Evaporator freezing up

[USM]

I am new to the forum but really need all of your help. I have a 2007 Yukon SLT with rear air and something is wrong with my A/C that I can't figure out. The air blows very cold (high 40's low 50's) out of the vents for a short period of time (around 20 minutes or so). Then the air just stops. I found out that the evaporator and the accumulator are freezing up (I pulled the blower motor out and stuck my hand in and felt the ice). I can turn the compressor off for a few minutes and it defrost then starts working again. I have put my manifold gauges on it and I feel like I am getting good readings (on a normal system without rear air). The temp outside was 74f and I am getting right at 40lbs on the low side and about 195lbs on the high side. I have checked the temp on both sides of the orifice tube and they seem normal. Very cold on one side and very hot on the other. I have also checked the drain tube and it is flowing well. The compressor NEVER cycles off. That doesn't seem normal to me. I am at a loss. Any help would be greatly appreciated.

 

[Bowtiebug]

Possibly a bad low pressures switch or slightly low on freon , the unit should cycle off before freezing up

 

[livingez_123]

Bad pressure/temp switch.

 

[USM]

Thanks for the responses. If the freon is low, do you know what ranges my A/C manifold gauges should be reading for the correct amount of freon?