Why is there a sign usually on hills that says cars need to turn off AC?

[Ifonecapa]

Sorry. Might sound silly but what is this for?

 

[mikidymac]

The A/C condenser in front of the radiator is a big heat generator and removes heat from your vehicle and deposits the hot air in front of the radiator. The A/C pump also puts an additional load on your engine.
So the additional heat from the A/C condenser, load form the A/C compressor and the load from going up the hill generates a lot more heat that your cooling system can't get rid of so your car can overheat going up steep hills. Thus the sign warning to turn off A/C. It isn't as much of an issue on modern well maintained vehicles especially if you aren't towing anything.

 

[rldavis]

Hmmm never seen a sign like that in the Southeast, course us rednecks wouldn't understand it either...

 

[Chester]

The only place I've seen those signs is when driving to and through Death Valley. But yes. to prevent engine overheating.

Chester

 

[Ifonecapa]

It isn't as much of an issue on modern well maintained vehicles especially if you aren't towing anything.

I see that's why it's rare to see nowadays.