Cherokee 4WD L6-242 4.0L VIN M FI (1988)
Brake Proportioning/Combination Valve: Description and Operation
Fig. 13 Proportioning Valve/Pressure Differential Switch
PROPORTIONING VALVE/PRESSURE DIFFERENTIAL SWITCH
The proportioning valve/pressure differential switch is mounted next to the master cylinder/power booster (fig. 13) and operates normally during
standard braking mode.
The valve consists of a one-piece housing containing a rear brake proportioning valve and a front pressure differential switch. Hydraulic pressure
from the master cylinder is applied through opposite ends of the valve, providing balancing forces which keep the piston centered in the cylinder
bore. The pressure differential switch is in the "off" position as long as the piston remains centered. If pressure loss in either the front or rear brake
system occurs, a pressure differential of 70-300 PSI will cause the piston to shuttle toward the low pressure side of the valve. As the piston moves,
ramps on the piston force the switch contact plunger upward closing the switch contacts. This action completes the electrical circuit between the
"red" brake light on the instrument panel and ground.
The pressure differential switch is activated by pressure differential in both braking modes. The pressure differential switch is a single pole, single
throw switch. The continuity through the electrical circuit (including the two pin connector) is checked each time the ignition switch is turned to
the start position by illuminating the red light on the instrument panel.
During normal operation the valves in the modulator are positioned to allow proportioned rear brake pressure to the rear brakes. When the system
goes into the anti-lock mode, the proportioning valve and master cylinder are isolated to enable pressure modulation of the brake system.
