Greeting again everyone,

Now that summer is pretty much here I've had a chance to start using the A/C in my car... or not.

I've wanted to put off turning on the A/C for as long as possible (by not pressing the big A/C button and having the light come on) for fuel efficiency and whatnot. By turning the temperature dial all the way to the cold position and turning on cabin air circulation, I've discovered that the air coming out of the vents becomes very cold, and the noise of the engine also becomes more pronounced which makes me wonder if the A/C clutch has been engaged, even though I can't really feel a difference in acceleration. By toggling the A/C button/light, I can't really feel a difference either. My previous understanding of the temperature dial as established by my leaking heater core is that all the dial really does is adjust the position of a blower vent to control how much air passes over a heat exchanger.

If I return the air circulation mode back to outside air without touching the temperature dial, the temperature warms up, and the engine noise settles back down.

I've read about these cars automatically engaging the A/C unit when you use the defroster, but are there any other circumstances in which the car will override user choice and engage the A/C unit without turning on the A/C light?

Thanks!

 

Yup using the recirculated setting will also turn on the ac without the light turning on.

 

yep ive experienced this before, with the interior circulation it automatically turns on the a/c

 

Yep again.

And from the owners manual:
@(Recirculation): Press to turn the recirculation
mode on. An indicator light comes on to show that
recirculation is on.
This mode recirculates and helps to quickly cool the air
inside the vehicle. It can be used to help prevent
outside air and odors from entering the vehicle.
The air conditioning compressor also comes on.
Recirculation is not available in floor, defog and defrost
modes. If the recirculation button is pressed, the
indicator light flashes five times and outside air is
delivered. Operation in this mode during periods of high
humidity and cool outside temperatures can result in
increased window fogging. If window fogging is
experienced, select the defrost mode.

 

Ah, mystery solved. Thanks for quoting the manual there, and thanks for your answers guys! I guess I should do more extensive tests for performance/fuel economy now that I know the A/c was on the whole time. I checked the digital MPG readout which was around 29 MPG, so I guess I wasn't taking a huge hit...

 

Really until the outside temps approach or pass 100 degrees, you should notice little to no differance in performance or mileage.

 

Huh? If the compressor is running what difference the temp?

 

if the a/c was a simple fluid heat exchange system, yes temp would not have a impact (other than boiling the fluid) . car a/c uses a phase change gas/fluid. got an evaporator = cold, condenser = hot . when freon "evaporates" (phase change, fluid to gas)in the evap. can carry a lot more B.T.Us with out to much system pressure rise. then compressor compresses gas, condenser condenses & we go around again. lot of heat= lot of gas= lot of power to compress & condense hot gas. ain't no such thing as a free lunch. pump a little heat = little power, lot's of heat=lots of power ... physics suck

 

Yep, Jersey got it right, simply put higher temps = higher head pressures = increased resistance to turn the compressor. See the same in my home AC, higher temps = increased current draw.