Brake Fluid Pressure Sensor/Switch: Description and Operation

NOTE: The Proportioning & Differential (P & D) Valve consists of a proportioning valve and differential switch.








REGULATING FLUID PRESSURE
The proportioning valve regulates the brake fluid pressure in the rear brake line, which is the same both on the master cylinder side and wheel cylinder side up to the break-point in the graph (F/S) but once the fluid pressure on the master cylinder side exceeds that point, that on the wheel cylinder side increases as shown by the graph.





OPERATION
Till the master cylinder side pressure (Pm) reaches (F/S), the spring force (F) is higher than the force caused pressure (F = Pm*S) and so Pm is transmitted to the wheel cylinder side as it is through the opening between the plunger and cup. As Pm rises higher than (F/S), F. exceeds F and the plunger moves to the right as shown in the figure to close the above opening.
When Pm rises even higher, the plunger moves to allow the opening. Then the wheel cylinder side pressure (Pw) increases and the opening is closed. Such operation cycle is repeated and Pm is transmitted to the wheel cylinder as Pw as shown by the above graph.





When the front brake line fails (when bleeding fluid from the brake line), the bypass line is opened to stop the proportioning valve so as to increase effect of the rear brake.

DIFFERENTIAL SWITCH
When a failure occurs in either the front or rear brake line, the differential switch, detecting a pressure difference in the brake line, turns ON to transmit the failure to the ABS control module. After the brake system is repaired, depressing the brake pedal will apply pressure to set the switch automatically (at OFF position).