Your car's climate control system is two systems that share some components. Understanding how they work helps you recognize when something is wrong before a small problem becomes an expensive one.

How the A/C system works

The heart of your A/C is the compressor — a belt-driven pump that compresses refrigerant and circulates it through the system. Compressed refrigerant moves to the condenser (the unit at the front of the car, in front of the radiator), where it releases heat and converts to liquid. It then flows through the expansion valve, which drops the pressure rapidly and causes the refrigerant to become very cold. That cold refrigerant passes through the evaporator inside the dash, where the blower motor pushes cabin air over it. The air cools, and that cooled air enters the cabin.

The system also removes humidity from the air as it cools — that's why you see condensation dripping from under a car when the A/C has been running.

Common A/C problems

Not cooling — usually low refrigerant or a failing compressor. A pressure test will tell you which.

Cooling and then warming up — low refrigerant that drops pressure as the system runs, or a cycling switch issue. Often the system will cycle on and off visibly (you can hear the compressor clutch engaging and disengaging).

Strange smells — a musty smell from the vents usually means mold or bacteria growth on the evaporator. This is common in humid climates and can be treated. An acrid chemical smell can indicate refrigerant escaping, though refrigerant itself is odorless — what you smell is the compressor oil.

Grinding or squealing when A/C is on — the compressor clutch or the compressor itself is failing.

How the heating system works

Unlike the A/C, the heating system doesn't generate heat — it borrows it. Once your engine reaches operating temperature, coolant circulates through the heater core (a small radiator inside the dash). When you request heat, a blend door opens to direct cabin air over the heater core, and the hot coolant warms the air. The blower motor then pushes that warm air into the cabin.

Common heating problems

Heat takes very long to warm up — often a thermostat that's stuck open, allowing the engine to run cooler than normal. Your temperature gauge will also read lower than usual.

Heat is weak or inconsistent — a clogged heater core (from degraded coolant leaving deposits) reduces flow through the core. A flush may clear it; replacement is sometimes necessary.

Heat on one side only, cold on the other — usually a blend door actuator failure. The motorized door that controls air mixing is stuck.

Sweet smell in the cabin — coolant smell means the heater core is leaking coolant into the dash. This is a significant repair but one worth catching before the coolant contaminates more of the interior.

Defroster not working — front defrost (HVAC) relies on the climate system working properly. Rear defrost uses an electrical grid on the glass that can fail at individual heating elements. Check both before winter.

Seasonal recommendations

Run your A/C for a few minutes in spring to check that it's working. A few minutes in the fall to make sure the defroster clears properly on both front and rear. These small checks catch problems while there's still time to address them before you need the system urgently.

At ProAuto

We service complete climate systems — A/C and heating — for all vehicle types. We handle both R-134a and R-1234yf refrigerants. If your A/C or heat isn't performing as it should, come in and we'll diagnose it properly.

ProAuto | 6100 W Dempster St, Morton Grove, IL 60053 | (847) 232-7777

Serving Morton Grove, Skokie, Niles, Glenview, Northbrook, Evanston, Wilmette, Winnetka, Des Plaines, and Park Ridge.