Automatic Parking Brake: Motorhome
Vehicle uses 2 types of automatic parking brakes. Hydraulic automatic apply system and electric automatic apply system. Both systems use a spring loaded actuator to set the parking brake when shift lever is placed in Park. Hydraulic automatic apply system uses power steering pump pressure to release parking brake. Electric automatic apply system uses an electric-hydraulic pump and reservoir to release the parking brake. Electric-hydraulic pump and reservoir are located in a component box inside the right frame rail behind transmission.
On the hydraulic automatic apply parking brake system, when shift lever is in Park, engine running, and manual parking brake foot lever in released position, fluid flows from steering gear to port SR on relay valve and out to port TW to the control valve supply port SC. See Fig 1. Once the system is charged, pressure should be 130-150 psi (9.1-10.5 kg/cm2 ). Any excess fluid will be discharged through port "R" and back to the pump. The supply port or charge port SC is blocked off due to control valve position in Park mode.
Any pressure previously built-up in control valve flows through control valve out port EC back through the relay valve, triggering release of pressure from port "D" of relay valve through port ER to reservoir. See Fig 1. This allows the spring controlled actuator to apply parking brake. The spring will apply brake by travelling as far as the brake adjustment demand requires to balance brake apply and spring force.
When valve is released from Park position, fluid charge at shift control valve port SC is diverted to port DC. The shift control port EC is blocked off. Fluid charge at relay valve port SR is diverted to port "D", pressurizing parking brake system and actuator. See Fig 1. Fluid pressure working against the spring pressure in actuator releases parking brake.
The manual foot lever should still be applied whenever vehicle is shifted into Park. This will alert operator of need for adjustment in the parking brake system.
On the electric automatic apply parking brake system, when shift lever is moved from Park, movement is transferred from transmission shift linkage to cam actuated switch. See Fig 2. Cam actuated switch closes allowing current flow to solenoid valve and pressure maintenance switch. Solenoid valve closes and holds system pressure. If system pressure is less than 1200 psi (83 kg/cm2 ) pressure maintenance switch closes and turns on pump motor. When fluid pressure reaches approximately 1600 psi (110 kg/cm2 ) pressure maintenance switch opens and shuts off the pump. Fluid pressure overcomes spring tension and moves the actuator piston. Movement is transferred to the parking brake through a cable and the parking brake is released. Moving shift lever to Park or turning the ignition off de-energizes parking brake solenoid and dumps fluid back into the reservoir. See Fig 2. As pressure decreases, spring in the actuator moves the piston and applies the parking brake through the cable.
If system pressure is less than 450 psi (31 kg/cm2 ) or electric-hydraulic pump motor is running due to pressure maintenance switch being closed, AUTO PARK warning light comes on. Pressure maintenance switch and parking brake light switch supply battery positive to the auto park light circuit. Releasing parking brake opens the switches and turns AUTO PARK light off.