RAM 2500 Truck 4WD V10-8.0L VIN W (1997)
Hydraulic Control Unit: Description and Operation
Rear Antilock Valve Circuit Operation
GENERAL INFORMATION
If the Controller Antilock Brake (CAB) senses that rear wheel speed deceleration is excessive, it will energize a isolation solenoid by providing
battery voltage to the solenoid. This prevents a further increase of driver induced brake pressure to the rear wheels. If this initial action is not
enough to prevent rear wheel lock-up, the CAB will momentarily energize a dump solenoid (the CAB energizes the dump solenoid by providing
battery voltage to the solenoid). This opens the dump valve to vent a small amount of isolated rear brake pressure to an accumulator. The action of
fluid moving to the accumulator reduces the isolated brake pressure at the wheel cylinders. The dump (pressure venting) cycle is limited to very
short time periods (milliseconds). The CAB will pulse the dump valve until rear wheel deceleration matches the vehicle deceleration rate or the
desired slip rate programmed into the CAB. The system will switch to normal braking once wheel locking tendencies are no longer present.
A predetermined maximum number of consecutive dump cycles can be performed during any one antilock stop. If excessive dump cycles occur, a
Diagnostic Trouble Code (DTC) will be set and stored in the CAB memory. If during an antilock stop, the driver releases the brake pedal, the
reset switch contacts will open. This signal to the CAB is an indication that pressure has equalized across the Rear Wheel Antilock (RWAL)
valve. The CAB will then reset the dump cycle counter in anticipation of the next antilock stop. Additionally, any fluid stored in the accumulator
will force its way past the dump valve, back into the hydraulic circuit and return to the master cylinder.
A fuse internal to the CAB, provides a fail-safe device which prevents unwanted control over the isolation and dump solenoids. The fuse is in
series with the isolation and dump solenoids output circuits. If the internal fuse is open, the CAB cannot provide voltage to energize either
solenoid and antilock stops are prevented. If the fuse is open, the braking system will operate normally but without antilock control over rear brake
pressure.
CIRCUIT OPERATION
RWAL Valve
The rear wheel anti-lock (RWAL) valve contains an isolation solenoid, a dump solenoid and a reset switch. Each is powered by the Controller
Anti Lock Brake (CAB) on separate circuits.
Circuit B108 from cavity 8 of the CAB feeds the dump solenoid. The isolation solenoid is powered on circuit B101 from cavity 1 of the CAB.
Circuit Z8 provides ground for both solenoids.
Circuit Bill from cavity 11 of the CAB connects to the reset switch. The case grounded RWAL valve provides ground for the reset switch.
Circuit B120 from the ABS pump relay supplies voltage for the isolation and dump solenoids in the Rear Wheel Antilock (RWAL) valve. The
Controller Antilock Brake (CAB) activates the solenoids by providing separate ground paths for each.
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Circuit B254 connects to cavity 26 of the CAB and provides ground for the rear dump solenoid.
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Circuit B252 connects to cavity 28 of the CAB and provides ground for the rear isolation solenoid.
The RWAL valve has one reset switch that provides an input to the CAB. Circuit B19 from the reset switch connects to CAB cavity 19.
