ASE S7 Air Conditioning Systems & Controls (School Bus)

A school bus A/C system uses R-134a. Which refrigerant identifier and service practice applies?

• AR-134a is a non-flammable HFC requiring SAE J2788 recovery/recycling equipment
• BR-134a is a flammable HFO requiring spark-proof tools only
• CR-134a is the same as R-12 and uses mineral oil
• DR-134a requires no recovery because it is environmentally inert

Before opening any school bus A/C refrigerant line for service, the technician must first:

• AAdd dye to the system
• BRecover the refrigerant with certified recovery equipment
• CRun the engine at high idle for 10 minutes
• DDisconnect the bus batteries

A manifold gauge set connected to a school bus A/C system reads high on the low side and low on the high side with the compressor running. The most likely cause is:

• AAir in the system
• BOvercharged system
• CInternal compressor problem (failed reed valves or worn pump)
• DRestricted orifice tube

The correct way to determine the refrigerant charge for a school bus A/C system is to:

• ACharge until the low side reads exactly 30 psi
• BAdd one full 30 lb cylinder per bus
• CCharge until the sight glass clears
• DCharge by weight per the manufacturer underhood label or service data

A bus A/C system is being evacuated. The purpose of the vacuum (evacuation) step is to:

• ARemove air and boil off moisture before recharging
• BTest compressor clutch operation
• CPressurize the system for a leak test
• DAdd refrigerant oil to the system

Tech A says a school bus with dual (front and rear) evaporators normally holds more refrigerant than a single-evaporator car system. Tech B says the additional rear lines and evaporator increase total system oil requirements. Who is correct?

• ATech B only
• BBoth Tech A and Tech B
• CNeither
• DTech A only

During an A/C performance test on a school bus, ambient is 90°F and the center duct discharge reads 65°F. This indicates:

• AA faulty thermostat only
• BNormal cooling performance
• CPoor cooling performance requiring further diagnosis
• DAn overcharged system only

Non-condensable air in a school bus A/C system most commonly causes:

• ANo effect on system pressures
• BA short cycling low-pressure switch
• CLower than normal high-side pressure
• DHigher than normal high-side pressure and reduced cooling

Subcooling is best described as the:

• ACooling of liquid refrigerant below its saturation (condensing) temperature
• BPressure drop across the orifice tube
• CHeat absorbed by the receiver-drier
• DTemperature rise of refrigerant vapor leaving the evaporator

Superheat is measured at the:

• ACondenser inlet
• BEvaporator outlet
• CReceiver-drier
• DCompressor discharge port

A school bus A/C system shows frost forming on the liquid line just after the orifice tube and poor cooling. This most likely indicates:

• AA failed compressor clutch coil
• BA normal condition at low ambient
• CA restriction at the orifice tube screen
• DAn overcharged system

When recharging a school bus A/C system, refrigerant oil should be:

• ANever added under any circumstance
• BReplaced with engine oil if A/C oil is unavailable
• CAdded in unlimited quantity for safety
• DAdded in the type and amount specified for the system and replaced components

R-1234yf is increasingly used in newer vehicles. Compared with R-134a it is:

• AA mildly flammable HFO with a much lower global warming potential
• BA non-flammable CFC banned for mobile A/C
• CIdentical in pressure and oil to R-12
• DCharged by sight glass only

A refrigerant identifier should be used before recovery on a bus A/C system to:

• AMeasure system superheat
• BDetect contaminated or blended refrigerant before it enters the recovery machine
• CSet the compressor clutch air gap
• DCalculate evaporator airflow

A bus A/C system reads high on both the low and high side gauges with poor cooling. The most likely cause is:

• AOpen compressor clutch circuit
• BLow refrigerant charge
• COvercharge or condenser airflow/cooling problem
• DRestricted orifice tube

A bus A/C system reads low on both the low and high side with poor cooling and the compressor short cycling on the low-pressure switch. The most likely cause is:

• AAir in the system
• BCondenser fan inoperative
• COvercharge
• DLow refrigerant charge / leak

The preferred method to leak test a school bus A/C system after repair before recharging is to:

• APressurize with nitrogen and trace gas or use an electronic leak detector / UV dye
• BRun the bus until cooling fails
• CApply soapy water to the dash
• DCharge fully and watch the sight glass

Tech A says a deeper vacuum and longer evacuation time help remove more moisture from a large bus system. Tech B says moisture in the system can combine with refrigerant and oil to form acids. Who is correct?

• ATech B only
• BBoth Tech A and Tech B
• CNeither
• DTech A only

A pressure-temperature (P-T) chart is used during bus A/C diagnosis primarily to:

• AIdentify refrigerant oil type
• BSet blower motor speed
• CRelate saturation pressure to temperature to evaluate charge and condenser performance
• DMeasure clutch air gap

After replacing a leaking evaporator on a school bus, the technician should also:

• AReplace the engine thermostat
• BFlush the cooling system
• CReplace the alternator
• DReplace the receiver-drier or accumulator and add the proper oil amount

An A/C system with very low refrigerant charge often will not cool because:

• AThe low-pressure switch opens and disengages the compressor clutch
• BThe blower motor stops
• CThe condenser fan runs continuously
• DThe high-pressure switch opens

A UV dye is added to a bus A/C system to:

• AIncrease cooling capacity
• BHelp locate refrigerant leaks under a UV lamp
• CLubricate the compressor
• DLower the refrigerant boiling point

On a school bus with a rear A/C unit, weak rear cooling but good front cooling most likely points to:

• AA blown main A/C fuse
• BA failed main compressor
• CA problem in the rear evaporator circuit, rear expansion device, or rear blower
• DLow engine coolant

Which statement about charging a system by weight is correct?

