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Auto Park Brake Information

 AUTOMATIC APPLY PARKING BRAKE

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 Series16, 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 either the J72 system. All W Series 24,000 lb. GVWR chassis are equipped with the J72 system. Both systems components and operation are outlined in the following pages.

 

J71 Parking Brake Operation and Components

The parking brake on the 16,500#, 17,000#, and 18,000# GVWR chassis incorporates a unique automatic apply feature with an internal expanding parking brake.  The parking brake is spring applied and hydraulic released.  The electric/auto parking system controls the propeller shaft-mounted parking brake.  It consists of a park brake motor switch (pressure maintenance switch), park brake switch relay, electric/hydraulic pump, reservoir, high-pressure actuator, solenoid valve, light switch, park/neutral relay, park/neutral switch, and manual push/pull switch.

To release the electric/auto park brake, move the shift lever on the steering column from the park position.  Once the gear selector has been moved from the park position this opens the circuit to the park/neutral switch, 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 de-energizes the solenoid valve to dump fluid back into the reservoir.  As the pressure decreases, the spring tension in the actuator moves the piston and applies the park brake through the movement of 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 at partial release pressures.


The PARKING BRAKE LIGHT SWITCH is located on the back of the actuator assembly.  The switch controls the groundside of the “AUTO PARK” lamp.  The lamp switch closes when system is below 450 psI, turning on the light when the ignition’s on.


The PARKING BRAKE PUMP ASSEMBLY is located on a component box on the passenger 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 and 2002 models, the pump assembly was moved to the front driver’s side of the 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 and 2002 models, the pump assembly, including the solenoid valve, was moved to the front driver’s side of the 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 ACTUATOR 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 or 2002, 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+ current to the park brake motor switch and the solenoid valve.


The PARK BRAKE PULL SWITCH is mounted on the instrument 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 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.

 

J72 Parking Brake Operation and Components

The J72 system incorporates a unique full-circle parking brake. The automatic electric/hydraulic parking system controls the transmission/propeller shaft mounted parking brake. This system consists of a pump motor relay, vehicle ignition relay, electric/hydraulic pump, reservoir, proportional relief solenoid valve, pressure transducer, and an electronic control module (ECM). The ECM monitors seven inputs from the vehicle and controls the pump motor relay, auto-park light, park brake warning light, buzzer, proportional valve, and pressure transducer.


To release the parking brake, push in the park brake switch pull-button and move the shift lever on the steering column from the park position or select “D”, “N” or “R” on the push

button shift control (optional AllisonTM transmission control). Note that both situations must occur for the brake to release. Once the ECM receives the signals from the transmission and button contacts, the ECM closes the solenoid valve and closes the pump/motor relay allowing current to flow to the motor. The pump/motor then supplies fluid to the brake assembly. The brake assembly includes a friction disc clamped by a piston through force of an internal spring. The fluid pressure acts on the piston overcoming the spring tension that clamps the friction disc, releasing the brake. The pump motor will shut off when the transducer reads the fluid pressure of approximately 1400 psi. The solenoid valve holds pressure in the system while the brake is in a released state. The ECM monitors system pressure from the pressure transducer and energizes the pump motor when system pressure drops below approximately 1200 psi.


Putting the shift lever back to park, selecting “P” on the push button shift control, turning off the ignition, or pulling the push/pull switch prompts the ECM to de-energize the solenoid valve dumping fluid back into the reservoir. As the pressure decreases, the spring force clamps the piston back against the friction disc and applies the park brake.


The “AUTO PARK” WARNING LAMP turns on when the system pressure is less than 900 psi and the ECM receives the signal from the transmission.


The PARKING BRAKE WARNING LAMP turns on when system is below 900 PSI and when the parking brake pull-button is pulled out.


The PARK BRAKE PULL SWITCH is mounted on the instrument panel. This is a manual activation switch for the park brake. This switch is normally closed, pushed in. The body manufacturer determines final location of this switch.


The PARK BRAKE PUMP MOTOR RELAY is located above the fuse box on the front driver’s side of the radiator core support. It receives ground from the ECM and acts as the automatic control circuit for the high voltage current required to run the pump motor. When the ECM supplies ground to the relay switch, the contacts close to complete the feed circuit to the pump motor.


The IGNITION RELAY is located next to the park brake pump motor relay on the front driver’s side of the radiator core support. It monitors the vehicle’s ignition switch to turn on/off the supply of the fifteen and forty-five amp fused-lines that the ECM and electric motor use. If the relay was not present, the ECM would be continually powered since it would be directly connected to the batteries. 


The PUMP ASSEMBLY - This unit mounts on the driver’s side of the radiator core support and is self-contained/environmentally sealed. The AUTO/ELECTRIC/HYDRAULIC ACTUATOR controls the pressure in the park brake system to either set or release the park brake. The actuator consists of a mounting bracket and the following components:


The ELECTRONIC CONTROL MODULE or ECM is an analog logic device that monitors the various vehicle conditions then controls the actuator accordingly. It is the “black box” mounted underneath the actuator mounting bracket. It connects to the vehicle via a fourteen pin connector, to the proportional solenoid valve via a two pin connector, and a pressure transducer via a three pin connector. The ECM is powered by two different sources. First, it receives power primarily through a fifteen amp fuse. Secondly, the ECM receives power off of the forty-five amp fused-line that the pump motor relay also uses.


The PUMP/MOTOR ASSEMBLY consists of an electric motor, geared pump, aluminum port block, and a semi-transparent fluid reservoir. The port block holds the proportional solenoid valve and pressure transducer. The pump provides fluid pressure for the system. A pressure relief valve in the port block limits system pressure to around 1700 psi.


The PROPORTIONAL SOLENOID VALVE controls when fluid can return to the pump reservoir. When the parking brake is released, the valve is closed to hold pressure in the system. When the parking brake is applied, the valve opens to allow fluid to return to the pump reservoir. If the vehicle is at a speed greater than five miles per hour (5mph) and the pull button is pulled, the valve will remain open for a few moments and then release the pressure slowly for a gradual apply of the brake. NOTE: The park brake is NOT designed for dynamic stops; if a dynamic stop occurs, see the nearest service center for brake inspection.


The PRESSURE TRANSDUCER monitors the pressure of the system and relays that information back to the ECM. The critical pressure values are the cut-in and out for the motor which are 1200 psi and 1400 psi, respectively.