Braking Sensor/Switch: Description and Operation

DESCRIPTION
The pedal sensor is located on the power brake booster overload relay. The pedal sensor sends a signal to the control module telling it what position the brake pedal is in (i.e. how hard the driver is braking).





OPERATION
The control module uses the signal to indicate if there is a fault in the hydraulic system if the pedal travel is too great. The control module also uses the signal to decide when to start and stop the hydraulic system pump so as not to affect the brake pedal position too much when the solenoid valves are working. The pedal sensor consists of six resistors connected in series and a seventh open position read off by a sliding contact, which means that the sensor resistance can have seven different values. With the brake pedal fully released (position 1), the sensor resistance is at minimum. With the pedal pressed right down to the floor (position 7), the resistance is infinite. Position 7 tells the control module if the pedal is travailing too far down, which means there may be a hydraulic fault in the system. When the brake pedal is depressed and ABS cuts in, ABS makes the brake pedal drop slightly, which the pedal sensor records. The hydraulic pump then cuts in to keep the pedal in the right position, and keeps working until the pedal sensor notes that the brake pedal has been pushed back up a step. This is to stop the pedal being pushed back up too much, which would make the pedal movement uncomfortable for the driver. If the control unit finds that the pedal sensor is in position 7 while ABS is on, it switches the hydraulic pump on and runs it for approx. 0.7 seconds. If this does not push the brake pedal back up at least one step (e.g. because there is a leak in the system) the hydraulic pump and ABS are switched off. As power brake booster overload relay tolerances vary as made, they are color-coded. To ensure that the pedal sensor is in the right position relative to the power brake booster overload relay, we use different length spacers between sensor and servo. The spacers are color-coded the same as the power brake booster overload relays.