If it has a wall thermostat it should have a freeze sensor. Very common for the sensor to fall away from the evaporator due to poor installation. Need to get up inside and check/reposition the freeze sensor. Remove all interior trim for access. Sensor needs to be direct on or in the evaporator coil fins.
While you are there check the plenum divider is properly installed. Again the divider is often poorly installed and will allow chilled air to immediately return to the intake plenum. Reposition and seal with metal foil tape from your local hardware store. See example here:
http://forums.woodalls.com/index.cfm/fuseaction/thread/tid/28955299.cfm