ASE H7 HVAC (Transit Bus) Feed

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A transit bus roof-mounted A/C unit will not cool. The technician's first step before connecting manifold gauges should be to:

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On a large transit-bus A/C system, refrigerant charge is most accurately set by:

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Tech A says R-134a and R-1234yf must never be mixed in the same system. Tech B says each refrigerant requires its own dedicated recovery/recycling equipment and service fittings. Who is correct?

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A high subcooling reading at the condenser outlet on a TXV transit-bus system, combined with a high head pressure, most likely indicates:

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Which lubricant is correct for an R-134a transit-bus A/C compressor specified for PAG oil?

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Before opening a transit-bus A/C system for component replacement, the refrigerant must be:

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A transit bus A/C system shows low side and high side both low, with poor cooling and bubbles where a sight glass exists. The most likely cause is:

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The purpose of the receiver-drier in a transit-bus TXV A/C system is to:

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A thermostatic expansion valve (TXV) on a bus evaporator controls refrigerant flow by maintaining a target:

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Tech A says a transit-bus system's high head pressure can be caused by air (non-condensables) in the system. Tech B says non-condensables are removed by a proper deep vacuum during evacuation. Who is correct?

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When evacuating a large transit-bus A/C system before charging, the primary reason for pulling and holding a deep vacuum is to:

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A bus A/C system holds a vacuum for two minutes, then the vacuum slowly rises (pressure increases) with the pump valved off. This indicates:

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The best method to locate a small refrigerant leak on a roof-mounted bus condenser is:

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A transit bus uses two A/C compressors driven by the engine. One compressor's clutch fails to engage while the other works. The first check should be the failed compressor's:

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Low evaporator superheat with the compressor running cold on the suction line and possible liquid slugging indicates:

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A large transit-bus condenser rejects heat most effectively when:

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The correct service order when replacing a leaking bus A/C compressor is:

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On a system that uses POE oil, the technician should know POE oil is typically specified for:

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Manifold gauges on an idling bus show normal-to-high low side, low high side, and weak cooling. The compressor is engaged. The most likely cause is:

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When recovering refrigerant from a large transit-bus system, the recovery is considered complete when:

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A bus A/C system performance test on a hot day yields a center-duct discharge temperature only a few degrees below ambient. The system holds a correct charge by weight and airflow is good. The next best check is:

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The compressor in a transit-bus A/C system primarily:

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Tech A says contaminated or moisture-laden refrigerant in a bus system can form acids that corrode components. Tech B says a saturated desiccant in the receiver-drier should be replaced whenever the system is opened for major service. Who is correct?

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Excessive evaporator superheat with low suction pressure and warm vent air on a TXV bus system most often indicates:

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Why do large transit buses commonly use roof-mounted A/C condenser/evaporator packages rather than a single under-dash unit?

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A refrigerant identifier should be used before recovering refrigerant from an unknown bus system mainly to:

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On an R-1234yf bus system, service equipment must include a refrigerant identifier and:

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During a charge-by-weight procedure, the technician should weigh refrigerant into a fully evacuated system using:

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A bus A/C system frosts the evaporator and airflow drops over time even though pressures look acceptable when first started. The most likely cause is:

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The high-pressure cutout switch in a bus A/C system protects the system by:

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A low-pressure cutout switch keeps the compressor from running when suction pressure is too low primarily to:

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When retrofitting or servicing, mixing PAG and POE oils in a single compressor is:

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A bus A/C system has correct charge by weight but high head pressure and high subcooling on a moderate day. The condenser fins are clean. The next likely cause to investigate is:

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The correct way to add refrigerant oil lost during a major component replacement on a bus A/C system is to:

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A sight of oil residue and dust accumulation at a hose fitting on a roof unit most likely indicates:

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After replacing a transit-bus evaporator, the technician should replace the receiver-drier or accumulator because:

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On a dual-evaporator transit bus, only the rear evaporator fails to cool while the front works. The most likely isolated cause is:

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The primary function of the bus A/C evaporator is to:

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When charging a large bus A/C system that takes many pounds of R-134a, liquid charging into the high side with the engine off is acceptable provided the technician:

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A bus A/C compressor runs but the suction line is warm and there is little temperature difference across the evaporator. Charge by weight is correct. This best indicates:

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Service fittings on R-134a and R-1234yf systems are intentionally different sizes/shapes to:

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Tech A says a noisy bus A/C compressor with metallic debris in the oil usually means the entire system should be flushed and the receiver-drier replaced. Tech B says you can just add refrigerant and return the bus to service. Who is correct?

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The main reason a transit-bus A/C system experiences severe duty compared with a passenger car is:

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A correctly performing bus A/C system at high ambient should show, at the evaporator, a:

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Subcooling is measured at the: