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What problem are you having?
1. Do you know if the system uses R-134A and what is outdoor temp?
Sounds like you probably have a restriction or blockage in the orifice tube or expansion valve
The low suction pressure indicates the refrigerant is not properly getting back to the compressor so low pressure switch is shutting it off
At 80 degree ambient, we really should be running 40 to 50
Manufacturer could have used a temp or a pressure switch but they are in direct corolation . A 20psig is about a 20 degree coil which would ice
Did compressor fail and you had to replace?
Honestly to me it sounds like there may be a small particle or something moving around in the system that is causing it .You might try adding just a bit of refrigerant just to make sure it is not due to low charge.We can be up as high as 210 on high side.
What does dealership say would cause the unit to cycle like it is?
Short cycling is usually caused by the pressure or temperature dropping below the pressures shown on the chart I sent. This would be anywhere below a 32 degree coil.
I can not answer why the dealership would say 18-20 is normal