Air conditioner thermostat working?

[nickcrowhurst]

The Coleman Delta TX air conditioner works efficiently to cool our trailer, but I'm not sure that the thermostat is working. The fan blower runs constantly and cold air blows out, but I never hear any part of the system cutting in or out. When the trailer is cool enough, we switch off the unit. There is a low/high cooling setting on a knob, but turning this does not appear to have any effect. Air conditioning systems are new to me, as an inhabitant of an island where 80 degrees is rare, even at the height of summer. Should I hear the compressor cutting in or out, or does the compressor just work at different rates for different amounts of cooling? If the thermostat is defective, is this an isolated part that is replaceable? Thanks. Nick.