A medium-duty truck A/C system cools poorly. The technician finds the discharge air at the dash vents is only 15°F below ambient and the high-side pressure is normal. Which is the BEST first diagnostic step?
Hvac System Diagnosis
Question · Medium
Technician A says low airflow at the vents on a sleeper-cab truck can be caused by a clogged cabin air filter. Technician B says it can be caused by an evaporator iced over from a stuck-open expansion valve allowing too much refrigerant. Who is correct?
Chapter 1: Exam Overview, Safety & Complaint Verification
1.1 ASE T7 Exam Format, Task List & Question Strategy
Study guide
During a performance test on a Class 8 truck A/C system at 1,500 rpm with doors open, the manifold gauges read 45 psi low side and 350 psi high side on R-134a with high ambient temperature. Vent temperature is high. What does this MOST likely indicate?
Question · Hard
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A truck owner reports the A/C 'works on the highway but blows warm at idle.' Which condition BEST explains this symptom?
Before connecting a manifold gauge set or recovery machine to a truck A/C system, the technician must FIRST identify which refrigerant the system uses. What is the safest way to confirm whether the system is R-134a or R-1234yf?
Question · Easy
A technician suspects a refrigerant leak on a heavy truck A/C system. Which method is the MOST reliable for pinpointing a small slow leak?
1.2 Shop Safety, PPE & Identifying the Truck's Power Source
A bunk/sleeper HVAC unit on a long-haul tractor will not cool while the auxiliary power unit (APU) runs, but the main dash A/C cools normally. Where should diagnosis focus FIRST?
A truck A/C system shows bubbles in the receiver-drier sight glass during operation in moderate ambient temperature. What does this MOST commonly indicate?
A heavy truck A/C system has normal pressures but the discharge air temperature does not drop until engine rpm is raised. Which component is the MOST likely cause?
Technician A says a noisy A/C system that quiets after a few minutes of operation may have a low refrigerant charge. Technician B says the same noise can be caused by a failing compressor bearing. Who is correct?
1.3 Verifying the Complaint: Interview, Records, TSBs, Noises & Sensory Checks
A truck's A/C produces a musty odor from the vents when the system first starts. What is the MOST likely cause?
When performing an A/C performance test on a medium-duty truck, which set of conditions gives the MOST representative readings?
A truck A/C system cools well for about 20 minutes, then gradually loses cooling while ice forms on the evaporator. After it warms up, cooling returns. What is the MOST likely cause?
A technician connects manifold gauges to an R-134a truck system. With the compressor off and ambient at 75°F, both gauges should read approximately what?
Chapter 2: HVAC Performance Testing, Gauges & Leak Detection
2.1 Systematic HVAC Performance Testing & Diagnostic Methodology
A driver complains the A/C only blows cold from the floor vents, not the dash vents, regardless of the panel setting. Which area should be diagnosed FIRST?
Which refrigerant is increasingly used in newer truck and equipment A/C systems because of its very low global warming potential (GWP)?
Ac System And Component
Why must R-134a and R-1234yf service equipment be kept separate and not interchanged?
What is the correct sequence when servicing a truck A/C system that requires component replacement?
2.2 Manifold Gauge Pressure Analysis & Diagnostic Logic
Why is a deep vacuum (evacuation) pulled on a truck A/C system before recharging?
After evacuating a truck A/C system, the technician closes the valves and the vacuum holds for 5 minutes, then slowly rises. What does this indicate?
Which type of compressor lubricant is specified for most R-134a truck A/C systems?
Which compressor lubricant is commonly required for hybrid or electric-drive A/C compressors that use an electrically isolated motor?
2.3 Superheat and Subcooling Analysis in Commercial Trucks
When replacing an A/C compressor on a heavy truck, why must the technician account for the oil distributed in the old components?
A magnetic-clutch A/C compressor will not engage. The clutch coil has correct resistance and battery voltage is present at the connector when commanded on. What is the MOST likely cause?
Ac Compressor And Clutch
How is the air gap on a typical magnetic A/C compressor clutch measured and adjusted?
Technician A says a swash-plate (axial piston) compressor is a common fixed-displacement truck A/C compressor. Technician B says a variable-displacement compressor changes its output without cycling the clutch. Who is correct?
2.4 Refrigerant Leak Detection, Purity & Identification
A truck A/C compressor failed catastrophically with metal debris in the system. Besides replacing the compressor, what is the MOST important additional service?
A new A/C compressor seizes shortly after installation on a heavy truck. Which is the MOST likely cause?
What is the function of the compressor clutch in a cycling-clutch truck A/C system?
An A/C compressor clutch cycles rapidly (short cycling) in cool weather. Which condition is the MOST likely cause?
Chapter 3: Refrigerant Management, Recovery, Oil & Charging
3.1 Environmental Regulations, Safety & Refrigerant Properties (R-134a & R-1234yf)
What is the function of a thermal expansion valve (TXV) in a truck A/C system?
A TXV-equipped truck system has very low low-side pressure, high superheat, frost only at the inlet of the evaporator, and poor cooling. Charge is correct. What is the MOST likely cause?
A TXV is stuck open (overfeeding) on a truck A/C system. Which symptom set BEST matches this fault?
On an orifice-tube A/C system, which component on the low side removes moisture and acts as a vapor reservoir to protect the compressor?
Ac Evaporator Condenser
3.2 Refrigerant Recovery, Recycling & Deep Evacuation
Why must the receiver-drier or accumulator be replaced whenever a truck A/C system is opened for major repair?
The condenser on a heavy truck is partially blocked by road debris and bugs. Which set of symptoms is MOST expected?
What is the primary function of the condenser in a truck A/C system?
What is the primary function of the evaporator in a truck A/C system?
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