28PSI suction pressure is way too low for normal operation.
Freezing pressures temperatures of low side and refrigerant
*** R12 factory fitted GM systems that use suction throttling valve [POA valve ] operate at 30PSI
***R134a freezing pressure temp 28 psi
In modern systems As automotive systems are a total compromise ie have variable suction line pressures due variable engine RPM constant rpms pressures are a myth . The thermostat saves the modern evaporator from freezing . The Tx valve plays a role but to a lesser degree.
At idle the compressor is hardly working . Pressures need to be taken at 1200rpm or higher . Often the TX valve can be seen operating and often during thermostat cycling the low side will dip well below 30psi all be it briefly .
Car makers use the low pressure cold temps [below ideal] to assist cooling. Many evaporators are to small for the job . This saves the car maker money .Effectively cheating the system .
Well designed systems donot have compromises and work very well in ALL ambients .
Operating pressure ideally ,,not always achieved
Low.... no lower than freezing pressure/temp of refrigerant
High... relevant to how hot ambient is,, ideally = 115f = 150-160psi