Brake warning light on when brakes are fine

[Phoenix39]

I have a red warning light on my dashboard that is constantly on (for about a year). It looks like this ((!)).
My e39 is European and from 97 - so it does not have yellow brake disk lights or the "check brake linings" text. It is the most "simple" dashboard. This is what it looks like:


It is the exclamation mark on the right, not the ((P)) for handbrake and not the yellow ABS light.
My manual says "brake pads/hydraulic".

The car was recently in at the shop, and they said they could find no faults with my brakes. I recently replaced my pads and wear sensors just to be safe. I also replaced the brake fluid, bleeded the system, and ensured the level is at max.

After all this I was hoping it was the sensor in the brake fluid reservoir, and I was suggested to take a bmw diagnostic to find out what the problem was - and the result was frustrating. There are no faults with the brake system! Fluid sensor is fine, abs is fine, no errors found for brakes...

What should I do? Is there something left to do before I just put it in at the shop and pay who knows how much?

[Phoenix39]

The diagnosis was done at a real bmw shop, by the way. Is there any way a sensor problem would not show up in the diagnosis? I'm hoping it could be the brake fluid sensor since I haven't replaced that yet, but the mechanic who did the diagnosis said that would turn up on his screen, instead of no errors...

[william540i]

Good day,

AFAIK, it is either low brake fluid (or faulty plunger in the MS in your case) or the wear sensor(s) have an open circuit. They don't have to be "worn" at the pad location to trigger a brake lining issue. Like you pointed out, if it is a brake pad issue, you will also get the message "check brake lining" when you have the HIGH cluster... Check the wires going to the pad wear sensors...

Good luck and let us know what the fix was please!

Cheers!!

[Phoenix39]

I talked to a BMW mechanic today, and he said it could be the wear sensors even if the computer didn't find an error in the diagnosis. I said I already replaced them, and he said that he would just do it again - because there could be a chance I put in a bad sensor... I don't know, it seems unlikely, and all the small expenses are starting to add up.
Is there a way to check my current sensor? The mechanic said it's possible with a multimeter - but I'm not sure how. The tip of it that goes into the caliper is supposed to be just plastic? Then when the plastic wears out there's a wire inside - but when the sensor is intact, I won't be able to measure?

I have the same issue with the brake fluid sensor - I'd hate to waste more money on parts that work fine. But I don't know how to test the old sensor. I tried removing the wire from the reservoir and inserting a paper clip inside the connector (some people online reported that this worked on their cars). But nothing happened, and I can't tell whether my sensor is fine or not...

I don't know how to test the wires without cutting them or destroying the plastic connectors... I saw people on Youtube poking the multimeter connectors through the plastic on the wire, but the sensor wires are so thin I'm afriad they're gonna break.
I'm pretty new at the electric troubleshooting so I hope someone who's done this can give me suggestion

[jstern]

Just going back to basics here. After you replaced the sensors, did you clear the fault in the computer. You do this by turning the key to the accessory position (position 2) and leaving it there without starting the car for at least 30 seconds-sometimes a bit more time is necessary. The dash light should go out after that time.

[Phoenix39]

Yes, I did the reset procedure several times.

[e39hamann]

Maybe there is a wiring issue somewhere? I had the red brake light on and fixed it by putting new sensors in (my rotors and pads aren't even a year old). I could potentially see how maybe the wiring after the sensors got messed up or even the connector that the sensors plug into.

[Djneff]

For brake light bulbs, did you double check the 3rd brake light? Those are easy to miss.

[Phoenix39]

It probably is a wire issue, and it should be easy to find out if I could measure wires with my multimeter. But the problem is that it's very difficult to find measure points! The plastic connectors are so thin I can't even fit a paperclip in them, so I would need to cut the wires and strip them to measure, but it doesn't make sense to ruin wires if they're not the problem. I don't know how mechanics check wiring without cutting them...

[Djneff]

It probably is a wire issue, and it should be easy to find out if I could measure wires with my multimeter. But the problem is that it's very difficult to find measure points! The plastic connectors are so thin I can't even fit a paperclip in them, so I would need to cut the wires and strip them to measure, but it doesn't make sense to ruin wires if they're not the problem. I don't know how mechanics check wiring without cutting them...

Use a thin sewing needle and then clip the needle.

[Phoenix39]

I found a way to measure today. I used one of the old break pad sensors - the pins also fit in on the brake fluid reservoir connector. So I cut the other end of the wire and used it for testing.

I checked the wire to the reservoir first (the sensor itself is inside the reservoir so I couldn't do anything with that). When I turn on the ignition, my multimeter gave a 12V reading - so I guess that means the wire is fine? I tried splicing the wires together to simulate a good sensor (by twirling them) but the warning light is still on. I've also tried resetting the light while simply unplugging it.

I measured resistance in both of my new brake pad sensors - and they're fine (the old ones were also fine). I measured volt at the wires that end up at the brake pad sensors - I also got a 12V reading here. I tried splicing the wires together like some people said (just for fun) and that didn't work either.

So from what I can understand, all sensors and wires are ok? I was expecting to find no power in the wires leading to either brake pad sensor or reservoir switch, or at least that one of my brake pad sensors were broken...

What can this mean? My battery went flat during this operation, and this turned on ABS and ASC lamp too... when the battery was charged, they went away again. My battery might be too old... but could this trigger the persistent brake warning light?

[ypahihi]

"What can this mean? My battery went flat during this operation, and this turned on ABS and ASC lamp too... when the battery was charged, they went away again. My battery might be too old... but could this trigger the persistent brake warning light?"

I know this is an old thread, but I am wondering if you ever found what the problem and how you resolved the issue.
I currently have a similar issue to your symptoms.