A7: Heating & Air Conditioning (Sample Test)

Both Technician A and Technician B are correct.

• Technician A is correct that a gurgling sound in the dashboard may be caused by an air pocket in the heater core. Air trapped in the heater core can disrupt the flow of hot coolant, leading to reduced heating performance and the noticeable gurgling noise.

• Technician B is also correct. A leaking head gasket can introduce combustion gases into the cooling system, creating air pockets throughout the engine—including in the heater core. These air pockets can impair coolant circulation, potentially causing overheating and other performance issues.

Therefore, both technicians provide valid points that are part of the same underlying problem—air entering the cooling system.

Both Technician A and Technician B are correct.

• Gaskets and O-rings are used in air conditioning systems to seal connections and prevent refrigerant leaks. These seals are critical for maintaining system pressure and ensuring optimal cooling performance.

• When replacing O-rings, it is essential to lubricate them with the correct type of refrigerant oil. Proper lubrication helps prevent damage during installation and ensures a tight, reliable seal. Using the wrong oil—or no oil at all—can cause the O-ring to tear, stick, or fail, leading to refrigerant leaks and potential system damage.

Therefore, both technicians are correct in emphasizing the importance of proper sealing components and correct lubrication procedures in air conditioning service.

The Engine Coolant Temperature (ECT) sensor is a variable resistor that monitors coolant temperature and sends a signal to the Powertrain Control Module (PCM). It receives a 5-volt reference signal from the PCM and changes the voltage output based on resistance, which varies with temperature. As the engine warms up, the voltage signal decreases, allowing technicians to compare this value to manufacturer specifications for diagnosis.

Refrigerant pressure sensors are transducers that convert refrigerant pressure into a variable voltage signal—typically ranging from 0.1 volts at low pressure (~0 PSIG) to 4.9 volts at high pressure (~430 PSIG). Modern vehicles often include pressure sensors on both the high and low sides of the air conditioning system.

To verify sensor accuracy, technicians can use a manifold and gauge set along with a scan tool. However, with a scan tool alone, it’s possible to monitor refrigerant pressures in many systems without the need for physical gauges.

According to the system scan shown in the image, the refrigerant charge appears adequate for normal operation. It’s also important to note that in most systems, the compressor clutch is disabled when ambient temperatures drop into the 30s°F, to protect the system from operating under unfavorable conditions.

The motor controller uses voltage reversal to control the direction of the motor, allowing it to move the blend door back and forth as needed. A potentiometer, integrated into the motor assembly, continuously monitors the position of the blend door, providing real-time feedback to ensure accurate and responsive control.

The A/C-heater control panel manages the operation of a vehicle’s heating and air conditioning systems. When servicing this panel, it’s important to follow proper safety procedures to protect both the technician and the vehicle.

In vehicles equipped with airbags, the negative battery cable must be disconnected, and a waiting period observed (as specified by the manufacturer) to prevent accidental airbag deployment during the repair process.

Self-diagnostic tests can be useful in identifying faults within the HVAC system, including a malfunctioning control panel. However, discharging the refrigeration system is not necessary when removing the A/C control panel—unless the panel is physically connected to the refrigerant components, which is uncommon in most designs.

Conclusion:
The statement “You must discharge the refrigeration system before the A/C control panel removal” is incorrect in typical service scenarios.

The cycling clutch switch is a key component in the air conditioning system that regulates when the compressor clutch engages and disengages. It allows the compressor to cycle on when cooling is needed and off when it’s not, helping to maintain proper system pressure and prevent freezing of the evaporator.

A faulty cycling clutch switch may cause the compressor to engage and disengage erratically, leading to inconsistent or reduced cooling performance. However, it would not directly cause the compressor to underperform during high-demand conditions, such as extremely hot weather—this type of issue would more likely be related to other system components like the compressor itself, refrigerant charge, or condenser efficiency.

The most likely cause of a vehicle overheating is a clogged radiator. Here’s why:

• Clogged Radiator: The radiator’s main function is to dissipate heat from the engine via coolant. If it becomes clogged with debris, rust, or scale, coolant flow is restricted, which reduces heat transfer and leads to engine overheating. This is a common issue, particularly in older vehicles or those with poor maintenance histories.

• Electric Cooling Fan Runs Continuously: While a malfunctioning fan can contribute to overheating, in this case, the fan running all the time is more likely a response to overheating—not the root cause. The system is attempting to cool the engine, but the underlying issue (like a clogged radiator) is preventing proper temperature control.

• Restricted Heater Core: A blocked heater core may reduce cabin heat output, but it has minimal impact on the overall engine cooling system. It is unlikely to cause engine overheating.

• Thermostat Stuck Open: A thermostat stuck in the open position generally leads to underheating or slower engine warm-up, not overheating. In fact, it often causes the engine to run cooler than normal.

The A/C compressor relay controls the power supply to the compressor clutch. If the relay becomes stuck in the closed position, it will continuously send power to the clutch, causing the compressor to remain engaged at all times, even when it should disengage. This can lead to poor A/C performance, increased wear, and potentially damage to the compressor or other system components.

While low refrigerant charge or an overcharged system can also result in poor cooling performance, they typically cause the compressor clutch to cycle on and off, rather than remain engaged. In these situations, the system struggles to reach or maintain the desired temperature, but the clutch does not stay continuously activated.

Both Technician A and Technician B are correct.

• Technician A is correct in stating that the gauge needle should remain stable after the recovery and evacuation process is complete. The purpose of evacuation is to remove all air and moisture from the system and create a deep, stable vacuum. If the needle rises after evacuation, it could indicate a leak or incomplete evacuation, both of which compromise system integrity.

• Technician B is also correct. A perfect vacuum equals 0 inHg (inches of mercury). Any movement of the needle—no matter how small, such as 0.01 inHg—suggests that some gas is entering the system, likely due to a leak or the presence of non-condensable gases. While this may not immediately affect system performance, it can lead to long-term reliability issues, such as moisture-related damage or reduced efficiency.

The statement “The throttle position sensor (TPS) is faulty or out of adjustment” is TRUE.

The TPS provides the engine control unit (ECU) with real-time data on throttle plate position, allowing the ECU to properly manage air-fuel mixture and ignition timing.

If the TPS is faulty or misadjusted, the ECU may receive incorrect throttle input, which can disrupt various engine control functions. In some cases, this may cause the A/C compressor clutch to disengage during acceleration, as the ECU interprets the incorrect signal as a need to reduce engine load or prevent stalling.