J72 Parking Brake Operation and Components

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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.

The J72 (option code) park brake system is made up of two major separate components or systems, the actuator and the brake assembly. Each of the components is serviced as an assembly.

The J72 system incorporates a unique full-circle parking brake. The automatic electric/hydraulic parking system controls the transmission/propeller shaft mounted parking brake.


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. 

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