Principles Of Operation - Air Suspension System: Notes

The ISCM (Integrated Suspension Control Module) adjusts the quantity of air in the springs to maintain the rear suspension at the required height. The ISCM calculates a target height for the rear suspension based on the average height of the front suspension. If the actual height of the rear suspension is outside the tolerance band for the target height, for a given length of time, the ISCM then adjusts the actual height to the target height. The normal tolerance band is ±9 mm (0.35 in.). This changes to ±3 mm (0.12 in.) when the vehicle is parked for 5 minutes with the engine running and all doors closed, or by putting the ISCM into a special mode using Jaguar approved diagnostic equipment. When a door is open the tolerance band changes to -5/+20 mm (-0.20/+0.79 in.). Tight Tolerance Mode can be set manually.

To decrease suspension height, the ISCM opens the exhaust valve in the air compressor assembly and the air spring valves in the valve block to release air from the air springs. To raise the suspension height, the module opens the air spring valves to introduce air into the air springs using air from the air reservoir and/or the air compressor.

When vehicle speed is 22 mph (35 km/h) or less, the ISCM normally uses air from the air reservoir when it needs to raise the rear suspension. This ensures that the occupants are not disturbed by noise from the air compressor assembly. However, if the rear suspension is more than 30 mm (1.18 in.) below the target height, and there is insufficient pressure in the air reservoir, the ISCM uses the compressor to lift the suspension. When the rear suspension is more than 50 mm (2 in.) low, the ISCM also sends a signal to the IC (Instrument Cluster) on the HS (High Speed) CAN (Controller Area Network) powertrain systems bus to display a Suspension Too Low message. If the vehicle is stationary the message is displayed with an amber triangle warning indicator; if the vehicle is moving a red triangle warning indicator is displayed with the message.