Heating and Air Conditioning A7 Flashcards

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?

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 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?

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 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?

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 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?

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.

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

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.

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?

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.