Pressure Differential Valve & Warning Lamp Switch

The pressure differential valve and warning lamp switch is part of combination valve to alert driver in event pressure drops in one of brake hydraulic systems. See Figure . The brake hydraulic system on this vehicle is vertically split, which means brake hydraulic system is equally split between front and rear brakes, with both front brakes on one hydraulic system and both rear brakes on the other.

This differs from brake hydraulic system on a front wheel drive car which is diagonally split. On a brake hydraulic system which is diagonally split, left front and right rear brake is on one hydraulic system and right front brake and left rear brake on other. The primary and secondary hydraulic circuit from master cylinder is separated into a vertically split hydraulic circuit by internal passages of combination valve. See Figure .

The pressure differential valve, which is internal to combination valve is used to monitor pressure in brake hydraulic system. The pressure differential valve functions by simultaneously sensing that hydraulic pressure in primary and secondary hydraulic circuit from master cylinder and front and rear brake hydraulic circuits is equal.

If pressure in brake hydraulic circuits is equal when brake pedal is depressed, differential valve will stay centered in combination valve and Red brake warning lamp will not be turned on. See Figure . If pressure in one of brake hydraulic circuits drops when brake pedal is depressed, drop in pressure will cause pressure differential valve to move off center in combination valve. As pressure differential valve moves off center it pushes plunger up into brake warning lamp switch. When plunger is pushed into brake warning lamp switch, a ground for Red brake warning light circuit is completed. Creating ground for Red brake warning lamp circuit will turn on Red brake warning lamp located in instrument cluster.

If Red Brake Warning Lamp is turned on, brake system requires immediate service. Since brake hydraulic systems are split front to rear, vehicle will retain half of its stopping capability in event of a failure in either half of system.

NOTE: The brake warning light switch is latching type. It will automatically center itself only after repair is made and brake pedal is depressed.

The instrument panel bulb in Red Brake Warning Light is automatically turned on each time ignition switch is turned to start position as a bulb check. The red brake warning lamp is also turned on when parking brake is set.