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Car AC Not Cooling?

Updated: Jan 14, 2021


Before you go digging in your couch for a load of change, expecting to pay hefty recharge or repair work, consider how your system works.

I’ll try and keep this somewhat simple. There is refrigerant and air conditioning hoses in your AC system, and that makes it cold, right? But why? It’s magic, right? The fluid just does things… Well, there is a gas in the lines, and it is condensed on what is called the high pressure side. It passes through a valve to the low pressure side, where it rapidly expands. The properties of this gas cause it to chill on rapid expansion. The chilled gas passes through a coil inside your cabin air box. When the air passes over the fins of the coil, it is cooled. We could go deeper into why this happens, but we won’t.

There is a cabin air filter somewhere in-line with this system, keeping pollutants and allergens from entering your car as the fan pulls in fresh air. If that filter is clogged, less and less air will pass. Eventually you may not have much air passing at all, and the little air that is passing through is now slightly condensed and warmer.  Also, without the proper airflow, the coil is unable to dissipate the chill to work efficiently.

I believe it is suggested to change the cabin air filter every 15,000 miles. When is the last time you changed yours? Chance are, never?

Take a look at the filter above. That came out of our Mercedes ML500 we purchased. I bet it wasn’t changed since 2004, when the car was new. You are breathing that in when you drive!

So, check your cabin air filter first. It’s cheap. It’s easy. And, you can do it yourself before running your vehicle in to a shop.

This process is the same in your house, but not changing your house filter could have VERY expensive consequences. Without good airflow, the coil gets colder and colder to the point of freeze-over. It literally encased in ice and frost, eventually all the way through your walls, down the pipes, and back to the pump outside. If not treated quickly, it will damage your pump and valve.

If it happens to you, turn off the AC, but leave the fan running. This will cause air to continue to pass over the coils to help thaw. Replace the air filter immediately, and leave the AC off until it’s fully thawed. Hopefully you caught it in time. Check your drain for.clogs at this time, because a lot of water is on its way to your pan.

Fact: If you have a heatpump/AC system, it uses the same refrigerant to do both jobs (heat and cool). The same way the gas cools when expanding, it heats when compressed. So, in the winters, the pump just opperates in reverse. Some may have Aux heat with an electric coil, but that’s usually to thaw the system or in case of emergency. The electric part is way more expensove to run, and less efficient at providing adequate heat. No, your car doesn’t work like a heat pump; at least none I’ve seen. It uses hot water from the engine to do that job.

Oh, and turning the AC all the way down doesn’t make it colder faster. The air coming out is always the same temp. The thermostat sets the cadence for how often the cold air is pumped out the vents by turning the compressor on and off to keep it at a certain temp. Just set the temp, blast the fan speed for a bit, and turn the fan speed down when you get to a comfy temp. Stop messing with the temp gauge. Having the compressor on all the time, to try and keep it at 50 degrees in your car on a 100 degree day, doesn’t help.


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