In most automotive repair shops, brake jobs are part of the daily bread-and-butter, straight forward repairs that involve parts replacement with little or no diagnosis. Occasionally, however, a job of this sort can throw you an unexpected curve. One such kink in the otherwise straight path of a brake job involves the BMW that comes in with the brake pad warning light on and, upon inspection, needs pad replacement at one or both ends.
After replacement and service of the rotors (if necessary) has been completed and the car is test driven, the pad warning is still glowing brightly in the instrument cluster. The wear sensors were not replaced because they looked fine -- no broken wires, no contact with the brake rotors. So why waste the customer’s money?
But how to reset that pesky warning light? All BMWs built after about 1980 have a pad wear warning light, and almost all of them work in the same way. A control unit built into the instrument cluster circuit board sends battery voltage out to the left front wear sensor, through that sensor, to-and-through the right rear sensor, and back to the cluster. If the control unit does not see a return voltage within approximately 1 volt of what it sent out, it turns on the warning light.
There is no reset for this system. The control unit will turn off the warning light when it sees the correct return voltage, which means a sensor wire that is broken or shorted to ground will turn on the warning -- but so will a high resistance in the circuit. If a sensor makes contact with a brake rotor and exposes even a tiny bit of the sensor wire, the resulting corrosion can cause enough of a voltage drop to turn on the warning light, even with no further contact with the rotor.
Disconnecting the sensors, in turn, and putting jumper wires across the pin contacts on the harness side (to see if the warning goes off) will usually determine which sensor is the culprit. If that doesn’t work, look for dirty harness connections and wires which may have rubbed against the wheel or brake caliper and damaged the insulation. These wires are usually a very light gauge and even a nick in the insulation can cause enough corrosion to set off the pad wear warning.
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