in a motor vehicle with a combustion engine , an electronic engine - management device , and an air - conditioning system with an air - conditioning control device , the air - conditioning compressor has reciprocating pistons whose displacement stroke is adjustable through one or more control valves . the electronic engine - management device has direct access to and priority control over the adjustment of the piston stroke , so that the electronic engine management device takes precedence over the air - conditioning control device in controlling the adjustment of the piston stroke .