The auto apply park brake control system receives power through the 10A AUTO PARK fuse 34, when the ignition is in RUN and START. The park brake switch closes and supplies power to the pressure valve solenoid and the pump motor switch when the gear shift selector is moved out of the park position. When the pressure valve solenoid is energized, the hydraulic pressure in the system is held to build up and overcome the spring tension which applied the parking brake. At the same time, the pump motor switch monitors the pressure in the system. When the pressure is below 7 619 kPa (1,105 psi), the switch closes to energize the pump motor relay and to light the AUTO PARK indicator. Once the pressure reaches 10 756  kPa (1,560 psi), the switch opens to de-energize the pump motor relay. The pump motor operates on the 60A AUTO PARK fuse 54.

The system also has a low pressure switch which opens at approximately 3 792 kPa (550 psi) and closes between 2 413 kPa (350 psi), and 3 103 kPa (450 psi). This lights the AUTO PARK indicator when the system is unpressurized (brake applied).

The auto apply parking brake system controls the propeller shaft-mounted parking brake. It consists of the following components:

Before attempting to diagnose the electric/auto park brake system, you must have a good understanding of electrical and hydraulic system basics. Without this basic knowledge, you will find it difficult to diagnose this system.

Some electrical basics, basic troubleshooting procedures and hints, and the use of circuit testing tools are discussed in Electrical Diagnosis.

The pump motor switch is mounted on the parking brake pump. It is a hydraulic pressure switch that operates within a certain pressure range turning the pump motor on and off. This generates the pressure needed to release the brake initially, and serves as a pressure maintenance device to keep pressure in the required operating range while driving. The switch closes when the system pressure is below 8 300 kPa (1,200 psi) and opens when the system pressure reaches approximately 11 000 kPa (1,600 psi). The switch operates the pump motor by applying B+ to the coil (control side) of the relay switch. This also applies B+ to the AUTO PARK indicator lamp which lights whenever the pump relay is energized.

To have B+ present at the pump motor switch two other switches must be closed:

The parking brake pump assembly is located in a component box on the passenger's side of the vehicle. The component box is on the inside of the right frame rail behind the transmission. It consists of an electric pump and fluid reservoir. The pump provides fluid pressure to a hydraulic cylinder that releases the brake. A pressure relief valve in the pump limits system pressure to 12 400  kPa (1,800 psi).

The parking brake solenoid valve is located on the pump body. The valve regulates fluid return to the pump reservoir. The parking brake is released by turning on the hydraulic pump to generate pressure, and closing the solenoid valve to hold that pressure in the system. The parking brake is applied by turning off the power, which stops pump operation and opens the valve to allow the fluid to return to the pump reservoir.

CAUTION:: Do not disassemble the actuator. Always service the actuator as a unit. The actuator contains a large spring under tension. Disassembling the actuator allows the spring to expand with great force, which can result in personal injury.

The actuator is located underneath the vehicle inside the left side frame rail, behind the transmission. The actuator is a spring-loaded device that operates the parking brake cable. A large spring inside the actuator applies the parking brake. The brake is released by applying hydraulic fluid pressure against a piston. When that pressure is great enough, the piston overcomes spring tension and pushes against the actuator to release the parking brake.

The park switch is located inside the transmission range (TR) selector. This switch is normally closed, open in PARK. The switch applies power to the park brake release mechanism when the TR selector is in any range other than PARK. By placing the range selector in PARK, the switch removes voltage from the release system resulting in application of the parking brake.

The parking brake pump motor relay is located in the component box underneath the vehicle. The relay coil receives B+ from the pump motor switch closing the contacts to complete the feed circuit to the pump motor. When the relay coil is energized, the AUTO PARK indicator is lighted.

The parking brake pressure indicator switch is a hydraulic ON/OFF switch located on the hydraulic pump. The switch controls B+ to the AUTO PARK lamp. This switch closes when the system pressure is below 3 100 kPa (450 psi) and turns on the light when the ignition is ON and the pressure is low, mainly when the brake is applied and there is no hydraulic pressure.

This is a switch mounted beside the release cylinder on the end of the actuator housing. This switch opens in an actuator overtravel condition causing the alarm relay (normally closed switch) to de-energize and complete ground for the park brake alarm and the diode network. That sounds the alarm and lights the BRAKE indicator lamp. Except in case of electrical malfunction, this alarm will be active only when the parking brake is applied and indicates the need for adjustment of the cable or service of the parking brake linings.