used home wiring nuts.

I like the "rear firing arresting net". Maybe a huge rear wing air brake? Chaff, and other anti missile measures?

Of course, you COULD follow the blue wire......and then we'd all have an answer to the mystery! ;):);)
 
#19 ·
Yes, air-brake wing would actually make sense. I wonder if there are aftermarket versions of this...

Or maybe this was a drag car and it deployed the parachute? :wroom:

Maybe I was thinking in the opposite direction, what would you want to turn OFF when you brake?
 
This is an older thread, you may not receive a response, and could be reviving an old thread. Please consider creating a new thread.
Top