When a Thermostat Goes Bad, Does the AC Still Work in the Car?

An air conditioning system is basically a series of heat exchangers, or radiators, with a compressor somewhere in the middle. All fluids on Earth -- including air and AC refrigerant -- contain a certain amount of energy in the form of heat. When you squeeze the fluid's molecules closer together, you increase the fluid's temperature. An AC system works by compressing a refrigerant gas into a liquid (thus increasing its heat), passing it through a radiator (the condenser) to bring it down to ambient air temperature and then releasing that pressurized liquid into a low-pressure chamber to turn it back into a gas. That super-chilled gas cools another heat exchanger (the evaporator); air passing through the evaporator picks up some of its coolness, flows into your car and keeps your ice cream from melting.

Car engines work best when kept at a certain temperature, typically between 160 and 240 degrees Fahrenheit. All metals expand as they heat, and different metals -- and different thicknesses of metals -- expand at different rates and by different amounts. The average engine maintains internal component clearances often less than 1/100th of an inch, which doesn't leave much room for metal expansion. Engineers must design an engine with the clearances at a specific temperature in mind, and that's usually somewhere around the boiling point of its coolant. A thermostat is a valve in the cooling system that closes when coolant temperatures are low, in order to recycle coolant through the motor and bring it up to the ideal temperature. When coolant temperature rises to the engine's ideal, the thermostat valve opens to send coolant through the radiator in order to maintain that temperature.

Under most circumstances, a bad thermostat will not have any effect whatsoever on the air conditioning system. The AC system is a completely separate entity; the only connection it has to the engine is at the compressor pulley, which derives its power from the crankshaft. Other than that, the AC system could care less what the engine is doing. The heater, however, is a different story. A car's heater core is like a second radiator. When you turn the heater on, a valve opens in the heater core line and allows hot engine coolant to flow into the core. Cabin air blows through the hot core and into your car to warm the occupants. A stuck-open thermostat will constantly send coolant through the radiator to cool, meaning that the coolant -- and thus the heater -- may never reach its full operating temperature.

While engine cooling systems and air conditioning systems are mechanically separate, computer controls can act as a link between between the two. Air conditioning compressors put a fair amount of stress on the engine, generally sucking up between 10 and 15 horsepower when active. If you're a physics geek interested in such things, that's an extra 424 to 636 British Thermal Units of heat energy per minute that the cooling system must dispose of in order to maintain equilibrium. Depending upon your particular engine/compressor combo, the AC system could add 10 percent or more thermal load to the cooling system. If your thermostat sticks closed and the engine begins to overheat, your car's computer may well shut the AC compressor down in a last-ditch effort to reduce the thermal load on the system. Of course, if your car's engine overheats by that much, then odds are that you've got bigger problems than liquefied Butter Pecan.