Dear Tom and Ray:

My electrical engineer friend told me when driving my car at approximately 40 mph not to turn on the air conditioner unless I put the car in Neutral first. He said it is too hard on the air conditioner clutch. There is nothing about this in the owner's manual.
Peggy

RAY: Well, he has a point, Peggy, although I don't really agree with his advice.

TOM: The speed of the air conditioning compressor is directly proportional to the speed of the engine. So if the engine is turning fast, and you suddenly turn on the AC, the clutch and compressor are going to be "jolted" into action, and that's not really good for them.

RAY: So by putting the car in Neutral, you lower the engine speed to idle, and thereby put less stress on the air conditioner clutch and compressor when you engage them.

TOM: The problem is it's not really safe to put the car in Neutral while you're travelling at 40 mph...or any mph, for that matter. So we'd recommend two alternatives. One is to turn on the AC before you're going 40 mph, like while you're stopped at a light. Or wait until you get to the next stop light.

RAY: The other alternative is to just turn on the air conditioner anyway, and don't worry about it. When you're driving on the highway, and sweat starts to drip off your nose, you don't want to pull over to a rest area just to turn on the air conditioner. I'd say just turn it on and don't worry about it.

TOM: Me, too. It may bother your passenger if he or she happens to be an electrical engineer, but the extra stress you put on the clutch and compressor is very minimal, and in my opinion, not worth sweating over (pun intended).