Brake pad warning light problem

[Steve L]

Dear Forum:
I have a 1987 325is and a few weeks ago, my brake pad warning light began to come on. It began intermittently at first, but is always on now.

My brake pads are fine and the sensors both look good. The front and rear sensors both have continuity of 18 ohms. If I put a jumper on the cable to the sensors, the dash warning light stays on.

I've looked thru my Bentley service manual for schematic information on this trouble light, but find very little information. I wonder if the circuit has lost ground, but can't find enough information on the wiring.

Does anyone have experience with a screwed up brake pad warning light? Does anyone know where I can find a schematic of the wiring for this system? It must be a simple system. Help appreciated.

Steve L

[george graves]

Might be corroded connector? Is that where you jumpered it? It might be corroded INSIDE the connector(harness side - not sensor side). Mine were, and so was my outside air temp sensor. +20 years of being exposed to the elements under the car can do that. None of those connectors are really sealed against the elements. Water + time, and the connector fails.

[jlevie]

The brake wear warning circuit is pretty simple in that the light comes on when there is an open in the circuit. You can find the wiring diagrams at http://wedophones.com/BMWManualsLead.htm

[1843lamp]

I have all new brake parts and four new sensors ,wiring and connections good checked with a meter ...time for the cluster to come out I think.

[101]

I had an intermittent brake lining light issue on the IX. Ultimately, a cluster rebuild took care of the issue. IIRC, it was a resistor for the circuit that went bad on the cluster.

[Steve L]

Thank you everyone for your thoughts. Mr. Graves, I looked at the connectors and think that the male/female plugs in the connector look pretty clean. It seems pretty clear that the problem is not with the sensors, themselves.

I know that this system is pretty simple. Perhaps there is a break in a wire leading from the sensor, but those two wires disappear into a harness at the top of the left fenderwell. Writer "101" mentioned the possibility of a bad resister in "the cluster". What does that refer to?

Meanwhile, I will go to the wegsite recommended by jlevie. Thank you all. I welcome additional thoughts on this.

Steve L

[Steve L]

Ok, I get it.....instrument cluster.

[101]

^^^ right. Instrument cluster.

After chasing my tail for months, I swapped clusters between cars and the problem stayed with the cluster, not the car. That's when I finally diagnosed the issue as something on the cluster.

Here's the verbiage that I received from Greg (Bavarian Restoration) when he fixed my cluster:

1. Brake lining light is controlled via switching transistor on main circuit board. I had this transistor replaced. This circuit (in the vehicle wiring) is one of the most finicky. The two sensors, as you may know, are located at the drivers front and passenger rear wheel wells. The connections can easily and commonly become oxidized. Even though the connections appear good, the oxidation can cause a high resistance leading to intermittent operation of the brake lining indicator. Regardless, that control transistor is replaced.

Good luck. I probably spent too much time and effort fixing this, but I'm one of "those kind" of people.

[Steve L]

Thank You Again

This is an excellent forum. Thank you to everyone who responded to my question. I've been working on cars for many years, but old age must be interfering with my thinking. I went to the website recommended by jlevie and I realized what I should have known all along. There should be about 12 volts of current going to the sensors. So my next step will be to put a volt meter on the front and rear wires to confirm that current is flowing thru the pad sensors. I can get out to my shop this weekend and we will see....

Thanks again for all of your wisdom.

Steve

[Mook613]

There is a resistor on the board (I think its right next to the blue plug) that is in-line for the sensors.

That resistor sits off the board to help dissipate the heat that it generates, but over many years the heat does cause the solder to eventually break away from the board.

You just need to re flow some more solder to the joints and it'll fix the issue.

[jeffnhiscars]

If anyone has a pic of that resistor ...

[Steve L]

Voltage to the Brake Pad Sensor

Dear Forum:

Thanks to jlevie, I obtained a schematic for the brake pad sensor circuit. That schematic indicated that the voltage going to the sensors should be about the same as battery voltage. I have a little less than 2 volts going to the front and rear sensors.

This would be a simple circuit, except for the part that runs thru the instrument panel. Does anyone have any thoughts about the lower than normal voltage in this circuit????

Steve L

[Mook613]

Dear Forum:

Thanks to jlevie, I obtained a schematic for the brake pad sensor circuit. That schematic indicated that the voltage going to the sensors should be about the same as battery voltage. I have a little less than 2 volts going to the front and rear sensors.

This would be a simple circuit, except for the part that runs thru the instrument panel. Does anyone have any thoughts about the lower than normal voltage in this circuit????

Steve L

Yes, check the resistor on the board!
It is a simple circuit as you noted, and if the solder holding that resistor has broken then the current cannot flow through it. It is an in-line resistor.