Cherokee 4WD L6-4.0L VIN S (1998)
During normal braking, the HCU solenoid valves and pump are not activated. The master cylinder and power booster operate the same as a vehicle
without an ABS brake system.
During antilock braking, solenoid valve pressure modulation occurs in three stages, pressure increase, pressure hold, and pressure decrease. The
valves are all contained in the valve body portion of the HCU.
Pressure Decrease
The outlet valve is opened and the inlet valve is closed during the pressure decrease cycle.
A pressure decrease cycle is initiated when speed sensor signals indicate high wheel slip at one or more wheels. At this point, the CAB closes the
inlet then opens the outlet valve, which also opens the return circuit to the accumulators. Fluid pressure is allowed to bleed off (decrease) as
needed to prevent wheel lock.
Once the period of high wheel slip has ended, the CAB closes the outlet valve and begins a pressure increase or hold cycle as needed.
Pressure Hold
Both solenoid valves are closed in the pressure hold cycle. Fluid apply pressure in the control channel is maintained at a constant rate. The CAB
maintains the hold cycle until sensor inputs indicate a pressure change is necessary.
Pressure Increase
The inlet valve is open and the outlet valve is closed during the pressure increase cycle. The pressure increase cycle is used to counteract unequal
wheel speeds. This cycle controls re-application of fluid apply pressure due to changing road surfaces or wheel speed.
