Brake Fluid Pressure Sensor/Switch: Description and Operation

COMBINED PRESSURE WARNING SWITCH

The pump generates a pressure in the system which acts on the tappet (1). The tappet moves against a spring (2). At a pressure of 130 bar the warning switch (3) is actuated.

First, the brake warning light will be extinguished followed a short time after by the ABS warning light. The tappet (1) is moved further by pressure and opens the pressure switch (4) at a pressure of 180 bar to open the pump relay coil circuit so stopping the pump motor.

If, after a number of brake applications or normal leak down, the system pressure falls below 140 bar the pressure switch (5) closes and the pump will operate till the cutoff pressure of 180 bar is reached. In this way, the system pressure is maintained within the range of 140 to 180 bar. If due to a hydraulic failure or a electrical failure, a pressure drop below 105 bar occurs, the brake warning switch (4) contacts close. The brake warning lamp lights, simultaneously the ABS warning switch (3) contacts open and the circuit between terminals 9 and 10 of the control module is broken. The electronic control unit is partially inhibited, ABS control for the front wheel brakes is switched off and the ABS warning light is illuminated