When to turn on car AC?

[Loobed]

Most people don't know this, but newer (GM) vehicles turn the AC compressor on when the knob is in the "defrost" position. That is one reason why you lose mpg in winter. I remove my AC fuse in the winter.

The AC compressor uses up the same amount of power whether you have the cabin fan on full blast or on the lowest setting. Get the inside of the car as cold as possible than turn the AC knob off, not just the fan. You should also select the recirculate inside air button rather than try to cool hotter outside air.

 

[subiedriver]

When its 95 outside and 120 inside my car, my A/C gets turned on right after I start the car, fan on max with recirc on. Within 2 or 3 miles, the car is cooled down tremendously.

 

[Nick R]

Originally Posted By: Loobed
Most people don't know this, but newer (GM) vehicles turn the AC compressor on when the knob is in the "defrost" position. That is one reason why you lose mpg in winter. I remove my AC fuse in the winter.

The AC compressor uses up the same amount of power whether you have the cabin fan on full blast or on the lowest setting. Get the inside of the car as cold as possible than turn the AC knob off, not just the fan. You should also select the recirculate inside air button rather than try to cool hotter outside air.

ACtually a lot of new GM vehicles, and vehicles period, have a clutch on the compressor, and also use variable-displacement compressors, so they use less power when they don't need full cooling ability, and have very little to no parasitic drag when turned off.

 

[antonmnster]

My Saabs from the 80s would engage the compressor even when cranking. But compressors really don't offer much resistance when they're turning too slow to compress anything. My e36 BMW would delay by about 10 seconds (too long!); my e46 waits about 2 seconds after startup.

It's weird to hear talk of leaving a/c year around. There's about 4 months around here when you CAN'T have the a/c run because it's too [censored] cold. And I suppose it's not surprising that that all of my family's early a/c cars never lasted more than 5 years before their compressors seized up.

 

[dakota99]

Heck all this talk about how to turn on your a/c, i hardly ever use mine. Maybe it has something to do with loving tropical weather, id much rather roll my windows down. Although i do try to use it sometimes, just to keep things lubed up in the system. Oh yeah can't forget my dad, every time i take him somewhere he demands to have it running.

 

[Colt45ws]

I dont worry about it. The engine never notices. I leave it on Automagic 74F most of the time.
On my old Cavalier I would wait a bit since the engine would have to rev up to a grand to prep for the load.

 

[SteveSRT8]

On almost ANY car with or without AC the entire electrical load is switched off during cranking. Then the load is allowed once the car starts.

All the way back to 1968 my GM vehicles ran the compressor momentarily in almost any position, no matter the temp. Running it in the defrost setting is normal on almost any platform.

Here it's air on max all the time, and I can't recall a time when my ac is off at all.

 

[Indydriver]

My wife leaves the a/c on at all times. It doesn't cause any mechanical problems but her vehicles always start smelling "stale" within weeks of purchase. I never have my a/c on when starting my car and if I use it at all, I make sure to turn it off a few blocks from home to get all the cold air out of the ducts to prevent condensation and mildew and that stale smell. I also close my windows very time I park it. Result? Still have that 'new car smell' three years in.

 

[K20FA5]

Originally Posted By: Nick R
If I start my car, the A/C is generally on, and I don't worry about it. I also drive off within 10 seconds of starting it usually.

 

[rslifkin]

I always make sure the A/C, stereo, etc is off during cranking, but I don't hesitate to turn it on immediately afterwards.

 

[mechtech2]

What do you think happens when you are cruising on the highway with the AC on? From zero to 2,500 or more RPM instantly!

It's just life in the big city, folks.

 

[FZ1]

I always start the car with the ac off. I turn it on after a few blocks,while at a stop.

 

[ComfyShorts]

Originally Posted By: SteveSRT8
On almost ANY car with or without AC the entire electrical load is switched off during cranking. Then the load is allowed once the car starts.



Wow about time somebody said it, I almost lost faith in BITOG.

 

[Nick R]

Originally Posted By: ComfyShorts
Originally Posted By: SteveSRT8
On almost ANY car with or without AC the entire electrical load is switched off during cranking. Then the load is allowed once the car starts.



Wow about time somebody said it, I almost lost faith in BITOG.


