Types of Auto Apply parking brakes used on the Workhorse Chassis
The parking brake on the P Series 16,500 lb., 17,000 lb., 18,000 lb. as well as the W Series 18,000 lb. and 24,000 lb. GVWR chassis incorporates a unique automatic apply feature. The parking brake is spring applied and hydraulic release. The automatic electric/hydraulic parking system controls the transmission/propeller shaft mounted parking brake. Workhorse utilizes two designs of automatic apply park brake systems, the J71 and J72. All P Series 16,500 lb., 17,000 lb. and 18,000 lb GVWR chassis were equipped with the J71 system through the 2004 model year. Starting in the 2005 model year the 17,000 lb. and 18,000 lb. chassis became equipped with the J72 system. All W Series 24,000 lb. GVWR chassis are equipped with the J72 system.
The J71 system has an internal expanding driveline parking brake, located on the back of the transmission. When engaged, it immobilizes the driveline. This brake is spring applied and pressure released. Meaning it takes hydraulic pressure to release the parking brake. This pressure is created by a hydraulic pump that is either located inside the passenger side frame rail, or under the hood of the RV.
Reason for Use
This parking brake is installed by the factory due to the rating of the parking pawl being exceeded by the vehicles GVWR. Because of this weight rating difference, the parking pawl is not installed in the transmission of these chassis, therefore there is no real "Park" position in these transmissions. So without the auto apply parking brake, "Park" is actually "Neutral".
This system consists of:
- Park brake motor switch (pressure maintenance switch)
- Park brake switch relay
- Electric over hydraulic pump
- Fluid reservoir (part of pump assembly)
- High-pressure hydraulic cable actuator
- Pressure solenoid valve
- Warning light switch
- Park/neutral position relay
- Park/neutral switch
- Manual push-pull switch
To release the auto park brake, move the shift lever on the steering column from the park position and push in the park brake switch push-button (if equipped). Note both conditions must occur for the brake to release (if equipped). Once these conditions exists, the circuit to the park/neutral switch opens in turn closing the park/neutral relay allowing current to flow to the solenoid valve. The solenoid valve closes and holds system pressure. The park/neutral relay also supplies current to the park brake pump motor switch (pressure maintenance Switch). The park brake motor Switch closes at pressures below 1200 psi turning on the pump to supply fluid pressure to the actuator. The actuator has a large spring inside that applies the parking brake. Fluid pressure overcomes spring tension and moves the piston in the actuator. This movement is transferred to the parking brake through the parking brake cable. When the fluid pressure reaches approximately 1600 psi the park brake motor Switch opens and the pump shuts off. The solenoid valve is holding.pressure. The park brake motor switch opens and closes depending on system pressure. Putting the shift lever back to park, turning off the ignition, or pulling the push/pull Switch, drops 12vdc to the pump motor switch, and de-energizes the solenoid valve. This means the fluid is allowed to dump back into the reservoir. As the pressure decreases, the spring tension in the actuator, it moves the piston and applies the park brake via the parking brake cable. The "AUTO PARK" WARNING LAMP turns on when the system pressure is less than 450 psi or when the electric/hydraulic pump is running due to the park brake motor Switch being closed. The lamp will flash if pressures are fluctuating.
The PARKING BRAKE LlGHT SWITCH
Is located on the back of the actuator assembly. The Switch controls the power side of the "AUTO PARK" lamp. The lamp Switch closes when system is below 450 PSI turning on the light when the ignitions on.
The PARK BRAKE PULL SWITCH
Is normally mounted on the dash panel. This is a manual activation switch for the park brake. This switch is normally closed, pushed in. When pulled, this opens the circuit to the park/neutral relay and in turn the solenoid valve allowing fluid pressure to return to the reservoir applying the park brake. The body manufacturer determines final location of this switch.
The PARKING BRAKE PUMP ASSEMBLY
Is located in a component box on the passengers' side of the vehicle on 1999 and 2000 model year chassis. The component box is on the inside of the right frame rail behind the transmission. On 2001 through 2004 P Series models the pump assembly was moved to the front driver's side radiator core support. The pump assembly consists mainly of an electric pump and fluid reservoir. The pump provides fluid pressure for the system. A pressure relief valve in the pump limits system pressure to 1800 psi.
The PARK BRAKE MOTOR SWITCH
(pressure maintenance switch) mounts to the parking brake pump assembly housing. It is a hydraulic pressure switch that operates within a certain pressure range turning the pump motor on and off. The park brake motor switch closes when system pressure is below 1200 psi and opens when system pressure reaches approximately 1600 psi. The park brake motor switch applies B+ to the coil side (control side) of the park brake motor relay.
The PARKING BRAKE SOLENOID VALVE
switch is a solenoid and valve assembly located in the component box in the underside of the vehicle on 1999 and 2000 model year chassis. On 2001 through 2004 models on the pump assembly, including the solenoid valve was moved to the front driver's side radiator core support. The solenoid controls when fluid can return to the pump reservoir. When the parking brake is released, the valve closes to hold pressure in the system. When the parking brake is applied, the valve opens to allow fluid to return to the pump reservoir.
The PARK BRAKE PUMP MOTOR RELAY
Is located in the component box underneath the vehicle on the inside of the passenger frame rail in 1999 and 2000 model year chassis. On 2001 and 2002 models the pump assembly, including the relay, was moved to the front driver's side of the radiator core support. It receives B+ from the park brake motor switch and acts as the automatic control circuit for the high voltage current required to run the pump motor. When the park brake motor switch supplies B+ to the relay switch, the contacts close to complete the feed circuit to the pump motor.
Is located on the inside of the right frame rail in the component box in 1999 and 2000, underneath the vehicle, The location of the actuator did not change in 2001 through 2004 but the other components that were located in the component box were relocated. The actuator is a spring loaded device that operates the parking brake cable. A large spring inside the actuator applies the parking brake. When hydraulic fluid pressure is applied against the actuator piston it overcomes the spring tension and pushes the brake cable to release the parking brake.
The PARK/NEUTRAL POSITION SWITCH
Is located on the left side of the transmission near the middle. When the column shifter is in the park position the switch is closed to supply current to the park/neutral relay opening the circuit to the park brake motor switch and the solenoid valve, When the column shifter moves out of the park position the switch opens. Once the circuit is open this allows the park/neutral relay to close providing B+ to the park brake motor switch and the solenoid valve.
The PARK/NEU1RAL POSITION SWITCH RELAY
Is located above the engine, mounted to the right hand side of the driver's island on 1999 and 2000 chassis; refer to the service manual for location on 2001 and newer chassis. It is a normally closed switch that is open when the park/neutral position switch is in the park position supplying B+ current to the relay. When the column shifter moves out of the park position the park/neutral position switch opens. Once the circuit is open this allows the park/neutral relay to close routing B+ current to the park brake motor switch and the solenoid valve.