Installation: Technical Bulletin Truck Height Control Valve Instruction Manual

Overview

The Truck Height Control Valve is a pneumatic suspension component that maintains the vehicle’s specified ride height by regulating air pressure in the suspension air springs. It is designed for heavy-duty trucks with tandem or triaxle air suspensions and can be configured for left-hand or right-hand control-arm positions, as well as short- or long-linkage applications.

An integrated dump-valve function provides a convenient connection for a cabin-operated suspension dump control. This design can simplify air plumbing by reducing the need for separate components.

Specifications

• Application: Truck air suspension ride-height control
• Control-arm configurations: Left-hand or right-hand
• Installation positions: Vertical or horizontal valve mounting, according to vehicle configuration
• Air supply requirement during operation: More than 550 kPa (80 psi) to open the pressure protection valve
• Air ports: SUP supply port, DEL1 delivery port, and DEL2 delivery port
• Delivery connections: One or both delivery ports may be used; an unused delivery port must be plugged
• Control-arm nut torque: 3 Nm (27 in. lbs.)
• Mounting-nut torque: 10 Nm (88 in. lbs.)
• Link-rod and fine-adjustment fastener torque: 3 Nm for the fine-adjustment bolt and 10 Nm for link-rod nuts and locknuts
• Ride-height measurement tolerance: Within ±3 mm of the specified dimension

Detailed Description

The valve uses a mechanical control arm connected to the suspension through a link rod. When the control arm is horizontal, the system is normally at its preset ride height and the valve does not intentionally add or release air.

Moving the control arm upward opens the supply path and sends compressed air to the air springs. This raises the suspension. Moving the arm downward opens the exhaust path and releases air from the air springs, lowering the suspension. The exhaust outlet beneath the interface cap is separate from the dump-air vent located beneath the rear valve cover.

The SUP port receives air from the vehicle supply through a pressure protection valve. DEL1 and DEL2 are the suspension delivery ports. The integrated dump function receives a signal from the cabin dump control and vents the suspension through its dedicated dump circuit.

For tandem suspensions, the valve is generally installed on the forward drive axle. For triaxle suspensions, it is generally installed on the centre drive axle. A vehicle manufacturer’s plumbing arrangement takes priority because some suspension configurations require a different layout.

Most suspension groups use one height control valve. If two valves are required, both must be the same design and must be installed in matching relative positions on opposite sides of the same axle. The suspension bill of material number, normally found on the label near the tyre placard inside the vehicle door cavity, helps identify the correct plumbing arrangement.

Safety

• Do not modify, rework, or substitute valve components unless the installation instructions specifically require it.
• Use approved parts and hardware. Modified or substitute components may fail to meet the required specifications and could contribute to loss of vehicle control, injury, or property damage.
• Do not alter the vehicle’s pneumatic-system settings or specifications. Unapproved changes can affect safe operation and may affect warranty coverage.
• Work with the vehicle unloaded on a stable, level surface.
• Support or lower liftable auxiliary axles before beginning work.
• Chock the front wheels and ensure the brakes are released when freeing suspension joints.
• Do not overinflate the air springs after full suspension extension. Excess travel or pressure can damage shock absorbers and air springs and may create a personal-injury hazard.
• Use the vehicle or suspension manufacturer’s ride-height specification rather than assuming a generic measurement.

Setup and Installation

Prepare the Vehicle

1. Park the unloaded vehicle on a firm, level surface.
2. Support or lower any liftable auxiliary axles.
3. Free the suspension joints by slowly moving the vehicle forward and backward several times without applying the brakes. Stop with the brakes released, then chock the front wheels.
4. Locate the specified ride height in the vehicle documentation, suspension label, or manufacturer information. If the specification cannot be found, use the existing ride height only as a temporary reference.
5. Measure ride height from the underside of the chassis frame rail to the centreline of the axle controlled by the valve. For dual-valve systems, measure at both valve locations.
6. Before removing the old valve, record the valve orientation, control-arm position, air-line locations, and identifying dimple. Photographs can help document the original arrangement.

Remove the Existing Valve

1. Disconnect and remove the link-rod assembly.
2. Move the control arm below horizontal to exhaust air from the suspension.
3. Disconnect all air lines from the valve and remove the valve from its mounting.

Install the Replacement Valve

1. Use the vehicle manufacturer’s instructions and the removal notes to select the correct valve orientation, identifying dimple, and control-arm position. Incorrect orientation can prevent the valve from operating correctly.
2. Attach the control arm to the valve body with the alignment dimple in the correct position. Apply thread-locking compound to the shaft thread and tighten the Nyloc nut to 3 Nm.
3. Rotate the control arm fully in both directions three to five times. This helps remove adverse effects from storage.
4. For a metric valve, install the air fittings before mounting where practical. Use new fittings with pre-applied sealing compound when available. If sealing compound is needed, apply only a small amount to the threads. Do not use Teflon tape or pipe-sealing compound. Imperial versions use integral push-to-connect fittings.
5. Confirm that the link rod fits the control-arm mounting hole. Enlarge the hole only when required by the specified linkage arrangement.
6. If an adjustable arm is supplied, assemble it in the centre position and tighten its adjustment bolt to 3 Nm.
7. Attach the valve to its mounting bracket with the supplied nuts and tighten them to 10 Nm.
8. Mount the valve as close to level as possible. Replace bent or worn brackets, bolts, nuts, or linkage components before completing the installation.
9. Connect the supply line from the pressure protection valve to SUP.
10. Connect an air-spring line to DEL1 or DEL2. Plug the unused delivery port if only one port is required.
11. Charge the vehicle air system.
12. Raise the control arm approximately 20 degrees above horizontal. The air springs should begin to inflate. Monitor the suspension to prevent overextension or overinflation.
13. Lower the control arm approximately 20 degrees below horizontal. The air springs should begin to deflate through the suspension exhaust outlet beneath the interface cap.

