I thought all systems had an evap temp switch, including those using an EPR. EPR's have a setpoint of around 24psi at low load, give or take a psi or two. At that pressure, the saturated temp of R-12 is well below freezing. The reason the evaporator usually doesn't freeze is the temperature drop between the refrigerant on the interior of the evaporator tubes and the exterior surface. That temp gradient is dependent on the amount of airflow across the coil as well as the temperature and humidity of the air. It is possible for the evap coil to freeze on an EPR system if the return (inlet) air temp is sufficiently low and the air if fairly humid. Condensation is an insulator, and it tends to restrict airflow, so if the airflow is low (blower on low speed), the cabin is fairly cool (cool return air flowing into the coil), and the coil is good and wet, the clinging condensate can start to freeze around the tubes. Once it starts, it spreads because the developing frost further restricts airflow.
A properly set evap temp switch on an EPR system won't normally open. Only if conditions are right to allow frost to form and the exterior coil temp drops to a few degrees below freezing would the switch open, cutting out the compressor until the coil defrosts and warms up to a few degrees above freezing. In essence, the switch operates just as a freeze 'stat in an EPR system, rather than actually being the primary evap temp control.