Yup. Because the A/c compressor clutch draws a decent amount of current when it is engaged, it is thus not engaged when the engine is cranking.

 

[ComfyShorts]

Originally Posted By: Nick R
Originally Posted By: ComfyShorts
Originally Posted By: SteveSRT8
On almost ANY car with or without AC the entire electrical load is switched off during cranking. Then the load is allowed once the car starts.



Wow about time somebody said it, I almost lost faith in BITOG.


Yup. Because the A/c compressor clutch draws a decent amount of current when it is engaged, it is thus not engaged when the engine is cranking.


Its not just that, look at any modern vehicles ignition switch diagram, when turned to crank position most components don't even receive power until key returns to run. What components get power removed varies with manufacturer, but try it yourself turn on radio while key in accessory and crank the car, the radio will probably go quiet.

 

[TMoto]

Originally Posted By: Loobed
Most people don't know this, but newer (GM) vehicles turn the AC compressor on when the knob is in the "defrost" position. That is one reason why you lose mpg in winter. I remove my AC fuse in the winter.

I believe all vehicles engage the a/c when set to defrost, and that's been the case for quite some time.

The reason is the a/c dries the air. Some may not have experienced this, but if your windshield starts to fog up and it's humid, turning on the defroster without a/c can make it worse.

 

[addyguy]

I have no idea if it's necessary, but I 'usually' turn off my AC before I turn my engine off, and give it 10-15 seconds before I turn the engine off.

When I turn on my car, I let it idle for 10-15 seconds before turning AC on. I don't know if this does anything helpful, it would just seem to reduce stress on the engine, everything doesn't have to come on 'at once'.

I don't always do this, sometime I leave the AC on, so it is on when I start the car.....

 

[OVERKILL]

Originally Posted By: TMoto
Originally Posted By: Loobed
Most people don't know this, but newer (GM) vehicles turn the AC compressor on when the knob is in the "defrost" position. That is one reason why you lose mpg in winter. I remove my AC fuse in the winter.

I believe all vehicles engage the a/c when set to defrost, and that's been the case for quite some time.

The reason is the a/c dries the air. Some may not have experienced this, but if your windshield starts to fog up and it's humid, turning on the defroster without a/c can make it worse.


Correct. They all do it.

 

[Hokiefyd]

I turn on the A/C in March and turn it off in November.

 

[buickman50401]

Originally Posted By: Loobed

Most people don't know this, but newer (GM) vehicles turn the AC compressor on when the knob is in the "defrost" position. That is one reason why you lose mpg in winter. I remove my AC fuse in the winter.

The AC compressor uses up the same amount of power whether you have the cabin fan on full blast or on the lowest setting. Get the inside of the car as cold as possible than turn the AC knob off, not just the fan. You should also select the recirculate inside air button rather than try to cool hotter outside air.

Its been this way for a long time. My 87 Cutlass Ciera ran the A/C when in defrost... for a good reason. You're trying to remove moisture from the windshield. While sometimes its on the outside, just as often in the winter its also condensing on the inside and its helpful to have the A/C engaged to remove that moisture from the air in the cabin.

You're speaking from the experience of someone who lives in California. Come to a place where we have an actual winter season with temps below 20deg and you'll find out exactly why that A/C engages with defrost. When its winter time and you get in the vehicle on extremely cold days, the vapor from your breath will start to frost over the inside of the windshield and side windows within less than a minute. Even with at A/C running on defrost, sometimes you'll still get frost on the windows on sub-freezing days especially on trips of any significant length. There are times in the winter that I'll run with the A/C turned on running full blast with the temp control turned on the warm to hot side just to get that moisture out of the car.

It seems like a fools errand to follow your advice regarding turning on the AC to turn the cabin into an icebox and then turning it off running it in recirc mode. For one, many cars don't have a separate recirc button and the recirc mode is only engaged when the A/C is on "max".

Secondly, you're going to be turning your A/C on and off constantly in hot weather. You're in an environment whose confines consist of about 50% glass and rest is relatively thermally uninsulated from the external environment. It doesn't take more than a few miles of travel for the temp to rise significantly with the A/C off regardless of if you have it on recirc.

I would hate to ride in a car with you. Mmmm, nice cold air. Okay we're turning the A/C off to wait until we start to sweat again and then we'll turn it back on.