Dear Tom and Ray:

Nearly 40 years ago, I was told that it was a bad idea to turn on the air conditioning when driving at highway speeds. I was advised that the AC should be turned on only when the car was idling or moving very slowly. I guess the idea was that starting the AC when the engine was operating at high RPMs would jolt it and put a lot of sudden strain on the belt-driven AC parts. I have always followed that old advice, and I get funny looks when I slow to 20 mph to turn on the AC. Is it a bad idea to start the AC when not idling or moving slowly? Was it EVER a bad idea? Am I the victim of a 40-year-old joke?

Thanks! -- David

TOM: Bad news, David. For at least the past 40 years, while you were being so careful, your AC was cycling ITSELF on and off at highway speeds.

RAY: That's how the air conditioner works. It has a clutch, and it engages based on demand, and then disengages itself when the demand or other conditions have been met. This happens at all speeds, from idle to highway.

TOM: For example, the AC also will cut out when you start the engine so that all the battery power can go to the starter. And it'll cut out if you put a large demand on the engine for power. So if you're entering a highway, or climbing a hill on a highway, the AC may shut off during that hard acceleration to allow all available engine power to go to the wheels.

RAY: So the air conditioner is designed to be started and stopped at all kinds of speeds. And it's doing that whether or not you turn it on and off.

TOM: So you can forget all about this advice. And -- let me do some calculations -- it's been 40 years, so hopefully your father is still around. He's obviously the one who gave you this lousy advice, so give him the good news, too. Tell him he can now enjoy his golden years by turning on the AC at any speed he wants to.