I live in Pune and surrounded by ghat sections wherever I go. The fact which I believe is listen to your car. You will be the first one to know if the car is struggling to climb uphill with a/c on. If it climbs up easily don't continue to drive with a/c otherwise shut it off. My Ritz ascends easily on the ghat section with a/c on but i personally shut it off while going uphill. I have been driving cars from the good old days of premier padmini (With steering-column- mounted lever and floor gear, both a/c and non a/c versions) that lady use to get short of breath while going uphill with a/c.
As a practical experience of a novice I got stucked up in Hyd roads at at 30 degree slop trying to mount when all of a sudden a crazy man crossed the road and had to apply brakes. Being a learner I had a horrible experience. I shifted my gear to 1st and tried to move the car but the car kept rolling down. I was really nervous and struggling to find out a way. With much effort nothing happened.
Called up my friend who was just 2 mins away from the point of incident and he came running.
What he did let me narrate here:-
First thing he put off the AC
Next he put his legs on the brake and cluth and slowly release the clutch
One point the car tried to move, he immediately started pumping slowly the accelerator.
Once it got some momentum he release the hand Brake and Walla we were up.
I do this everyday now and without any glitch.
He told me put off the AC while on slope else you would miss acceleration.
Please let me know if this approach is correct or not?
Actually i went there to ask them that my AC turn off if i drive car on mountain area specially suddenly i get good height road. Then tata mechanic he told me this, even showed me . Before i had no idea :P .
i did not feel engine straining with AC on Up hill on Scorpio. but in Bolero i can feel the difference and the engine strains a little.
as for my 800 it was a 5 speed powerful than normal one (with vacum booster brakes.) the strain reduced a lot when i changed the AC gas to R134 (non-CFC).. it cost me 8000/- 3 years back. but it did the job the load was reduced.
now i have a K10 wagon r, and yes i do see strain with AC so i just switch it off and downshift to make it easier on the car
in my indica and wagon r i used to switch off the ac when climbing uphill and overtaking. but with my vento i rarely have switched off the ac to get more power. the strain on the engine if ac is switched on i feel is only negligible and is of no consequence on the life of the engine
generally all the ac has a thermostat which is a rotary knob to control the cooling ,at full position it runs for maximum cooling at intermediate levels it cuts the power to compressor when certain level of cooling is achieved.
so once the cabin is cooled the ac would not operate as long as heat is build up again.instead of that if if we turn of ac frequently the cooling of the cabin and compression part is much disturbed.so it would need to work a lot to maintain that .
in short frequent startup of compressor eats lot of power compared to running one.
so it is always a good idea to maintain an optimum level with ac running automatically
that said.above is not an auto climate controlled thingie Acc is digitally computer controlled with lot of sensors and comparitive measurements of temperature you wont need to fiddle with it as it does what it says.
AFAIK cars now-a-days do not have a thermostat like the room AC or the refrigerators. ( I do not know about High end cars, but cars like maruti zen, swift santro, esteem, accent, indica, indigo, honda city, manza, etc, dont have it.)
The temperature control knob only operates the hot air vent, so if we turn it clockwise or slide it towards the right, it opens the warm air inlet progressively. The AC compressor is still on till you turn it of manually.
Even in Auto AC/climate control, when we set the temperature for the cabin, the temperature is controlled by mixing warm air when the temperature in the cabin goes lower than the set temperature. But the compressor keeps running.