NVQ L4 Automobile AC Technician flashcards

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

A2L refrigerant classification

A safety classification for refrigerants with lower flammability, including R-1234yf. Requires ignition-source control, ventilation, and leak detection during service.

Refrigerant asphyxiation risk

Halogenated refrigerants displace oxygen in confined spaces, causing oxygen depletion. Always ventilate and use gas detection when working in pits or enclosed bays.

Thermal decomposition products

Heating refrigerant on flames or hot surfaces produces hydrogen fluoride and carbonyl fluoride, which are toxic. Purge with nitrogen before brazing and wear PPE.

Cylinder hydrostatic test interval

Refrigerant cylinders require periodic hydrostatic testing, typically every five years, and must never be filled above marked capacity or tare plus charge weight.

Recovery versus recycling

Recovery removes refrigerant from a system. Recycling cleans it on site by removing oil and particulates but does not restore it to new-product specification.

Reclamation specification

Reclamation restores refrigerant to AHRI 700 purity through laboratory processing and analysis. Only reclaimed refrigerant may be resold or returned to a different owner's system.

Cylinder 80 percent fill rule

Recovery cylinders must not exceed 80 percent of water capacity by weight, calculated from tare weight plus refrigerant capacity, to allow for thermal expansion.

Refrigerant identifier use

An identifier tests refrigerant purity before recovery into a shared cylinder. Contaminated or mixed refrigerant must be sent for reclamation, never blended.

PT chart use

Pressure-temperature charts convert measured pressure to saturation temperature. Compare to ambient to judge charge level before diagnosing a fault.

Restriction diagnosis

Low suction pressure with high discharge pressure and a hot compressor suggests a restriction in the expansion valve, orifice tube, or receiver-drier.

Electronic leak detection

Heated diode or infrared detectors sense refrigerant traces in air. More sensitive than UV dye and suitable for slow leaks, but require calibration and a clean environment.

Nitrogen pressure test

Pressurizing the system with dry nitrogen to 10-15 bar and watching for decay locates leaks without adding moisture. Never use oxygen or shop air.

Vacuum hold test

Evacuate below 500 microns and isolate. A slow rise that plateaus indicates moisture boiling off; a continuous rise indicates a leak.

Receiver-drier replacement rule

Replace the receiver-drier or accumulator whenever the system is opened to atmosphere, because the desiccant saturates and releases moisture and acid into the circuit.

PAG oil hygroscopicity

PAG oil absorbs moisture rapidly from air. Keep containers sealed, use the specified viscosity, and never mix with mineral oil or unapproved oils.

O-ring material compatibility

HFC refrigerants require HNBR or neoprene seals. Mineral-oil-compatible seals swell or degrade with R-134a and R-1234yf, causing leaks.

Evacuation target level

Evacuate to 500 microns (0.67 mbar) or lower and hold for at least ten minutes. This removes moisture and non-condensable gases before charging.

Charge by weight

Weigh refrigerant into the system using a scale. Pressure-based charging is unreliable because pressure varies with ambient temperature and condenser airflow.

Subcooling target

Subcooling equals condenser outlet temperature minus saturation temperature at high-side pressure. A target of 5-10 K confirms adequate charge and condenser performance.

Superheat target

Superheat equals suction temperature minus saturation temperature at low-side pressure. A target of 5-10 K at the evaporator outlet indicates correct charge.

Performance test conditions

Set max cold, blower high, windows open, engine at 1500-2000 rpm, and ambient above 20 degrees Celsius. Record vent temperature, typically 4-10 degrees.

Three-wire pressure sensor

A three-wire sensor outputs a voltage proportional to refrigerant pressure. Diagnose by comparing measured voltage to the manufacturer's pressure-voltage chart.

Evaporator temperature sensor role

The evaporator temperature sensor prevents icing by cycling the compressor. A failed sensor causes rapid cycling, no engagement, or evaporator freeze-up.

CAN bus AC fault

Communication faults on the CAN bus can disable compressor operation. Scan all modules for communication codes before condemning the compressor or clutch.

Blend door actuator fault

A stripped gear or lost calibration in a blend door actuator causes temperature differences between vents. Recalibrate or replace the actuator after verifying linkage.

Cabin air filter restriction

A clogged particulate or carbon filter reduces airflow and cooling performance. Measure pressure drop across the filter and replace when above specification.

Recirculation mode trade-off

Recirculation reduces cooling load but raises cabin humidity and fogging risk. Fresh-air mode is required for effective defogging and demisting.

Defrost mode operation

Defrost uses heater core flow plus AC dehumidification. Blend door position and AC operation both affect clearing speed and windshield fogging.

R-1234yf GWP value

R-1234yf has a global warming potential of about 4, compared with about 1430 for R-134a, and is classified A2L due to lower flammability.

R-744 CO2 system pressure

R-744 carbon dioxide systems operate at pressures up to about 130 bar. They require dedicated high-pressure equipment, hoses, and CO2-compatible oil.

EV heat pump service

Electric vehicle heat pumps use the refrigerant circuit for heating and cooling. Service requires high-voltage safety training and manufacturer-specific procedures.

Service record retention

Job cards should record refrigerant type, charge weight, oil type and quantity, vacuum level, leak test result, and vent temperature, retained per local regulations.