For 1990-2009 cars

### Air Control System Description

The suspension air control system uses air drawn from the vehicle air system in order to pressurize the air spring assemblies through the height leveling valve.

The correct vehicle ride height is maintained through use of a frame-mounted height leveling valve.

The pressure protection valve is located on the front service reservoir. The pressure protection valve protects the air supply to the brakes if a failure occurs in the air suspension system. If a loss of air occurs forward or upstream of the protection valve, air pressure within the air spring system will be partially maintained until repairs can be made. If a loss of air occurs rearward or downstream of the protection valve, the air in the brake system will be protected but the air pressure in the air springs will be lost.

### Hendrickson HAS Air Suspension Description

The Hendrickson HAS Series single axle air suspension is made up of two air springs (7). The air springs are supported on main support beams (8), which are fastened to the axle and to the frame. A torque rod (14) from the frame bracket to the spring seat (11) or bottom axle cap (10) provides stability. Additional stability is provided by a transverse torque rod (20) from the frame to the axle housing. The shock absorbers (4) help to control jounce and rebound during vehicle operation.

### Height Leveling Valve Description

The height leveling valve is located on the frame. The height leveling valve attaches to the cross channel by a fixed link. The valve senses changes in the ride height due to varying loads. The valve either will add air or exhaust air from the air springs until the correct ride height is reached.

### Air Suspension Relief Valve Description

The air suspension relief valve permits deflation of the rear suspension air springs. The air suspension relief valve is manually operated by the air suspension dump switch located on the dash panel. The supply port on the air suspension relief valve closes when the air suspension dump switch is actuated. This allows the air from the air springs to exhaust and the springs to deflate. The exhaust port closes and the supply air is directed to the air springs when the air suspension dump switch is deactivated. The air springs are then allowed to inflate to normal ride height.

### Air Suspension System Operation

Voltage is applied to the air suspension dump switch when the ignition switch is in RUN. When the air suspension dump switch is manually closed, voltage is then applied to the air suspension relief valve. Since the air suspension relief valve is permanently grounded, the air suspension relief valve will be energized until the suspension dump switch is manually opened. The air suspension dump switch indicator lamp receives a voltage signal from the air suspension dump switch when it is activated.