Don't turn on the A/C at 40 MPH or above?
People who spread this kind of nonsense need to get a life. Yes....slam the compressor on and off at 5000 RPM and you will stress it out, but unless you are drag-racing and winding out first gear to redline, most cars at 40 MPH will be doing only a couple of thousand RPM ( usually no more than about 3500 even in second ).....no sweat. In fact, turning the A/C on often actually HELPS the engine, in spite of the lower gas mileage and added drag. Many cars today with A/C have a second electrically operated fan that comes on either concurrently with the compressor or when a coolant temperature sensor triggers it. The second fan adds more air going through the radiator ( which is usually not a problem at high vehicle speeds anyway ) and helps to keep engine temperature down which would normally go up with the extra compressor load.

 

Does that negate the saying that turning on the heater to MAX when the car is overheating (A/C off of course)?

What about the theory that running the A/C on with the heater set to max can cause 'damage' ? The only thing I can think of is that the temperature fluctuations might affect the plastics in the ducts or on the dash,,but who knows.

 

my 2000 prelude, probably because it doesnt have a digital climate control, mine is just a slider switch, youre probably right on that

 

The clutch on the compressor is engaged by an electromagnet, under the control of the climate control system which can cycle it on or off. When starting the clutch is disengaged while the engine is cranking.

Particularly in hot weather, you don't want to cycle the clutch at high RPM because head pressure on the compressor is very high on the intake side, making it VERY hard to turn over. Under these conditions the clutch (or the drive belt) will slip excessively, causing wear and premature failure.

In normal cruising at high speed, your engine is only turning about 2500-3000 R's, easily handled by the clutch.

Lvangundy is right, turning on the heater full blast is a somewhat effective way to lower your coolant temperature, provided it hasn't already started to blow to the overflow bottle. Rolling down the windows and suffering a little (or a lot) beats walking.

I've had the opportunity to test this a time or two - Once in a brand new (1967) Mustang racing an identical Cougar across Arizona at speeds approaching 120. We were pretty evenly matched, neither gaining much of an advantage on the other until I tried this old hot rodder's trick. I found I could keep the hammer down without sending the radiator cap into low earth orbit, allowing me to pull ahead of the young ladies in the other car by turning off the AC and putting the heater on FULL. The airflow over the heater core had the effect of adding a small auxiliary radiator to the cooling system, despite the fact that it's output was INSIDE the car.

Of course it couldn't go on all day like this, and I backed out of it to breathe the engine a bit and let them take the lead when the ladies had the bad luck to boil their kitty over just as they blew by probably the most surprised member of the AZ Highway Patrol. He pulled us both over, chewed them out, then came up to me. Spotting my seabag on the back seat, and me in a dress blue Navy uniform (wool), he waved me on, going back to their car, pulling out his ticket book. I guess seeing me in a wool uniform, nearing heatstroke in the 115-degree heat, with my HEATER on, he figured I'd suffered enough.