The parking brake is not designed for use in place of the service brakes. Apply the parking brake only after the vehicle is brought to a complete stop, except in an emergency.
The three types of parking brake systems used are listed below.
The parking brake control module (PBCM) contains the system microprocessor. A display on the side of the PBCM provides system operation and diagnostic information. During normal operation, a percentage of system operating pressure is displayed. A reading of 10 or less indicates the parking brake is applied. A reading of 90 or greater indicates the parking brake is released. A diagnostic port on the PBCM allows for displaying DTCs and DTC information. An LED on the PBCM illuminates when the PBCM is commanding the parking brake pump motor relay ON.
The parking brake pump assembly consists of an electric pump and a fluid reservoir. The pump supplies hydraulic pressure needed for system operation. A pressure relief valve in the pump limits system pressure to 12,400 kPa (1800 psi).
The pressure sensor mounts in the housing of the parking brake pump assembly. The sensor is a 3-wire potentiometer which operates from a 5-volt reference. The signal voltage from the pressure sensor is low, around 0.5 volts when there is no pressure in the system, for instance, when the parking brake is applied. The sensor signal voltage increases as system pressure increases. At 11,000 kPa (1600 psi), the pressure signal to the PBCM is about 3.5 volts.
The parking brake solenoid valve controls the flow of hydraulic fluid within the system in order to allow the application or release of the parking brake. The valve energizes, which closes the value, when the parking brake is released in order to allow the pump to build pressure in the system. The valve opens when the parking brake is applied in order to allow hydraulic fluid to return to the reservoir.
The parking brake actuator is a spring-loaded device which applies the parking brake. When hydraulic pressure from the pump overcomes this spring tension in the actuator, the actuator piston moves and releases the parking brake.
Important: It is not necessary to cut the cable in order to release the parking brake.
The parking brake cable transfers the movement of the actuator to the bellcrank in order to move the parking brake shoes. The actuator end of the parking brake cable has a 2-piece fitting which allows the parking brake to be released in emergencies or for service if the hydraulic system fails.
The travel switch is a normally-closed switch which opens if the actuator piston travel is excessive during parking brake application. If the travel switch is detected as being open at any time, a DTC is set. Adjustment of the parking brake often corrects the conditions for setting travel switch DTCs.
A dash mounted parking brake switch opens when pulled out, causing the parking brake to apply, and closes when pushed in.
The parking brake relay energizes in order to close the parking brake solenoid valve and inform the PBCM that the parking brake switch is closed. The PBCM then turns ON the pump motor in order to release the parking brake. Not all vehicles equipped with an electro-hydraulic parking brake utilize a parking brake relay. On certain systems, the parking brake solenoid valve is powered directly from the parking brake switch.
The PBCM energizes the parking brake pump motor relay in order to turn ON the pump motor.
The PBCM illuminates the APPLY PARK BRAKE indicator when the ignition is OFF and the parking brake is released. The indicator remains ON until the parking brake is applied.
The PBCM illuminates the SERVICE PARK BRAKE indicator in order to inform the vehicle operator when a parking brake related DTC sets. The PBCM supplies voltage to the lamp in order to illuminate the indicator.
Ground is applied to the PARK BRAKE indicator lamp in order to illuminate the indicator when the parking brake is applied and the ignition is ON.
When the vehicle operator pushes the parking brake switch IN, the parking brake switch closes, energizing the parking brake relay. The voltage on the parking brake pressure valve solenoid supply voltage circuit energizes the parking brake pressure valve solenoid, in order to block fluid flow back to the reservoir, and informs the PBCM that the parking brake switch is in the "released" position. The PBCM then sends voltage to the parking brake pump motor relay via the parking brake motor relay control circuit in order to turn ON the pump motor. As the pump motor runs, pressure builds in the actuator. System pressure is monitored by the parking brake pressure sensor. Once pressure in the actuator overcomes the actuator spring pressure, the actuator piston moves and releases the parking brake. When the pressure sensor detects system pressure is high enough to hold the parking brake released, the PBCM de-energizes the parking brake pump motor relay in order to turn OFF the pump motor.
On vehicles equipped with an Allison automatic transmission and a column mounted gear selector (MUQ), the parking brake applies automatically when the gear selector is placed in park or neutral. The PBCM receives inputs from the park/neutral position (PNP) switch and the park brake switch in order to properly apply and release the parking brake. The parking brake switch automatically opens in park or neutral.
The electro-hydraulic parking brake is applied by spring pressure inside the actuator assembly. When the parking brake switch is pulled OUT, the parking brake pressure valve solenoid de-energizes, causing the valve to open, allowing all fluid to return to the reservoir. Spring pressure moves the actuator piston in order to apply the parking brake.
The manually operated parking brake system is applied using the parking brake lever. This applies pulling force to the cable and the parking brake bellcrank, rotating the cam assembly in order to apply the parking brake.
The air brake system parking brake is applied by pulling the dash-mounted push-pull parking brake valve button out. This exhausts the spring parking brake chamber hold-off air through the spring brake release valve.
