Today: Rita's mechanic is full of hot air when it comes to the AC.

Dear Car Talk

Dear Car Talk | Oct 01, 2011

Dear Tom and Ray:

I was told today at an auto-body-repair shop that the temperature dial on my car (the one that is half-blue and half-red) is NOT a thermostat for the air conditioner, and that I should NEVER touch it when the air conditioner is on. I was told that it does work as a temperature control if you are venting in air from outside or running the heater, but not for the AC.

They said that unless a car has digital controls (mine doesn't), I can't adjust the temperature of the AC. I just have to turn it on and leave this dial at the coldest setting, and use the fan speed to adjust how much cooling I get. Is this correct? I had never heard this before. This information was given to me in response to my report of smelling Freon when I adjusted that dial while running the AC.

-- Rita

TOM: Forget everything they told you, Rita. You can adjust that dial anytime you want.

RAY: That dial operates the heater control valve. All it does is add warm air to the passenger cabin -- regardless of whether the AC is on.

TOM: When you turn on the AC, all it does is blow cold air. There's no way to control the "temperature" of the air that comes out of the AC unit itself.

RAY: So how do you control the temperature in your passenger cabin? There are three ways: You can turn the air conditioner off when it gets too cold. You can turn the fan speed up or down. Or you can blend in some warmer air until the temperature is where you want it. That's perfectly acceptable, and will do no harm to anything.

TOM: In fact, that's what the digital, or "climate control," systems do. When you set the cabin temperature to 72, it turns on the AC, and then blends in warm air from the heater until the thermostat reads 72.

RAY: Blending in warm air also is the only way to comfortably defog your windows in the winter. When there's moisture on the insides of the windows, you need to use the air conditioner to remove that moisture. But if it's 6 below zero out and your butt cheeks are already freezing together, that can be mighty uncomfortable. So what do you do? You turn the temperature control valve up into the red, and blend in hot air. That way, you get warm, conditioned air (i.e., warm, dry air). Your windows clear up, and your teeth don't break from chattering.

TOM: One final thought. You say you smelled Freon when you added heat. What you may be smelling is coolant, which would be coming from a leaking heater core. So ask another mechanic (not these knuckleheads) to check that out for you.

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