Here's another possibility:
http://members.tripod.com/~jbabs714/acrefrig.htm
"If your system runs low on refrigerant it will freeze up because the lack of pressure means the evaporator (cold coil) will be under the 32 degree mark, thus freezing the water."
If wiring checks out then this may be the case.
Edit....looks like I was typing when DHoward was....