ASE S7 Air Conditioning Systems & Controls (School Bus) cheat sheet
ASE S7 Air Conditioning Systems & Controls (School Bus) · Keep the key ideas within reach.
A/C System Inspection, Diagnosis, and Repair
Static pressure with system off at 80°F ambient: roughly 70-90 psig on both high and low sides when properly charged.
Low side too high and high side too low together indicate a weak or worn compressor, not a charge problem.
Both gauges low with bubbles in the sight glass indicate an undercharge; both high with no subcooling indicate an overcharge.
Temperature differential across the evaporator coil of 15-25°F is normal; below 10°F suggests low refrigerant or restricted airflow.
A sweating or frosted suction line at the expansion valve outlet points to an overcharge or a stuck-open expansion valve.
Pressure test for leaks with dry nitrogen plus trace refrigerant; electronic leak detectors find leaks down to about 0.1 oz/yr.
Performance test: vent temperature 35-45°F below ambient at 1,500-1,800 engine rpm with blower on high and doors open.
Manifold gauge set hoses: blue to low side, red to high side, yellow to vacuum pump or recovery machine.
Refrigerant Recovery, Recycling, and Handling
Section 609 of the Clean Air Act prohibits venting refrigerant to the atmosphere and requires certified technicians for motor vehicle A/C service.
Recovery cylinders must never be filled above 80% of gross weight capacity; use a scale, not the float gauge alone.
Recovered refrigerant may be recycled on site but not reused in a different customer's vehicle unless it meets SAE J2788 purity.
R-134a recovered refrigerant must be identified by the SAE J1990 or J2788 machine's oil separation and filter-drier stages.
Never mix R-12 and R-134a; contaminated mixtures must be destroyed, not recycled.
Evacuation to 500 microns (29.9 inHg) and holding for 10 minutes confirms a leak-free, moisture-free system.
Recovery cylinder must be DOT-approved, gray with a yellow top for R-134a, and never filled by liquid without a scale.
Refrigerant contact with skin causes frostbite; wear gloves and eye protection and ventilate the work area.
Compressor and Drive Systems
Compressor clutch coil resistance is typically 3-5 ohms; infinite resistance indicates an open coil.
Clutch air gap specification is usually 0.014-0.030 in (0.35-0.75 mm); excessive gap prevents engagement.
A seized compressor may shear the drive belt or slip the clutch; check for a locked-up clutch hub before condemning.
Compressor oil is PAG (polyalkylene glycol) for R-134a; never use mineral oil in an R-134a system.
Oil charge is typically 4-8 oz total; adding oil requires draining and measuring from the removed component.
Low suction and high discharge together with a hot clutch indicate internal compressor wear or a restricted discharge line.
Noise on engagement that stops when the clutch disengages points to clutch bearing or pulley misalignment.
System must have a low-pressure cutoff switch to protect the compressor from running with low refrigerant.
Condenser, Evaporator, and Refrigerant Lines
Condenser must have unobstructed airflow; a clogged condenser raises high-side pressure and reduces cooling.
High-side pressure above 2.5 times ambient plus 30 psig indicates condenser airflow restriction or overcharge.
Evaporator coil must be clean and drain pan clear; a clogged evaporator drain causes water on the floor and fogging.
Suction line should be insulated from the evaporator outlet to the compressor to prevent condensation and heat gain.
Liquid line restriction shows as a temperature drop across the restriction; frost at the orifice confirms it.
Line fittings use O-rings compatible with R-134a; lubricate O-rings with PAG oil, never with mineral oil.
Evaporator inlet and outlet temperature difference of 15-25°F indicates proper refrigerant flow; less than 10°F suggests low charge.
A/C hoses are barrier-type for R-134a; standard R-12 hoses leak R-134a through the wall.
Expansion Devices and Refrigerant Flow Control
TXV controls superheat at the evaporator outlet; typical superheat is 5-10°F.
A stuck-open TXV floods the evaporator, causing low superheat, sweating suction line, and possible compressor slugging.
A stuck-closed TXV starves the evaporator, causing low suction pressure and no cooling.
Orifice tube systems use an accumulator with a desiccant bag and a fixed orifice; TXV systems use a receiver-drier.
TXV sensing bulb must be tightly clamped to the suction line and insulated; loose bulb causes hunting.
Superheat = evaporator outlet temperature minus evaporator saturation temperature at suction pressure.
Subcooling = liquid line temperature subtracted from condenser saturation temperature; 10-15°F indicates a full charge.
Orifice tube color codes identify size; wrong tube causes incorrect flow and pressure imbalance.
Electrical and Electronic Controls
Blower motor resistor pack uses series resistors to provide low, medium, and high speeds; an open resistor kills lower speeds only.
Blower motor draws high current; a seized motor blows the fuse or the resistor.
A/C pressure switch opens the compressor clutch circuit on low or high pressure to protect the system.
Ambient temperature switch prevents compressor operation below about 32-40°F to avoid evaporator freeze-up.
Clutch relay coil is controlled by the PCM or HVAC control head; check for 12V at the coil and ground at the control side.
Blower motor current draw is typically 10-20 amps on high; higher indicates a dragging motor.
Use a wiring diagram to trace the clutch circuit; a blown fuse may indicate a shorted clutch coil or pinched wire.
HVAC control head uses a blend door actuator and mode door actuator; a failed actuator causes temperature or mode faults.
HVAC Air Distribution and Blower Systems
Blower motor resistor pack provides low, medium, and high speeds; high speed often bypasses the resistor.
Cabin air filter restricts airflow when clogged; replace per manufacturer interval, typically 15,000-30,000 miles.
Mode door directs air to floor, panel, or defrost; a failed actuator leaves air stuck in one mode.
Blend door mixes hot and cold air; a stuck blend door causes no temperature control.
Airflow restriction at the evaporator causes low cooling and possible evaporator freeze-up.
Blower motor must be checked for free rotation; a dragging motor draws excessive current and can melt the connector.
Recirculation door position affects cooling performance; recirculation mode improves cooling in high ambient.
Defrost mode uses A/C to dehumidify; the compressor runs in defrost to clear the windshield.
Heating and Defrost Systems
Heater core uses engine coolant; a clogged core causes poor heat and unequal inlet/outlet temperatures.
Blend door actuator controls heater core flow through the air box; a failed actuator gives no heat or full heat.
Defrost performance requires the A/C compressor to run to dehumidify the air; a failed compressor causes fogging.
Heater control valve regulates coolant flow to the core; a stuck-closed valve causes no heat.
Coolant level and thermostat affect heater output; a stuck-open thermostat causes low heat.
Defroster door directs air to the windshield; a failed mode door leaves air at the floor.
Heater core leak shows as coolant smell in the cabin and fogging on the windshield.
Rear heater systems on school buses use a separate blower and core; check both for heat output.
Safety, Environmental, and Regulatory Compliance
Section 609 certification is required to service motor vehicle A/C systems; Section 608 covers stationary equipment.
Venting refrigerant to the atmosphere is illegal; recovery is required before opening the system.
R-134a is a greenhouse gas; use recovery equipment and never release it to the atmosphere.
Wear safety glasses and gloves; liquid refrigerant causes frostbite and eye damage.
Recovery cylinders must be DOT-approved and never overfilled beyond 80% capacity.
Work in a ventilated area; refrigerant displaces oxygen and can cause asphyxiation in enclosed spaces.
Refrigerant containers must be stored upright and away from heat sources; never heat with an open flame.
Used refrigerant oil must be disposed of per local regulations; never pour it down a drain.