Report Refrigeration - Engine Driven Systems - DIY in Articles Posted February 18, 2015 What I did not mention in the article, is that some systems have 2 expansion valves - one for the fridge, one for the freezer, and separate hold-over tanks. If this is the case, you may well (likely) will have a flow adjustment, often a pressure regulator, to ensure that one compartment does not get all the refrigerant. It's basically a balance for the system. Also, if your system has no pressure switch, you may (SHOULD!) have a temp controller for the Compressor, to ensure the unit switches off when it's fully cold. To just run the compressor until you switch off the engine is likely to cause the system to run at such a vacuum that air is sucked in on the LP side, bringing moisture with it, and ruining the efficiency of the system. Suitable controllers can be had for about $45.00.