A/C System Inspection, Diagnosis, and Repair
Start With the Complaint and the Basics
A school-bus A/C diagnostic should always begin with confirming the customer complaint, then a visual inspection before any gauge is connected. Check belt condition and tension, compressor mounting, condenser and evaporator fins for debris or mud, line routing and chafing, and every electrical connector in the compressor clutch circuit. Record ambient temperature and humidity; performance specs are meaningless without them.
The Pressure-Temperature Relationship
Understanding that pressure and temperature track each other on the high and low sides is the core skill. Static (compressor off, system equalized) pressures should correspond roughly to ambient temperature on the refrigerant's PT chart. If static pressure is far below ambient saturation pressure, suspect a low charge; if far above, suspect overcharge or a restriction with trapped heat.
Traps to Watch
Questions often present a system that "cools poorly" with normal-looking low-side pressure. Look for a restricted expansion device, a blend-air or heater-control fault letting hot coolant warm the evaporator, or an inoperative condenser fan. Do not jump to "needs refrigerant" - a system that is not cycling and shows high low-side pressure is often overcharged or has a weak condenser airflow. Also watch for questions that describe gauge readings taken with the engine off versus running; the interpretation changes completely.
Worked Rule of Thumb
With the compressor engaged and stabilized: low side should be in the range that produces evaporator temperatures just above freezing (commonly cited around 25-45 psi for R-134a depending on conditions), and high side should be roughly ambient temperature plus 30-60°F converted to pressure. If low side is low and high side is low, suspect undercharge. If low side is low and high side is high, suspect a restriction. If both are high, suspect overcharge or airflow.