Condenser, Evaporator, and Refrigerant Lines
Heat Exchange Fundamentals
The condenser rejects heat to ambient air; the evaporator absorbs heat from cabin air. Both depend on airflow and clean fins. On school buses, condensers are often roof-mounted or remote-mounted with electric fans, and evaporators are in overhead units. Mud, dust, and debris on the condenser reduce capacity and raise high-side pressure. A restricted evaporator (dirty coil, blocked filter) causes low airflow and poor cooling with possible coil icing.
Lines, Hoses, and Components
Know the layout: compressor discharge to condenser, condenser to receiver-drier (or accumulator on orifice-tube systems), then to the expansion device, evaporator, and back to the compressor suction. Lines must be routed away from exhaust and moving parts; chafing and vibration cause leaks. O-rings and spring-lock or block fittings must be lubricated with the correct refrigerant oil and not over-tightened. The receiver-drier/accumulator stores refrigerant and removes moisture; it should be replaced whenever the system is opened for major service.
Traps to Watch
A frequent trap: high-side pressure high and cooling poor, and the question points to the expansion valve - but the real cause is a condenser fan not running or a condenser packed with debris. Another trap is a frosted suction line mistaken for a charge problem when it indicates a restriction or overcharge. Also watch for questions about line temperature: the liquid line should be warm, the suction line cool; a hot suction line suggests low charge or a restriction.
Worked Rule of Thumb
Use the touch test: the condenser inlet should be hot and the outlet noticeably cooler; the evaporator outlet (suction) should be cold and sweating. If the condenser outlet is as hot as the inlet, airflow is inadequate. If the suction line is warm, suspect low charge or a restriction upstream.