Air Conditioning, Heating, and Cab Systems
Core concepts
Understand the refrigeration cycle: compressor, condenser, expansion device, evaporator, and refrigerant states. Know refrigerant handling, recovery and recycling, leak detection, and system pressures. Heating uses engine coolant through a heater core and blend door. Cab systems include pressurization and filtration for dust exclusion, seals, and blower controls. Electrical controls and sensors (pressure switches, thermistors) manage compressor cycling.
Traps
Questions often confuse high-side and low-side pressures or assume low refrigerant always means a leak without checking for a restriction. Another trap is adding refrigerant without recovering and weighing it. Do not vent refrigerant to atmosphere. Cab dust intrusion often traces to pressurization or filter/seal faults, not the A/C.
Rule of thumb
Low side low and high side low suggests low charge; high side high and low side high suggests poor condenser airflow or overcharge. Always recover, evacuate, and recharge by weight per specification. Check cab filters and door seals for dust intrusion before blaming the HVAC.
Worked example
A cab cools poorly at low engine speed but improves at high speed. Check condenser airflow and fan operation; restricted airflow raises high-side pressure and reduces cooling. Clean the condenser and verify fan clutch or electric fan operation before adding refrigerant.