• ACharge until the compressor stops cycling
• BWeight charging only applies to TXV systems
• CAlways overshoot the spec by 20% for hot climates
• DUse a calibrated charging scale and the exact specified weight for that system

During recovery on a bus A/C system, the recovery machine repeatedly rises back into a positive pressure after shutting off. This usually indicates:

• ARefrigerant is still boiling out of the oil and more recovery time is needed
• BThe high-pressure switch failed
• CThe orifice tube is missing
• DThe recovery is complete

A bus A/C system holds a vacuum for the required time during evacuation but loses vacuum when the gauges are isolated overnight. This indicates:

• AThe system is sealed and ready to charge
• BA small leak that must be located and repaired before charging
• CThe refrigerant oil is contaminated
• DThe compressor clutch is faulty

PAG oil and POE oil are both used with R-134a systems. A key reason a manufacturer may specify POE oil is:

• AIt increases refrigerant pressure
• BIt is cheaper than PAG
• CIt is more compatible with certain compressor designs and hybrid/electric A/C systems
• DIt eliminates the need for a receiver-drier

The duct temperature 'split' used in performance testing refers to the:

• ADifference between engine and ambient temperature
• BDifference between front and rear evaporator size
• CDifference between high-side and low-side pressure
• DDifference between inlet air temperature and outlet (discharge) air temperature at the ev…

A bus A/C system with a slightly low charge typically shows:

• ABubbles in the sight glass (if equipped) and reduced cooling
• BExcessively high high-side pressure
• CCompressor seizure
• DHigher than normal cooling

Which is the correct service sequence after locating and repairing a refrigerant leak on a bus?

• ARecharge, then evacuate, then leak test
• BRecover (if needed), repair, replace desiccant, evacuate, then recharge by weight
• CEvacuate, recharge, then recover
• DRecharge, run, then add dye

On a TXV (thermostatic expansion valve) bus system, low cooling with low-side pressure too low and high superheat usually indicates:

• AAn overcharged system
• BA shorted blower resistor
• CTXV starving the evaporator (stuck closed, lost bulb charge, or restriction)
• DTXV flooding the evaporator

The magnetic clutch on an A/C compressor functions to:

• AFilter refrigerant moisture
• BControl blower speed
• CRegulate refrigerant pressure
• DEngage and disengage the compressor drive from the engine accessory belt

The compressor clutch air gap is measured to:

• AVerify the clutch will engage and disengage reliably within specification
• BMeasure belt tension
• CCheck refrigerant oil level
• DSet refrigerant charge

A compressor clutch will not engage. The clutch coil tests open (infinite resistance). The correct conclusion is:

• AThe clutch coil is good
• BThe clutch coil is open and must be replaced
• CThe refrigerant is low
• DThe blend door actuator failed

Tech A says a clutch that slips under load can cause poor cooling and a burnt smell. Tech B says excessive clutch air gap can prevent full engagement and cause slipping. Who is correct?

• ATech A only
• BTech B only
• CBoth Tech A and Tech B
• DNeither

A condenser on a school bus is often roof- or skirt-mounted and larger than a car's because:

• AIt reduces refrigerant charge
• BIt replaces the receiver-drier
• CBuses do not need much cooling
• DThe system must reject heat from multiple/large evaporators serving a big cabin

A bus A/C system has high high-side pressure on a hot day. The condenser fins are packed with debris. The correct repair is to:

• AClean the condenser to restore airflow and heat rejection
• BReplace the compressor
• CBypass the high-pressure switch
• DAdd refrigerant

A receiver-drier is used in systems with a:

• AVariable blower only
• BThermostatic expansion valve (TXV)
• CFixed orifice tube only
• DNo metering device

An accumulator is used in systems with a:

• ABelt-driven blower
• BThermostatic expansion valve
• CFixed orifice tube
• DNo desiccant

A bus evaporator that frequently freezes over (ices up) and blocks airflow most likely has a problem with the:

• ACompressor mounting bracket
• BRefrigerant identifier
• CCondenser fan
• DEvaporator temperature/thermostatic control or cycling control

A scroll-type A/C compressor differs from a reciprocating piston compressor in that it:

• ACompresses refrigerant with orbiting scrolls instead of pistons and reed valves
• BCannot be used on buses
• CRequires R-12 only
• DUses no lubricant

A variable-displacement compressor controls cooling capacity by:

• ACycling the clutch on and off only
• BChanging piston stroke/displacement based on system demand
• CVarying refrigerant type
• DChanging the orifice tube size

A noisy A/C compressor with metal debris found in the system at disassembly requires that the technician also:

• AReplace only the orifice tube
• BOnly replace the compressor
• CFlush the system or replace affected components and the desiccant to remove debris
• DAdd extra refrigerant

On an orifice-tube system, the orifice tube's function is to:

• AStore excess refrigerant
• BDrive the compressor
• CRemove moisture from the refrigerant
• DMeter high-pressure liquid into the evaporator as a low-pressure spray

A TXV regulates refrigerant flow based on:

• AEvaporator outlet temperature/pressure (superheat) via a sensing bulb
• BBattery voltage
• CAmbient humidity only
• DEngine RPM

A TXV sensing bulb that has lost its charge will most likely cause the valve to:

• AHold a perfect superheat
• BDefault toward closed, starving the evaporator and reducing cooling
• CIncrease refrigerant charge
• DOpen the high-pressure switch

A bus front evaporator cools well but the rear evaporator does not cool and the rear expansion device is suspected. The best first diagnostic step is to:

• AReplace the rear blower motor
• BReplace the compressor immediately
• CCheck rear evaporator inlet/outlet temperatures and rear circuit refrigerant flow
• DAdd refrigerant until rear cools