ASE H7 HVAC (Transit Bus) flashcards
ASE H7 HVAC (Transit Bus) · Build recall with a focused review session.
32 cards. Read the term and the definition here. The deck at the top of this page is optional.
- Refrigerant R-134a safety classification
- R-134a is an A1 refrigerant per ASHRAE 34, meaning low toxicity and no flame propagation. It is heavier than air and can displace oxygen in confined spaces.
- Section 609 certification
- Section 609 of the Clean Air Act requires technicians who service motor vehicle air conditioning systems to be certified in refrigerant recovery and recycling.
- Recovery cylinder fill limit
- Never fill a recovery cylinder beyond 80% of its capacity by weight. Use a scale and follow DOT regulations to prevent hydrostatic rupture.
- Evacuation vacuum level
- A system should be evacuated to 500 microns (29.9 inHg) and held for 15-30 minutes to remove moisture and non-condensables before charging.
- Compressor clutch air gap
- The air gap between the clutch plate and pulley is typically 0.4-0.8 mm (0.016-0.031 in). Excessive gap prevents magnetic engagement.
- High-pressure cutoff switch
- A safety switch that disengages the compressor clutch when system pressure exceeds a set threshold (typically 400-450 psig) to prevent damage.
- Condenser function
- The condenser transfers heat from high-pressure refrigerant vapor to the outside air, causing the refrigerant to condense into a liquid.
- Filter-drier location
- The filter-drier is located on the liquid line between the condenser and expansion valve. It removes moisture and debris and must be replaced when the system is opened.
- Superheat definition
- Superheat is the difference between the actual refrigerant temperature and its saturation temperature at the same pressure. It indicates complete vaporization.
- Orifice tube system
- A fixed orifice tube meters refrigerant flow. It requires an accumulator on the suction side to store excess refrigerant and prevent liquid slugging.
- Blower motor resistor
- A resistor block reduces voltage to the blower motor for lower speeds. An open resistor causes loss of some blower speeds.
- Thermistor operation
- A thermistor is a temperature sensor whose resistance decreases as temperature increases (NTC). It is used to sense evaporator temperature.
- Heater core function
- The heater core uses engine coolant to warm air for passenger heating and defrost. A clogged core causes poor heat output.
- Blend door purpose
- The blend door controls the mix of heated and cooled air to achieve the desired temperature. A stuck door causes temperature control issues.
- Performance test procedure
- Measure vent temperature, ambient temperature, and humidity. Compare to specifications. Low vent temperature with normal pressures indicates proper operation.
- Leak detection methods
- Use electronic leak detectors, UV dye, or soap bubbles. Evacuate and recharge with trace refrigerant if needed to locate leaks.
- PAG oil hygroscopicity
- PAG oil absorbs moisture from the air. Keep containers sealed and use fresh oil to prevent moisture contamination in the system.
- Refrigerant identifier
- An SAE J177 or J2912 refrigerant identifier verifies the type of refrigerant in a system before recovery to prevent mixing.
- Compressor oil type
- R-134a systems use PAG oil. Adding oil requires knowing system capacity and compensating for oil removed with components.
- Evaporator icing cause
- Evaporator icing can occur if the expansion valve is stuck open or the evaporator temperature sensor fails, blocking airflow.
- TXV superheat setting
- The thermostatic expansion valve regulates refrigerant flow to maintain about 5-10°F (3-6°C) superheat at the evaporator outlet.
- Pressure switch types
- Binary pressure switches are on/off; analog switches provide variable signals. Both protect the compressor by cutting off at high or low pressures.
- Defrost operation
- Defrost directs heated air to the windshield. The A/C compressor may run to dehumidify air for faster defrosting.
- Subcooling definition
- Subcooling is the difference between the actual liquid refrigerant temperature and its saturation temperature at the same pressure. It indicates proper charge.
- Recovery equipment certification
- Recovery equipment must be EPA-certified and capable of recovering at least 80% of the refrigerant in a system.
- Clutch coil resistance
- The compressor clutch coil typically has a resistance of 2-5 ohms. Check for open or shorted coil if the clutch does not engage.
- Suction line insulation
- The suction line should be insulated to prevent condensation and heat gain. A cold suction line indicates proper operation.
- Sight glass indication
- A clear sight glass on the liquid line indicates a full charge. Bubbles indicate low charge or a restriction.
- CAN bus termination resistance
- The CAN bus should have about 60 ohms termination resistance. Incorrect resistance indicates wiring or module faults.
- Cabin air filter
- A clogged cabin air filter restricts airflow. Replace at recommended intervals to maintain passenger comfort.
- Restriction diagnosis
- A restriction causes a temperature drop across the restricted component (e.g., orifice tube or filter-drier). Identify by touch or temperature measurement.
- System flush requirement
- After compressor failure, flush the system to remove debris. Use approved flushing agent and replace the filter-drier.