Portable electronic device for changing trailer height using the pneumatic suspension

A trailer vehicle function control system for a trailer. The system comprises a control valve (32) operable by an electronic control unit (13) provided with a standards compliant communications interface adapted to communicate wirelessly with a portable electronic device. In use, if the electronic control unit (13) is enabled to receive vehicle control functions from a portable electronic device, the signal strength of the standards compliant communications interface is controlled to a level to ensure that the portable electronic device is within a predetermined distance from the communications bus.