dash brake light on

My dash brake light is on. Can't find anything obvious. Just did the brakes last year, new rotors, pads, calipers work fine, replaced sensor wire, pad thickness is fine. I'm baffled. Any suggestions??

I have the same problem. Once in a while while driving for a long time it will go out until I shut off the car. Then next time it is on again.
I have discussed this with a BMW master tech and he suggested it could be a broken conductor in one of the wires to the sensors. Difficult to diagnose and repair.

Similar issue with my speedometer being intermittent but works after driving about 20 minutes.

Bob in Lake Havasu

Try unplugging the connectors to the wires in the pads and spraying some WD-40 or similar on the contacts. Maybe a little corrosion is preventing good contact in the plug.

John

The brake wear sensors are fed 12v via Fuse-10 whenever the ignition switch is in the Run or Start position. The circuit flows in a loop first to the right-rear brake, then to the front-left, and then back to the indicator switch inside the instrument cluster. It is one continuous loop feeding 12v back to the instrument cluster.

[attachment 245 BrakeWearSensor.jpg]

The wear sensors each contain a thin wire embedded in a plastic housing. When the brakes pad wear out, the plastic surrounding that thin wire gets worn down until the wire either touches the brake rotor (thereby grounding the circuit), or the wire actually gets cut (thereby breaking the circuit). Either way the instrument cluster no longer sees 12v at the end of the circuit and that triggers the warning light.

If the warning light is lit, that means the circuit is either grounding out somewhere, or is broken.

With the key in the Run position, measure voltage at the left-front wear sensor connector. If you do see +12v there, then you know the right-rear sensor is good too as the circuit goes through that one first. If you don't see +12v at the left-front, then the issue is somewhere upstream, i.e. at the right-rear. If you don't see +12v at the right-rear sensor either, then work back from there.

If you do see +12v at both of the sensor connectors, then check that the circuit continues unbroken to the instrument cluster. It comes back to the instrument cluster via a Blue/Green wire to Pin-26 on C2 (the White 26-pin connector on the back of the cluster). Again, it is one of the end pins on those large connectors. The end pins are the most prone to cracking the pin's solder joint on the instrument cluster circuit board if the connector is wiggled back and forth rather than pushing straight on when plugging/unplugging those large connectors.

If the warning light only comes on intermittently, I'd suspect a cracked solder joint on that Pin-26. Otherwise the most likely culprit is the connector at the brake sensor.

I chased an intermittent brake warning light long enough that I eventually gave up and simply removed that light bulb from the cluster. I have my wheels on and off frequently enough, checking the brakes each time, that I really don't need that warning light to tell me whether the brake linings are wearing too thin.

Quote
Ferdinand
The brake wear sensors are fed 12v via Fuse-10 whenever the ignition switch is in the Run or Start position.

That reminds me. Fuse 10 also feeds the backup lights, and the wire for the backup light switch is the one that can rub against the driveshaft until it gets a bare spot and grounds out on the shaft if it isn't properly secured.

Are you having trouble with your backup lights too? If you are, check to see if the wire is touching the driveshaft. Wire tie it to the shift linkage if it's hanging loose. (Manual transmission for sure, automatic may be different).

John

Thanks guys. I started at the left front, no voltage. Went to right rear, checked at the plug, had voltage going in. Sprayed it with some electrical cleaner spray, then wd40 and it worked! Thanks again. Love this website.

Dave

Thanks Ferdinand. That detail may help me find the problem. I need to look into the cluster for the speedometer problem too.

Bob in Lake Havasu

Quote
Ferdinand
I chased an intermittent brake warning light long enough that I eventually gave up and simply removed that light bulb from the cluster. I have my wheels on and off frequently enough, checking the brakes each time, that I really don't need that warning light to tell me whether the brake linings are wearing too thin.

Yeah, I did something similar. For years I tried to keep those sensors working, but the sensor wires were constantly breaking so most of the time I ended up driving around with the warning light on. Finally I just gave up, cut the wires on the sensors, and connected them together. Presto, no more annoying light, and it's easily reversible if I ever want to go back (which I can't imagine.) I also have the wheels off often enough that I can check the pads with my eyes.

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Dave
'91 325iX