ASE Practice Test (A7: Heating & Air Conditioning) 2 — Questions and Answers

Question 1: An R-134a A/C system has low suction pressure (low side) and high discharge pressure (high side) during operation. Refrigerant charge is verified as correct. What is the MOST likely cause?

System is overcharged with refrigerant
A restriction in the liquid line or expansion device (Correct answer)
Compressor suction valves are leaking
Condenser fan motor has failed

Correct answer: A restriction in the liquid line or expansion device

Low suction pressure combined with high discharge pressure indicates a restriction in the refrigerant flow between the high and low side — typically in the liquid line, filter/drier, orifice tube, or expansion valve. The restriction prevents refrigerant from reaching the evaporator, dropping low-side pressure.

A/C system pressure diagnosis requires understanding the refrigerant circuit. The compressor draws low-pressure vapor from the evaporator (suction side) and pumps it as high-pressure vapor to the condenser. In the condenser, the vapor condenses to liquid. The liquid passes through the filter/drier (or accumulator) and then through the expansion device (orifice tube or TXV), where it drops in pressure and temperature before entering the evaporator. A restriction in the liquid line (between the condenser and the expansion device) or a plugged orifice tube/TXV blocks refrigerant flow. Upstream of the restriction (the high side), refrigerant accumulates — causing high discharge pressure. Downstream of the restriction (the evaporator and suction line), refrigerant is starved — causing abnormally low suction pressure. The evaporator may also frost over due to the extreme pressure drop. Overcharging would cause both high and low side pressures to be elevated. A failed condenser fan would cause high discharge pressure but suction pressure would be normal or slightly high, not abnormally low. Leaking compressor suction valves would cause low suction pressure but also low discharge pressure (the compressor cannot build pressure efficiently). The combination of low suction and high discharge specifically points to a flow restriction between the two sides.

Question 2: A technician connects manifold gauges to an R-134a system and finds both high and low side pressures equalize to approximately 70 psi with the engine off and A/C off on an 80°F day. What does this indicate?

The system is at normal static pressure for R-134a at 80°F (Correct answer)
Both pressures equalizing confirms a refrigerant leak somewhere in the system
The compressor has failed and cannot separate high and low side pressures
The system has moisture contamination causing pressure equalization

Correct answer: The system is at normal static pressure for R-134a at 80°F

Static (equalized) pressure in an A/C system with the compressor off should reflect the saturation pressure of the refrigerant at the ambient temperature. R-134a at approximately 80°F has a saturation pressure of about 70 psi, so this reading is normal and expected when the system is properly charged.

When the A/C compressor is not running, the system reaches a static state where both the high and low sides equalize to the same pressure. This pressure is the saturation pressure of the refrigerant at the current temperature — the pressure at which the refrigerant exists in equilibrium between liquid and vapor at that temperature. For R-134a, the saturation pressure follows a predictable temperature-pressure relationship. At 70°F, the saturation pressure is approximately 61 psi. At 80°F, approximately 70 psi. At 90°F, approximately 80 psi. At 100°F, approximately 92 psi. A reading of 70 psi with both gauges equalized on an 80°F day is exactly what is expected for a properly charged R-134a system. This confirms refrigerant is present, the system is sealed, and the refrigerant is at the correct phase equilibrium temperature. If static pressure is lower than expected for the ambient temperature (for example, 30 psi at 80°F when it should be 70 psi), it indicates a low charge (refrigerant loss). If static pressure is zero, the system has lost its charge completely. If static pressure is much higher than expected, the system may be overcharged or contain air (non-condensable gases). Pressure-temperature charts are essential tools for this diagnosis.

Question 3: When an R-134a A/C system is recovering refrigerant, the recovery machine indicates the system contained 0.8 lbs instead of the 1.4 lb specification on the underhood label. What should the technician do AFTER completing service?

Charge the system to the specified weight and return the vehicle without leak testing
Perform a leak test before recharging, then charge to specification weight (Correct answer)
Charge the system with the recovered 0.8 lbs only, as adding more may overcharge
Recharge to 1.4 lbs, test the system, and note the discrepancy in the repair order

Correct answer: Perform a leak test before recharging, then charge to specification weight

A system with significantly less refrigerant than specified has a leak. Recharging without finding and repairing the leak will result in the customer returning with the same complaint. The correct procedure is to find and repair the leak, then evacuate and recharge to specification.

A/C system refrigerant loss occurs only through leaks — refrigerant does not degrade, wear out, or get consumed in normal system operation. When a recovered charge is 0.6 lbs below the 1.4-lb specification (a loss of approximately 43%), there is clearly a significant leak or a series of leaks in the system. The correct diagnostic and repair procedure is: complete refrigerant recovery; perform system leak testing using an electronic leak detector and/or UV dye under UV light; repair all identified leaks (O-rings, Schrader valves, component seals, refrigerant lines, condenser, evaporator); pull a deep vacuum (500 microns or less) to remove moisture and check system integrity; and recharge to the specified weight. Skipping the leak test and recharging wastes refrigerant and the customer's money — the system will simply lose the new charge through the same unrepaired leak(s). Charging with only the recovered amount is wrong because the recovered amount represents the leak-reduced charge, not the correct amount. Noting the discrepancy without addressing it is not an acceptable repair. Evacuation to a proper vacuum (typically 500 microns or less on a micron gauge) before recharging is essential to: boil off any moisture that entered the system (moisture at system temperatures forms ice crystals that block the orifice/TXV and causes corrosion); and remove air (non-condensable gases that raise high-side pressure and reduce cooling efficiency).

