Brake pad sensors

Copperman05 · Jan 10, 2011

Hi All,

The brake pad warning light came on on my S2 recently during our recent cold weather snap. It has now gone out again. Is this usual? A faulty sensor? I would assume that once the sensors have made contact and turn the warning light on they would normally stay on until replaced?

Your thoughts/experiences are appreciated.

Edd

scam75 · Jan 11, 2011

Correct, it will be a faulty sensor. I've seen quite a few break right at the plug as they get old. Could be one on it's last legs. Quite common.

You can make up a test sensor by just cutting an old one and joining the two wires together. You can then plug this into each corner to see if your fault goes away.

Alternatively tie a not in all four of them and get new pads when they start grinding!

Stuart

JamesO · Jan 11, 2011

I was under the impression that the fault light comes on when the circuit breaks as the disk wears through the sensor wires, it does seem to be just a loop of wire set in a plastic block.

Mine are currently unplugged and the light is on. Not looked at mine any further yet as its pretty much last on the list of things to fix, but I would have thought if it was looking for a short to ground then only one wire would be used not a pair. Unless a short or open circuit will put the light on, not sure its that clever though.

So perhaps yours are a bit corroded causing an intermittant fault, or one of the plugs is loose. Or you could just replace all 4..

scam75 · Jan 11, 2011

It is only one wire that doubles back to the plug. The light comes on if the circuit is broken therefore unplugged sensors will bring the light on the same as bridging the terminals puts it off again.

When the pads wear out the wire is broken by the disc and the light comes on.

Stuart

robwright · Jan 11, 2011

I think but will have to check that it actually wears through the insulation of the wire to create a circuit. Thus when you remove the sensor from the disc the light should extinguish. Although it is not that simple. The ECU controls when the light come on under given criteria. I.e I think if you stop the car for less than 3 minutes it will not come back on again. I experienced this recently with mine. Again not 100% sure will have to check the diagram.

scam75 · Jan 12, 2011

When a wire breaks the light comes on regardless of what the disc is doing to the sensor. This would not be the case if it required the disc to complete the circuit. Plus bridging the terminals makes it think the pads are fine and the light will go off.

There is no more to it, it's a simple system from what I can see. Circuit complete, sensor light off, circuit broken, sensor light on. I've sorted a few sensor issues in the last 6 months!

Stuart

Copperman05 · Jan 12, 2011

So does this mean my sensors are broken or my brake pads need replacing?

Edd

Diver944 · Jan 12, 2011

ORIGINAL: Copperman05

So does this mean my sensors are broken or my brake pads need replacing?

Edd

You'll need to actually look at them to determine this Edd. If you have more than 2mm of pad left then its probably a loose sensor

robwright · Jan 12, 2011

ORIGINAL: scam75

When a wire breaks the light comes on regardless of what the disc is doing to the sensor. This would not be the case if it required the disc to complete the circuit. Plus bridging the terminals makes it think the pads are fine and the light will go off.

There is no more to it, it's a simple system from what I can see. Circuit complete, sensor light off, circuit broken, sensor light on. I've sorted a few sensor issues in the last 6 months!

Stuart

Stuart I have just dug out the diagrams and although sketchy they would seem to back up what you said. Each sensor is connected in series. From my own experience what I don't understand is when I changed my front pads after the light came on I reinstaed the sensors as they weren't very worn. The light never came on, which you would presume it would had the sensor circuit been broken. Just a thought that came to me now. Could it be that once the sensor wire insulation has been worn it grounds via the disc thus having the same effect as making the circuit open circuit? I.e it follows the easiest or first route it sees to ground via the disc before it makes it back to the instrument cluster.

scam75 · Jan 12, 2011

It makes sense Rob and certainly is plausible. The wire would have to go through that stage before severing completely and could give the impression of open circuit as you describe. That may be what happened to you.

Or you might just have had a bad connection somewhere!

Stuart