Brake Light Switch: Description and Operation

Brake Light Switch







Similar to previous models an electronic brake-light switch (Hall-sensor)is used. It is powered from KL R ON. Its signal is provided as an analog input.

The brake switch has a level of 0 volts in the non-actuated state and approx.8-9 volts when the brake is depressed (dependent on the system voltage).

Fault Detection

There are two different fault scenarios for the brake-light switch: short circuit to ground and open circuit.

Short circuits to ground can only be safely detected with terminal R OFF. If a low level is detected instead of the anticipated high, there is a short circuit and the brake-light switch is determined to be faulty. This self-diagnosis is only performed after entry into power-down mode so long as no light function is activated while the K-CAN-S bus is inactive.

A possible open circuit is not electrically distinguishable from an actuated brake. With terminal 15 ON and information on brake pressure (DSC PT-CAN ZGM K-CAN-S LM) but no detected brake-light-switch actuation, the brake-light switch is also condemned as faulty. The fault entry "Brake-light switch faulty" is made in both cases.

Limp-Home Functions

The function of the brake light is assured even when the LM is in limp-home mode. Internal circuitry connects the brake-light-switch output directly to the lamp drivers. The function rules out the possibility of PWM operation.