What is easier on your A/C?
 

So, speaking from a fuel efficiency standpoint, what's the better way to run your A/C? Turn the fan speed dial down or raise the temperature dial? Or is once your A/C engaged, it pretty much wastes the same energy no matter what?

Ehm, for cars that just have an off/on AC clutch, you might as well turn on the fan at full blast so you can get as much cooled air in, versus turning the fan on low and having the AC do the same amount of work for not as much cooled air. If you have any hills on your commute that you can go down in gear above your idle fuel cut, turn on the AC for free cooling.

I usually set my fan to max recirculate, run the AC for a few minutes, turn it off till it heats up again, then cycle it back on. If your car is insulated well enough I figure one could get back with bursts of AC

Raising the temperature dial likely leaves that comnpressor running, and allows hot water to flow through the heater core. Not very efficient for normal cooling use...

I leave the fan up pretty high, with the air recirculating in the cabin, and cycle the compressor off and on manually.

I'm glad I have manual controls (set the fan speed, not the temp) and I just run AC full blast for a minute than leave it off as long as I can. Lately I was just dealing with it being a bit warm, or open both drivers side windows 2". I found that with both windows open the air circulated out instead of just passing by the window. If I open the drivers front and passengers rear theair crosses through even more.

I love this site. It is full of cheapskates like me who think alike

I use my air conditioning as air brakes alot. Only run it while slowing down. I leave the fan on recirculate and just cool the ac core down momentarily. I will always turn it off while accelerating. And if I need to cool it down a little bit I will only do so when at a steady speed or slowing.

I hate to hijack this thread but how hard would it be to put a vaccum activated control on the A/C On button so you could set it to come on at above a certain vaccum level? So if you are only pushing on the gas pedal less than a certain amount the A/C comes on.

I used to have a/c....

Anyway, about fan speeds, remember that cars have flow through ventilation, so your cooled air is being blown out of the car by the fan. Once you get the car down to a halfway comfortable temperature you're best off with the lowest fan speed setting you can tolerate.

Unless you set the ducts to "recirculate", the faster you run the fan, the more air you're blowing through the car. Kind of like using your home a/c with the vent control set to "open" (or a window open). You'll feel lots of nice cold air on your body but you're paying to cool that air down. If you keep the car windows shut and can get by with the fan on low speed I'm pretty sure the compressor will have to run less.

Ya, that is why I leave it set on recirc. Well, that and the whole humidity thing...

That would be challenging. You would need a vacuum switch that opens under no vacuum, and closes under high vacuum. It would need to be adjustable so you could set it to allow operation while cruising, and this would require that you measure the vacuum under different conditions to see what level you would need the switch to trip at. Or, you'd have to drive and adjust for a couple of weeks until it was working the way you wanted. And, it isn't necessary on modern cars, depending on make and model the AC is controlled by the computer, and the computer cuts compressor power under WOT conditions.

Actually wiring it in is easy enough, you just mount the switch near the compressor, cut one of the compressor control wires, attach both ends to the vacuum switch, then run a vacuum line to the intake manifold. You need direct exposure to the intake itself, not a ported line.

Might also look at a thermostat that would cut power to the compressor when the temp is cool enough. Automobile systems use a mix of full AC and heater core air to adjust the air temp coming from the vents, meaning that you are cooling the air, then heating it back up again. By setting a thermostat inside the car (needs to be a DC powered operated thermostat, home thermostats sometimes use AC power) you can run the temp at max cool, then control how often the compressor cycles by the thermostat. Kinda like the idea of using a deep freeze as a fridge. You'd wire the thermostat in the same way, the only difference being you'd use a relay in the AC line, and a control line off the thermostat. This way you only have to run a single 18ga wire to the engine bay, not two 12ga wires to go from the AC wiring to the thermostat and back.

On every car I have checked the compressor is cycled on an off by the pressure switch on the low pressure side of the evaporator. So when heat is adsorbed into the evaporator the low side refrigerant pressure rises, when the pressure rises sufficiently enough to activate the switch, the compressor runs. Once the evaporator is cooled sufficiently the corresponding low side pressure would have dropped and the compressor is switched off by the same switch.

Having said the above the most efficient way to continuously run the AC is on the lowest fan speed possible and also split the air between the floor and upper vents if you can. The reason to split the air is to help keep the return air as cold as possible to reduce the A/C run times. You do want to run on recirculate at all times and never ever turn on the heat. If its too cold on low shut it off and roll down the windows.