ASE H7 HVAC (Transit Bus) cheat sheet

Refrigerant R-134a is classified as A1 (low toxicity, no flame propagation) under ASHRAE Standard 34.

Section 609 of the Clean Air Act requires certification for technicians servicing motor vehicle air conditioning systems.

Ventilation: refrigerant vapor is heavier than air and can displace oxygen in low areas; use exhaust ventilation when leaks are suspected.

Pressure-temperature relationship: for R-134a, saturated pressure at 0°C (32°F) is about 30 psig; at 25°C (77°F) about 77 psig.

Personal protective equipment: safety glasses with side shields, gloves, and long sleeves are required when working with refrigerant or solvents.

Recovery cylinder filling: never fill a recovery cylinder beyond 80% of its capacity by weight; use a scale.

Refrigerant oil (PAG) is hygroscopic; keep containers sealed and use fresh oil to avoid moisture contamination.

System capacity is affected by ambient temperature, humidity, and airflow; performance tests must account for these variables.

Ozone depletion potential (ODP) of R-134a is 0; global warming potential (GWP) is approximately 1300-1430.

Leak detection: electronic leak detectors are sensitive to halogenated refrigerants; verify detector calibration before use.

Recovery equipment must be EPA-certified and capable of recovering at least 80% of the refrigerant in a system.

Evacuation: a system must be evacuated to a vacuum of 500 microns (29.9 inHg) and hold for 15-30 minutes to remove moisture and non-condensables.

Recovery cylinder: use only DOT-approved recovery cylinders; never fill beyond 80% capacity by weight.

Recovery process: liquid recovery is faster than vapor recovery; use the liquid port when possible.

Recycling: removes moisture, acid, and particulates from refrigerant; it does not restore to virgin specifications.

Refrigerant identifiers: use an SAE J177 or J2912 refrigerant identifier to verify refrigerant type before recovery.

Never mix refrigerants; mixed refrigerants cannot be recycled and must be destroyed.

Recovery machine filters: replace filter-drier and oil separator as recommended by the manufacturer.

Vacuum pump: use a two-stage pump with a vacuum gauge; isolate the pump from the system before turning off to prevent oil backflow.

System charging: charge refrigerant as a liquid (using a charging scale) to avoid fractionation of blends; R-134a is a single-component refrigerant and can be charged as vapor, but liquid charging is faster.

Compressor function: increases refrigerant pressure and temperature by compressing low-pressure vapor into high-pressure vapor.

Clutch operation: the electromagnetic clutch engages the compressor when 12V is applied to the coil; check for coil resistance (typically 2-5 ohms).

Clutch air gap: typical specification is 0.4-0.8 mm (0.016-0.031 in); excessive gap prevents engagement.

Compressor oil: PAG oil is used with R-134a; adding oil requires knowing the system capacity and compensating for removed components.

Belt tension: check deflection or use a tension gauge; a slipping belt can cause poor cooling and noise.

Compressor types: reciprocating, rotary vane, scroll, and swash plate; variable displacement compressors are common in transit buses.

High-pressure cutoff: compressor clutch is disengaged if pressure exceeds a set threshold (typically 400-450 psig) to prevent damage.

Low-pressure cutoff: clutch disengages if pressure falls below a set threshold (typically 20-30 psig) to prevent compressor damage from lack of refrigerant.

Compressor failure diagnosis: check for abnormal noise, oil contamination (black oil indicates overheating), and metal debris in the system.

Condenser function: rejects heat from the refrigerant to the outside air, causing high-pressure vapor to condense into liquid.

Condenser performance: affected by airflow, ambient temperature, and fin cleanliness; restricted airflow causes high head pressure.

Evaporator function: absorbs heat from the passenger compartment, causing low-pressure liquid refrigerant to vaporize.

Evaporator icing: can occur if the expansion valve is stuck open or the evaporator temperature sensor fails; ice blocks airflow.

Refrigerant lines: use O-ring seals; lubricate O-rings with refrigerant oil, not petroleum jelly.

Line routing: liquid line from condenser to expansion valve; suction line from evaporator to compressor; discharge line from compressor to condenser.

