Electrical and Electronic Controls
Control Architecture
School-bus A/C uses a control head (dash or overhead), relays, fuses, pressure switches, thermistors, and often a body control module or dedicated HVAC controller. The compressor clutch, blower, condenser fans, and blend doors are electrically controlled. Know how to read a wiring diagram, identify power and ground paths, and use a DVOM to test voltage drops rather than just continuity. Many "A/C won't cool" complaints are electrical: blown fuse, bad relay, open pressure switch, or failed blower resistor/module.
Sensors and Actuators
Pressure switches protect the compressor: low-pressure cutoff prevents operation with low charge; high-pressure cutoff protects against overpressure. Thermistors report evaporator temperature to prevent icing. Blend-door actuators position air-mix doors; a failed actuator causes temperature control faults. Know the difference between a switch (on/off) and a sensor (variable signal).
Traps to Watch
A frequent trap: replacing a relay because the clutch does not engage, when the real fault is a bad ground or an open high-pressure switch. Another trap is assuming a blower that works only on high speed has a bad motor - it is usually the resistor or blower control module. Questions may describe intermittent operation; look for loose connectors, chafed harnesses, or heat-sensitive components. Always verify power and ground at the load before condemning it.
Worked Rule of Thumb
For a non-working blower, check voltage at the motor connector on each speed setting. Voltage present on all speeds but no motor operation = bad motor. Voltage only on high = bad resistor/module. No voltage on any speed = check fuse, relay, and control head.