ASE S7 Air Conditioning Systems & Controls (School Bus) flashcards

32 cards. Read the term and the definition here. The deck at the top of this page is optional.

Section 609 certification

Federal Clean Air Act requirement that technicians servicing motor vehicle A/C systems be certified in refrigerant recovery and recycling.

Recovery cylinder 80% rule

Recovery cylinders must never be filled above 80% of gross weight capacity to allow for liquid expansion and prevent rupture.

Evacuation to 500 microns

Pulling a vacuum to 500 microns and holding for 10 minutes confirms the system is leak-free and moisture-free before charging.

PAG oil

Polyalkylene glycol lubricant required for R-134a systems; mineral oil is not miscible with R-134a and must never be used.

Clutch air gap

Clearance between the clutch hub and pulley, typically 0.014-0.030 in; excessive gap prevents magnetic engagement of the compressor.

Low-pressure cutoff switch

Switch that opens the compressor clutch circuit when suction pressure drops too low, protecting the compressor from oil starvation.

Compressor slugging

Liquid refrigerant entering the compressor, causing mechanical damage; often caused by an overcharge or stuck-open expansion valve.

Condenser airflow restriction

Blocked or dirty condenser raises high-side pressure and reduces cooling; check for debris, bent fins, and fan operation.

Suction line insulation

Insulating the suction line from evaporator to compressor prevents condensation and heat gain that reduce cooling performance.

Barrier hose

A/C hose with an inner liner that prevents R-134a from permeating through the hose wall; required for R-134a systems.

Evaporator temperature differential

Difference between evaporator inlet and outlet air temperature; 15-25°F indicates proper refrigerant flow and cooling.

TXV superheat control

Thermal expansion valve regulates refrigerant flow to maintain 5-10°F superheat at the evaporator outlet.

Superheat formula

Superheat equals evaporator outlet temperature minus the evaporator saturation temperature at the measured suction pressure.

Subcooling formula

Subcooling equals condenser saturation temperature minus liquid line temperature; 10-15°F indicates a full charge.

Stuck-open TXV

TXV that fails open floods the evaporator, causing low superheat, a sweating suction line, and possible compressor slugging.

Orifice tube system

Fixed-orifice system using an accumulator with desiccant; TXV systems instead use a receiver-drier on the high side.

Blower motor resistor

Series resistor pack that provides low and medium blower speeds; an open resistor disables lower speeds but leaves high speed working.

Ambient temperature switch

Switch that prevents compressor clutch engagement below about 32-40°F to avoid evaporator freeze-up and compressor damage.

Clutch relay test

Verify 12V at the relay coil and ground at the control side; a missing ground indicates an open control head or PCM circuit.

Blower motor current draw

Normal high-speed current is roughly 10-20 amps; higher current indicates a dragging motor or seized bearing.

Blend door actuator

Motor that mixes hot and cold air; a failed actuator causes no temperature control or air stuck at one temperature.

Mode door actuator

Motor that directs airflow to floor, panel, or defrost; a failed actuator leaves air stuck in one mode.

Cabin air filter interval

Cabin air filter replacement typically every 15,000-30,000 miles; a clogged filter restricts airflow and reduces cooling.

Recirculation door

Door that selects outside or recirculated air; recirculation mode improves cooling in high ambient temperatures.

Heater core diagnosis

A clogged heater core causes poor heat and unequal inlet and outlet hose temperatures; compare both hoses at operating temperature.

Heater control valve

Valve that regulates coolant flow to the heater core; a stuck-closed valve causes no heat from the HVAC system.

Defrost with A/C

Defrost mode runs the A/C compressor to dehumidify air; a failed compressor causes windshield fogging during defrost.

Rear heater circuit

School buses often use a separate rear blower and heater core; both front and rear systems must be checked for heat output.

Static pressure check

With the system off at 80°F ambient, both gauges should read roughly 70-90 psig when the system is properly charged.

Weak compressor symptom

Low suction pressure and low discharge pressure together indicate internal compressor wear rather than a charge problem.

Vent temperature test

Vent air should be 35-45°F below ambient at 1,500-1,800 rpm with blower on high and doors open.

Refrigerant frostbite hazard

Liquid refrigerant contacting skin causes frostbite and eye damage; wear gloves and safety glasses and ventilate the work area.