Suction line: should be insulated to prevent condensation and heat gain; a cold suction line indicates proper operation.

Filter-drier (receiver-drier): located on the liquid line; removes moisture and debris; must be replaced whenever the system is opened.

Pressure drop: excessive pressure drop across the condenser or evaporator indicates a restriction or dirty coil.

Expansion valve function: regulates refrigerant flow into the evaporator to maintain a superheat of about 5-10°F (3-6°C) at the evaporator outlet.

TXV types: internally equalized (uses evaporator inlet pressure) and externally equalized (uses evaporator outlet pressure).

Superheat calculation: superheat = actual evaporator outlet temperature - saturation temperature at that pressure.

Orifice tube: fixed restriction; system uses an accumulator to store excess refrigerant and prevent liquid slugging.

TXV diagnosis: if the valve is stuck closed, low suction pressure and no cooling; if stuck open, high suction pressure and possible evaporator icing.

H-block valve: combines expansion valve and distributor; used in some systems.

Refrigerant charge: undercharge causes low suction pressure and bubbles in the sight glass (if equipped); overcharge causes high head pressure.

Sight glass: on liquid line; clear indicates full charge; bubbles indicate low charge or restriction.

Accumulator: used with orifice tube systems; contains desiccant and oil bleed hole; must be replaced if system is opened.

Blower motor circuit: includes fuse, relay, resistor block (for speed control), and motor; check voltage drops across each component.

Blower resistor: reduces voltage to motor for lower speeds; open resistor causes loss of some speeds.

Pressure switches: binary (on/off) or analog; high-pressure cutoff and low-pressure cutoff protect the compressor.

Thermistor: used for evaporator temperature sensing; resistance decreases as temperature increases (NTC).

Actuators: blend door actuators (electric or vacuum) control temperature; mode door actuators control airflow direction.

HVAC control module: receives inputs from sensors and controls outputs; scan tool can retrieve trouble codes.

CAN bus: used for communication between HVAC module and other systems; check for proper termination resistance (about 60 ohms).

Relay testing: check coil resistance and contact continuity; a clicking relay may still have faulty contacts.

Sensor calibration: some sensors require calibration after replacement; follow manufacturer procedures.

Heater core: uses engine coolant to warm air; a clogged core causes poor heat output; check inlet and outlet hose temperatures.

Blend door: controls the mix of heated and cooled air; a stuck door can cause temperature control issues.

Defrost: uses heated air directed to the windshield; the A/C compressor may run to dehumidify air for faster defrosting.

Air distribution: mode doors direct air to floor, panel, or defrost; vacuum or electric actuators control doors.

Cabin air filter: restricts airflow if clogged; replace at recommended intervals.

Blower motor: variable speed control via resistor or PWM; check for seized bearings or worn brushes.

Passenger comfort: air temperature, humidity, and airflow velocity affect perceived comfort; HVAC system must maintain set point.

Heater control valve: controls coolant flow to heater core; a faulty valve can cause no heat or constant heat.

Air mix door calibration: some systems require recalibration after battery disconnect or actuator replacement.

Performance test: measure vent temperature, ambient temperature, and humidity; compare to manufacturer specifications.

Pressure readings: low side 25-45 psig, high side 200-250 psig (varies with ambient); abnormal readings indicate system faults.

Superheat and subcooling: superheat at evaporator outlet (5-10°F) and subcooling at condenser outlet (10-20°F) indicate proper charge.

Leak detection: use electronic leak detector, UV dye, or soap bubbles; evacuate and recharge with trace refrigerant if needed.

Restriction diagnosis: a restriction causes a temperature drop across the restricted component (e.g., orifice tube or filter-drier).

Compressor efficiency test: compare high and low side pressures; a worn compressor may not build adequate pressure.

Electrical diagnosis: use a multimeter to check voltage, resistance, and continuity; check grounds for corrosion.

Scan tool: retrieve HVAC trouble codes and live data (sensor readings, actuator positions).

System flush: required after compressor failure to remove debris; use approved flushing agent and replace filter-drier.