[old_master]

When the low side reaches 22 psi the clutch should disengage, (turn off). When low side pressure rises to 42 psi the clutch should engage, (turn on). This is controlled by a pressure switch near or at the accumulator. The switch can not be properly diagnosed for correct switching points unless the system is fully charged. With R134a refrigerant and R12 refrigerant, in this pressure range, the pressure and temperature are approximately equal. With that in mind, the AVERAGE pressure/temperature of the evaporator should not be lower than 32F. This keeps the evaporator above freezing so it will not ice up. While checking pressures, the windows should be up, doors closed, engine rpm around 1500, and adequate air flow across the radiator. There are variables involved with A/C performance such as ambient temperature and humidity. Both affect high side pressure readings and register temperature output. The high side pressure at 70F ambient should not exceed 225psi. The readings you have posted indicate the system may be low on refrigerant. Unfortunately, with a cycling clutch system such as yours, there is no way of knowing how much refrigerant is in the system which makes "toping off" dangerous to both the technician doing the work and to the components in the system. If the refrigerant charge is in fact low, the system should be leak tested and repaired as necessary before performing any service work. The proper way to charge the system is to drain, (recover) and evacuate at 29.9 inches of vacuum for a minimum of one hour, then recharge with the proper amount of the correct refrigerant and oil. More questions? Just ask. Hope this helps.