A7 Heating &
Air Conditioning
Airflow from all vents is weak at every blower speed, but A/C vent temperature measures 38°F. Blower motor current draw is normal. Which of these could be the cause?
Technician A says pulling a deep vacuum lowers the boiling point of moisture so it vaporizes and is drawn out of the system. Technician B says evacuation also removes acidic refrigerant oil. Who is right?
An R-134a system is performance tested at 85°F ambient with the engine at 1,500 rpm. Which of these could be the cause of these readings?
| Measurement | Specification | Actual |
|---|---|---|
| Low-side pressure | 25–35 psi | 10 psi |
| High-side pressure | 200–250 psi | 120 psi |
| Center vent temperature | 40–50°F | 68°F |
Which of these is the LEAST likely cause of engine overheating that occurs only in stop-and-go traffic?
A positive temperature coefficient (PTC) heater element in the HVAC case is used to:
Technician A says the blower motor resistor block is bypassed on the highest blower speed. Technician B says a solid-state blower control module varies motor speed using pulse-width modulation. Who is right?
During a 20-minute winter drive, coolant temperature stays near 150°F and the heater blows cool. Coolant level is correct. What is the MOST likely cause?
Coolant electrolysis is suspected of eroding a heater core. The correct test is to:
Water drips onto the passenger floor whenever the A/C runs in humid weather. Cooling performance and coolant level are normal. Which of these could be the cause?
A compressor clutch is slow to engage and slips under load. Clutch air gap measures 0.042 in., 0.045 in., and 0.041 in. Specification is 0.016–0.031 in. The correct repair is to:
Technician A says an A/C clutch coil measuring 3.5 ohms is open and must be replaced. Technician B says the clutch coil diode suppresses the voltage spike produced when the coil de-energizes. Who is right?
The desiccant sealed inside an accumulator is used to:
Which is the LEAST likely cause of vent air shifting to the defrost outlets during hard acceleration?
Heater output is lukewarm at every temperature setting. The engine reaches 200°F, both heater hoses are hot, and coolant level is correct. Which of these could be the cause?
R-1234yf recovery equipment built to SAE J2843 will not begin recovery until:
High-side pressure reads 340 psi at 85°F ambient with warm vent air. The condenser is clean, the fan runs, and the charge is correct. What is the MOST likely cause?
Technician A says a stepper-motor mode door actuator should be tested by measuring coil resistance with its connector attached. Technician B says an actuator that clicks repeatedly indicates a failed control module. Who is right?
The inside of the windshield fogs over in cool, damp weather. Any of these could be the cause EXCEPT:
A cooling system holds 15 psi for two minutes with no visible leak. On the tester, the 16 psi pressure cap releases at 9 psi. This indicates:
A three-wire A/C refrigerant pressure transducer signals the control module to:
The climate control display is blank and the blower will not run, but the radio and cluster work normally. A U-code is stored. Which of these could be the cause?
Technician A says a dual-zone system uses a separate blend door for each side of the HVAC case. Technician B says a dual-zone system uses two separate evaporator cores. Who is right?
A blower motor runs at full speed with the ignition on and the blower switch OFF. What is the MOST likely cause?
Which is the LEAST likely cause of a compressor clutch that engages, then drops out after a few seconds?
A blower motor turns slowly at every speed setting. The most useful next test is to:
Reading Your Results: A7 Heating and Air Conditioning, Test 1
A score by itself only tells you whether you passed. What follows is how to read the questions you missed on this test. A7 is really three systems sharing one dashboard, and most wrong answers come from diagnosing in the wrong one.
If you missed the questions about which system owns the complaint
Refrigeration, engine cooling, and air distribution all end up at the same vents, so the first job on any A7 question is deciding which of the three is at fault. Weak airflow at every blower speed with vent temperature at 38 degrees is an air-side problem, because the refrigeration side is clearly working. Lukewarm heat with the engine at 200 degrees and both heater hoses hot means the coolant is arriving and the blend door or the case is the problem. Water on the passenger floor with normal cooling and normal coolant level is condensate that cannot drain, not a leak from either system. Before choosing an answer, ask whether the evidence points at refrigerant, at coolant, or at air, because each one has a completely different set of causes.
If you missed the questions built on a measurement
Three questions here hand you readings and expect you to interpret them against what the system should be doing.
If you missed the electrical questions
A large share of A7 is really electrical work that happens to be inside an HVAC case. A blower that runs at full speed with the switch off means something is supplying the motor around the control, since the module or relay is stuck on rather than failing to turn on. A blower that turns slowly at every speed is a resistance problem in the feed or the ground rather than a speed control fault, because every speed is affected equally. A clutch coil measures a few ohms and is normal at that value, so a resistance reading only condemns a coil when it is open or shorted. When an HVAC complaint involves a motor or a clutch, measure the circuit before replacing the component.
If you missed the EXCEPT and LEAST likely questions
Four appear on this test, and missing more than one is a reading habit rather than a knowledge gap. Work out what the three real causes share before you look for the odd one. On the stop-and-go overheating question, three answers all reduce the cooling the system provides when airflow depends on the fan rather than road speed, and one is a fault that would show up at highway speed too. On the windshield fogging question, three answers all leave moisture in the cabin air, and one does not affect humidity at all. Naming the shared trait first turns a four-way guess into a single check.