"It's a bad idea to run your air conditioner when you're driving faster than 50 mph." Fact or myth?

Dear Car Talk

Dear Car Talk | Apr 01, 2007

Dear Tom and Ray:

On a recent cross-country trip (South Carolina to Nevada) with my boyfriend, we had an air-conditioning issue, which I'm hoping you can help us with. I wanted to make a cell-phone call, so I put the windows up (for quiet) and turned the AC on. We were cruising at approximately 80 mph. At the next stop for gas, the car was making a funny noise, which said boyfriend decided was the compressor. Then the AC did not work any longer. Plus, the car was smoking. So he took it in for service the next day -- and $1,000 later, we have another compressor, among other AC parts. The man at the repair center said you should NEVER turn your AC on going faster than 50 mph. Everyone I've talked to says they've never heard of such a thing, and they've all done it. And I have done it myself, with no repercussions. So is the repairman right? Is this true? We were driving a 1998 Jeep Wagoneer. It has had AC problems anyway -- could it have been an existing problem? Or did my turning the AC on at 80 mph cause this? I am going to
pay for it, because I feel so bad, but I would like to know for future knowledge. Thank you! -- Julie

TOM: Don't pay for it! It wasn't your fault, Julie.

RAY: No, the timing was pure coincidence. The air-conditioning compressor was already on its last legs before this incident. It just happened to kick the bucket after you used it. You are completely blameless.

TOM: We've heard this myth before, about not turning on the air conditioner at highway speeds. The purveyors of this myth explain that, with the car going that fast, the belt is turning at high speed. And if you then switch on the AC, it will be "jolted" into action at a very high speed, and this will harm it.

RAY: This, as we say in the garage, is bullfeathers. Here's why. When the AC is running, it cycles on and off by itself anyway. So it's already turning itself on and off all the time, no matter how fast you're driving.

TOM: Right. So if what the repairman said was true, you wouldn't ever be able to use your AC on the highway. And we know that's not true. Otherwise, everybody you passed on the interstate would be dripping sweat, with their hair plastered to their forehead.

RAY: Plus, the major accessories are all shut off every time you accelerate hard, so that all of the engine's power can go to the wheels. When you back off the gas pedal, the AC comes right back on. The engine is always turning fast at that moment. I'm not saying that's great for the AC, but the compressor has its own clutch, and it's engineered and designed to be used that way.

TOM: So you can turn the AC on and off whenever you want, Julie. And if you want to contribute to your boyfriend's new compressor, feel free. But we call this a pre-existing condition. You're off the hook.

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