1. Operate the unit in high speed cool.
2. Maintain discharge pressure (shown in chart below) for 100 F (38 C) ambient.
3. Maintain box temperature near 35 F (2 C) or 0 F (-18 C).
4. Compare unit suction pressure with normal expected readings. Determine if it is higher or lower than expected normal pressures.
5. Check the suction line at the accumulator. A cool, moist line is normal. A frosted or abnormally dry line and out-of-range pressures indicate refrigeration problems.
The table below gives approximate suction and discharge pressure for common refrigerants with a 0 F (-18 C) or 35 F (2 C) box temperature and 100 F (38 C) ambient temperatures.
Control condenser air flow to maintain condenser temperature near a 100 F degree (38 C) ambient while reading either discharge or suction pressure.
Suction pressure is most accurate when it is below the influence of the throttle valve or ETV. For greatest accuracy, suction pressure on units with high pressure refrigerant must be taken with a box temperature at 0 F (-18 C). Examples of high pressure refrigerants are R-502, R402A and R-404A.
Units with modulation may have suction pressure affected by the modulation valve or ETV. This occurs while operating near setpoint with a box temperature above 24 F (-4 C).