Five free practice questions on HVAC controls: content area C of the ASE A7 Heating & Air Conditioning test, 19 of its 50 scored questions (38%). They are in the ASE format: four options, one best answer. Answer each one before opening the explanation. No account, no email.
01The questions
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A blower resistor block's connector is found melted around one terminal, and that terminal is loose and corroded. Why does a poor connection like this get hot enough to melt plastic?
- The current through the added resistance makes heat
- The added resistance raises the current in the circuit
- The corrosion forms a short to ground at the terminal
- The motor draws more current as its voltage drops
Show the answer
A. Correct. Heat at a connection equals the current squared times its resistance, and a blower carries enough current that a small resistance at a loose, corroded terminal makes a lot of heat at that one spot. The terminal and connector are replaced, not just cleaned, because the heat has weakened the terminal's grip.
Why not the others
B. Added resistance lowers circuit current, it does not raise it. The heat comes from the current that still flows being forced through the resistance at the bad terminal.
C. A short to ground would blow the fuse rather than slowly heat one terminal. Corrosion adds resistance in series with the circuit, and current through that resistance is what heats the connector.
D. A blower motor slows and draws less current when its voltage drops, because its fan load falls with speed. The heat comes from current flowing through the resistance at the loose terminal.
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On many vehicles, when the A/C is requested at idle, the PCM increases idle airflow and waits a moment before engaging the compressor clutch. Why?
- To keep idle speed steady under the new load
- To keep the clutch from slipping as it engages
- To lower the high-side pressure before engagement
- To keep the evaporator from icing at idle
Show the answer
A. Correct. The compressor takes noticeable engine torque the instant it engages, and at idle that load would pull engine speed down or stall the engine. Adding airflow first, then engaging the clutch, lets the engine carry the new load without a dip.
Why not the others
B. Clutch engagement depends on the coil's magnetic pull, not on engine speed, and a small rise in idle does nothing for it. The PCM adds airflow so the engine can carry the compressor's load.
C. The engine cannot lower high-side pressure before the compressor runs; the compressor is what raises it. The added airflow is there to support idle speed under the new load.
D. Icing is controlled from evaporator pressure or temperature, not by idle airflow. The PCM adds airflow so the engine does not sag when the compressor engages.
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After a heater case is reinstalled, its temperature cable must be set so the blend door seals at both ends of its travel. On this vehicle the cable housing is held by a clamp at the case. How is the cable set?
- Hold the lever and the door at matching end stops, then clamp
- Hold the lever and the door at mid-travel, then clamp
- Hold the lever at one end and the door at the other, then clamp
- Clamp the housing, then bend the door arm to meet its stop
Show the answer
A. Correct. With the control lever and the blend door both held at the matching end of their travel, such as full cold, clamping the housing ties the two together so the door seals there. The lever is then swept end to end to confirm the door seats at both stops.
Why not the others
B. Matching midpoints does not make sure the door reaches its seal at either end; any small error leaves it short at one stop. The cable is set with lever and door at the same end stop.
C. The lever and the door must be at the same end of travel. Set at opposite ends, the door would be held against one stop and could not follow the lever.
D. Bending the door's arm distorts the linkage and can crack the door or case. The cable is set by the housing's position in its clamp, with lever and door at the same end stop.
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On a car whose climate module requests the compressor and whose PCM controls the clutch relay, any of these conditions could keep the compressor clutch from engaging EXCEPT:
- high-side pressure above the PCM's limit.
- an outside temperature below the system's minimum.
- a cabin temperature above the setting.
- the blower switched off.
Show the answer
C. Correct. A cabin warmer than the set temperature is a call for more cooling, so it is a reason to run the compressor, not to stop it. The other three are protective limits or interlocks that hold the clutch off.
Why not the others
A. Many PCMs open the clutch relay when the pressure sensor reads above their high-pressure limit, to protect the system from damage. The clutch re-engages once the pressure falls back.
B. Many systems will not run the compressor below a minimum outside air temperature, so a cold day, or an outside temperature input reading falsely low, can keep the clutch off.
D. On systems that allow the compressor to run only while the blower moves air across the evaporator, switching the blower off cancels the A/C request, because the evaporator would soon ice with no airflow.
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Some climate systems meter coolant through the heater core with an electric valve driven by a pulse-width-modulated (PWM) signal from the module. How does the module change the coolant flow?
- By changing the signal's frequency
- By changing the signal's duty cycle
- By switching vacuum to its diaphragm
- By stepping its supply voltage up or down
Show the answer
B. Correct. A PWM signal switches the valve on and off rapidly, and the share of each cycle it is on (the duty cycle) sets the average valve opening, so the module can hold any flow between closed and fully open. A scan tool shows the commanded duty cycle in percent.
Why not the others
A. A PWM output normally runs at a fixed frequency; at the same duty cycle, a faster or slower rate gives the same average opening. Flow changes with the on-time share, the duty cycle.
C. Switching vacuum to a diaphragm is how a vacuum-operated heater valve works. This valve is electric, and its flow is set by the duty cycle of the signal.
D. A PWM output switches a fixed supply voltage on and off rather than raising or lowering it. Flow changes with the percentage of on-time, the duty cycle.
02Answer key
| Question | Answer |
|---|---|
| Q1 | A |
| Q2 | A |
| Q3 | A |
| Q4 | C |
| Q5 | B |
03What area C covers
This content area covers how the heating and A/C system is switched and controlled: blower motors and speed controls, compressor clutch circuits and the PCM strategies that engage or cut out the compressor, cooling and condenser fans, electric and vacuum door actuators, cables, ducts and outlets, heater coolant valves, automatic temperature control sensors and calibration, and network faults that affect climate control.
The ASE test specification calls this content area “Operating Systems and Related Controls Diagnosis and Repair”. It carries 19 of the 50 scored questions, 38% of the test.
- Diagnose blower motors, resistors, modules and relays
- Test compressor clutch coils, relays, diodes and switches
- Check PCM and load-based compressor control
- Test cooling and condenser fan circuits
- Diagnose electric and vacuum door actuators and doors
- Adjust control cables, linkages and heater coolant valves
- Test ATC sensors, modules and calibration
- Trace network faults that affect climate control
04What the questions turn on
Check the command first
When the scan tool shows the PCM commanding a fan or clutch on, the inputs behind that decision are cleared and the fault lies after the command. If the command never comes, look at an input, such as a pressure transducer reading low.
Clutch coil current
At a fixed voltage, coil current depends only on the coil's resistance, so current well above specification means shorted turns and a new coil. Low voltage at the coil cuts its current and magnetic pull, and the clutch slips under load.
Proving a vacuum circuit
A vacuum motor and its hose are hold-tested separately with a hand pump that is first proven tight. If the motor alone holds and the loss appears only through the hose, the leak is in the hose or its fittings, not the motor.
Calibration learns the door stops
After a power loss or an actuator replacement, the module drives each door to its stops and records the feedback reading at each. A door blocked short of its stop teaches the module a short range, so the door is checked before the actuator.
05The other content areas
The A7 test has three content areas. Each has its own page of free questions:
The ASE A7 Heating & Air Conditioning practice test page describes the full question bank. How these questions are written and checked: how our questions are made. Found a mistake? Tell us and we will fix it.
ToolboxPrep is an independent study site. It is not affiliated with or endorsed by ASE (the National Institute for Automotive Service Excellence). Our questions are original practice questions written to the published ASE test specifications; they are not actual test questions.