Connect and Align the Linkage

1. Use the valve to position the suspension near the specified ride height.
2. Fit the link rod into its upper and lower mounting holes and adjust its length if necessary.
3. For rose-joint linkage rods, install the specified washer between the rod and mounting points so the joint does not bind during suspension travel.
4. At ride height, check that the link and its mounts are vertically aligned and that the linkage angle does not exceed approximately 90 degrees.
5. Check the linkage at full suspension extension. With the lower connection temporarily detached, raise the valve arm until the air springs are fully extended, then reconnect the linkage. Confirm that the control arm cannot rotate past the vertical position. Relocate the valve if this condition occurs.
6. Allow the valve and linkage to return the vehicle to ride height, then make the final adjustment.

Check Ride Height

1. Drive the vehicle onto a level surface, free the suspension joints, and chock the front wheels without applying the parking brake.
2. Maintain the air system at full operating pressure during measurement and adjustment.
3. For the most repeatable check, disconnect the link rod while the vehicle is at ride height, lower the control arm for about 15 seconds, then reconnect the rod and allow the suspension to stabilise.
4. Measure from the underside of the chassis frame rail to the centreline of the axle controlled by the valve.
5. Use the adjustable lever for fine corrections when provided. Otherwise, loosen the rod locknuts and turn the link rod to lengthen or shorten it.
6. Repeat the 15-second lowering, reconnection, stabilisation, and measurement cycle until the specified height is obtained. Adjust both valves separately on a dual-valve system.
7. Tighten the fine-adjustment bolt to 3 Nm, the link-rod nuts to 10 Nm, and the rod locknuts to 10 Nm.
8. Apply a suitable foaming leak-detection agent to every air connection and fitting. Confirm that the fittings and fasteners are secure and that no bubbles indicate an air leak.

Alternative Ride-Height Measurement

When the axle centre is difficult to identify, measure both distances from the floor. First measure from the floor to the centre of the hubcap, designated as B. Then measure from the floor to the underside of the chassis frame rail, designated as C. Subtract B from C:

Ride height A = C − B

For example, a hubcap-centre measurement of 500 mm and a frame-rail measurement of 754 mm produce a ride height of 254 mm. Confirm that tyre pressures are even and correct before using this method.

Operation

During normal driving, suspension movement changes the control-arm position. The Truck Height Control Valve responds by adding air when the suspension is too low and exhausting air when it is too high. When the target height is restored, the valve returns to its neutral position.

A short period of air discharge from the exhaust outlet after the vehicle stops can be normal. The suspension may continue stabilising for several minutes, so this sound alone does not prove that the valve is leaking or defective.

Troubleshooting

Air Springs Do Not Inflate

• Verify that supply pressure exceeds 550 kPa (80 psi) and is sufficient to open the pressure protection valve.
• Check that the supply line is connected to SUP and that the delivery line is connected to DEL1 or DEL2.
• Inspect air lines for kinks, blockages, loose connections, or incorrect routing.
• Check that the cabin dump control has not been activated.
• Confirm that the control-arm position and identifying dimple match the selected valve orientation.
• Inspect the valve mounting and linkage for incorrect alignment or interference.

Air Springs Do Not Exhaust

• Move the control arm sufficiently below horizontal to open the exhaust path.
• Check that the valve is mounted in the correct orientation.
• Inspect the suspension exhaust outlet beneath the interface cap for obstruction.
• Check the linkage for binding, excessive angle, or interference that prevents control-arm movement.

Ride Height Is Incorrect

• Confirm that the air system is at full operating pressure.
• Repeat the controlled 15-second lowering and stabilisation procedure before measuring.
• Verify the manufacturer’s ride-height specification and measure from the correct frame-rail and axle reference points.
• Adjust the lever or link rod, then repeat the measurement cycle.
• On dual-valve systems, confirm that both valves have the same design and matching installation position, and adjust each location independently.

Suspected Air Leak

• Apply foaming leak-detection solution to the supply, delivery, and other air connections.
• Tighten loose fittings and fasteners, replace damaged lines or fittings, and retest.
• Do not mistake several minutes of normal stabilising exhaust after vehicle stop for a confirmed valve leak.

Pros & Cons

• Pros: Supports left- or right-hand layouts, accommodates different linkage arrangements, provides two delivery ports, and integrates a suspension dump function.
• Pros: Mechanical air control automatically corrects ride height without requiring an electronic controller.
• Cons: Correct operation depends on precise orientation, linkage alignment, air-line routing, and ride-height adjustment.
• Cons: Dual-valve installations require matching valve designs and careful adjustment at both valve locations.