Heating and Defrost Systems
Heat and Visibility
Heating on a school bus comes from the engine coolant passing through a heater core, with blowers moving air across it. Defrost directs warmed, dried air to the windshield. Coolant flow valves, blend doors, and blower operation control heat output. A clogged heater core, stuck coolant valve, or air pocket causes poor heat. Defrost performance is a safety issue - buses must clear the windshield for driver visibility.
Controls and Coolant
Know the coolant flow path: engine to heater core and back, controlled by a valve. A failed valve stuck closed means no heat; stuck open can add heat during A/C operation. Blower resistors and mode doors affect defrost airflow. Air in the cooling system prevents proper heat and can cause overheating. Verify coolant level and bleed air when servicing.
Traps to Watch
A frequent trap: no heat diagnosed as a bad heater core when the coolant valve is stuck closed or the system has air. Another trap is poor defrost blamed on the blower when the mode door is not moving to defrost. Also watch for A/C and heat interaction - a blend door or coolant valve fault can make the A/C seem weak because hot air is mixing in. Questions may describe heat that works at speed but not at idle; suspect coolant flow or air pockets.
Worked Rule of Thumb
If heat is weak, check coolant level and temperature first, then verify the coolant valve opens and the heater hoses are both hot (inlet hot, outlet warm). If both hoses are hot but airflow is cold, suspect a blend door or mode door fault. If the outlet hose is cold, suspect a clogged core or closed valve.