Question 4: A customer complains that the A/C blows warm air at idle but cools normally when driving at highway speeds. What is the MOST likely cause?

Low refrigerant charge
Condenser cooling fan not operating (Correct answer)
Evaporator icing over at idle
Compressor clutch slipping at low RPM

Correct answer: Condenser cooling fan not operating

When A/C works at highway speed but not at idle, the condenser is not receiving adequate airflow at low speed. At highway speed, ram air through the grille provides cooling. At idle, the system depends on the electric condenser fan. A failed fan prevents condenser cooling at idle, causing high-side pressure to rise and compressor cutout.

The A/C condenser requires adequate airflow across its fins to reject heat from the high-pressure refrigerant. This airflow comes from two sources: ram air (forced by the vehicle's forward motion) at highway speeds, and the electric condenser fan at idle and low speeds. At highway speeds, even a non-functioning condenser fan may be acceptable because ram airflow through the grille and across the condenser is sufficient to condense the refrigerant. The system works and the passenger compartment cools normally. At idle, ram air is essentially zero. The system must rely entirely on the electric condenser fan. If this fan has failed (motor burned out, fan relay failed, fuse blown, control signal absent), the condenser cannot adequately reject heat. The high-side pressure rises rapidly. Most systems have a high-pressure cut-off switch that shuts the compressor off when pressure exceeds a safe threshold (typically 400-430 psi for R-134a). When the compressor cuts off, the system stops cooling — blowing warm air. Diagnosis: verify the condenser fan operates when the A/C is on with the engine at idle. Listen for fan operation, visually confirm fan rotation, and measure voltage at the fan motor connector. Also check for the fan control relay and its fuse. The thermal management system (PCM-controlled fan strategy) should also be checked for proper operation.

Question 5: What refrigerant property makes the expansion valve (TXV) or orifice tube critical to A/C system operation?

They allow only the correct amount of oil to circulate with the refrigerant
They create the pressure drop that allows liquid refrigerant to evaporate and absorb heat (Correct answer)
They filter moisture from the refrigerant before it enters the compressor
They control refrigerant temperature to prevent evaporator freezing

Correct answer: They create the pressure drop that allows liquid refrigerant to evaporate and absorb heat

The expansion device (TXV or orifice tube) creates a controlled pressure drop as high-pressure liquid refrigerant passes through its restriction. This pressure drop causes the refrigerant to flash (partially vaporize) and drop in temperature, allowing the low-pressure refrigerant to absorb heat in the evaporator.

The refrigeration cycle operates on the principle that a refrigerant absorbs heat when it evaporates (changes from liquid to vapor) and releases heat when it condenses (changes from vapor to liquid). The expansion device is the component that creates the conditions necessary for evaporation in the evaporator core. High-pressure liquid refrigerant from the condenser enters the expansion device at relatively high temperature (but below the condensing temperature — it is subcooled liquid). The expansion device is essentially a controlled restriction — a small orifice. As the refrigerant is forced through this restriction, pressure drops dramatically. According to the pressure-temperature relationship of refrigerants, lowering the pressure lowers the boiling point. The refrigerant, which was a stable liquid at high pressure, suddenly finds itself at a pressure far below its boiling point at that temperature. It begins to flash (boil rapidly), absorbing large amounts of latent heat from itself and its surroundings. This is why the evaporator and the outlet line become very cold. A TXV (thermostatic expansion valve) modulates its opening to maintain a constant evaporator outlet superheat, providing better control under varying load conditions. An orifice tube is a fixed restriction with no moving parts, simpler but less adaptive. Both serve the fundamental purpose of creating the pressure drop required for refrigerant evaporation.

Question 6: A heater core is leaking coolant into the passenger compartment. The repair requires draining the cooling system. After the repair, the cooling system is refilled. What is the MOST important step to complete BEFORE checking for leaks?

Road test the vehicle to operating temperature
Bleed the cooling system of trapped air (Correct answer)
Check the coolant reservoir level and cap condition
Run the engine at high RPM to pressurize the system quickly

Correct answer: Bleed the cooling system of trapped air

After refilling a cooling system, air trapped in the system must be bled out. Air pockets prevent complete coolant fill, reduce cooling efficiency, and can cause localized overheating. Most modern systems have specific bleeding procedures using bleeder valves or by operating the heater with the engine warming up.

Cooling systems have complex passages through the engine block, cylinder head, heater core, and hoses. When the system is drained and refilled, air becomes trapped in high points in the system. This trapped air has serious consequences. Air pockets prevent coolant from circulating through those areas, creating hot spots in the engine and reducing heater performance. Air in the coolant passages near temperature sensors can cause false thermostat operation and incorrect gauge readings. Air in the heater core causes gurgling noises and reduced heat output. Different systems have different bleeding procedures. Some vehicles have bleeder screws at high points in the system (such as on the thermostat housing or heater hose fitting) that must be opened to allow air to escape as coolant is added. Others rely on running the engine with the heater on full heat, which opens the heater core circuit and allows air to circulate out through the overflow reservoir. Many technicians use a cooling system vacuum fill tool that draws a vacuum on the system before filling — this removes air before coolant is added, resulting in complete fill without air pockets. Regardless of method, proper bleeding is essential. A vehicle returned without proper air bleeding will likely show low coolant level on the first check (as air pockets collapse), and may experience the overheating or heater performance complaints that prompted the customer to bring it in.

ASE Practice Test (A7: Heating & Air Conditioning) 2

An R-134a A/C system has low suction pressure (low side) and high discharge pressure (high side) during operation.
Refrigerant charge is verified as correct.
What is the MOST likely cause?