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Why does my car stall when I turn on the air conditioner?

Turning the key in the ignition on a hot day usually means air conditioning isn't far away, but what happens when this process makes your car stall? Not only is this incredibly frustrating, it seems confusing too. Interestingly though, it's all about your A/C system's power needs.

What is the idle air control valve (IACV) and why do I need to know about it?

Your car air conditioning unit requires power from your engine. If its power needs are too great, your engine gets overpowered and your car stalls. Here's why:

In fuel injection vehicles, the idle air control valve or IACV decides how much air is allowed to bypass the throttle plate. Essentially, the valve can change your car's idling speed where necessary depending on the engine's needs.

Think of the valve like a train controller: It decides exactly what your car needs to idle happily. If the controller (IACV) isn't able to determine the air flow, isn't reading the air flow right or isn't able to do its job, the car won't idle and will stall.

What does the IACV have to do with my air conditioner?

Your air conditioning unit (also known as a heating, ventilation, air conditioning unit or HVAC) is all about air flow. It basically removes the air from the car's cabin, cools it and takes out the humidity, then puts it back into your car as cool air.

Your air conditioning is directly connected to your engine, which is where the IACV sits. Since the IACV works to manage the amount of air in the engine, it's directly linked to your HVAC system.

If your car is especially hot and you have cranked up the A/C, your IACV may not be able to manage the air exchange, struggling to let your car idle. So it stalls.

What if the problem isn't my IACV?

Here's where things get a bit tricky. The A/C system is pretty complex, and there are a number of things that cause your A/C system to require more power from your engine, overpower its supply, and stall your car. One of the main parts that can fail is the compressor, here's why:

A faulty compressor could be the problem with your air conditioning unit

Just like in your fridge, your car's A/C system uses refrigerant which can transition between liquid and gas form depending on temperature. But it's not only temperature that helps refrigerant change state, it's also your compressor.

The refrigerant starts as a gas then compresses into a liquid. The liquid then passes through the compressor where it becomes hot and becomes a gas again! It's this snazzy process of refrigerant passing back and forth through the compressor that pulls the hot air from your cabin, and replaces it with cool air.

So! Since the compressor is a pretty vital part of the cooling process, it's no surprise that a damaged compressor would require extra power to function. If its power needs overwhelm the output of your engine, you're engine can't function well and can stall. Simple right? Wrong.

Car heating and cooling systems are pretty complex, but in the Aussie heat and chilly winter they're essential to your daily commute. Don't wait to get your system fixed as it can lead to further damage which will cost you more in the long run. If your car is stalling when you activate the A/C, take it to the experts at your favourite heating and cooling repair shop. They understand this stuff far better, and will find the problem and offer a solution to get your back on the road and in the cool in no time.

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