Here is info I found on it.
J72 AUTOMATIC APPLY PARKING BRAKE
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"Â, "N"Â or "R"Â on the push
button shift control (optional AllisonTM transmission control). Note 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"Â 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"Â 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.
J72 ELECTRICAL-HYDRAULIC ACTUATOR
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"Â 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. Below is a detail of the pin connections of the ECM as well as the
solenoid valve and pressure transducer.
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.
