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05-03-2015
Most people know the myth that it's bad to turn your A/C on right away when you start your car. But after giving it some thought, it doesn't really make sense to me. All it does is engage the magnet in the compressor which makes the car work a little harder at idle. That's not much different than starting your car in the winter first thing in the morning. So is this myth not true or am I missing something?
05-03-2015
It doesnt matter, everything is computer controlled.
The PCM is smart enough to know what to do.
05-03-2015
never heard that one before in my life.
fire it up asap if you want. when i had my auto start in my truck i set the heat or a/c as needed to full blast so the cab would be warm or cooled off.
as with most a/c compressors it went bad at 100,000 miles just like everything else on the front of that engine.
05-03-2015
I grew up being told it's bad to do that, but I guess it doesn't matter. Was curious because it didn't line up with anything. Thanks!
05-04-2015
Back in the years of the Model T, I bet it was simply run off the button and therefore would engage as you were trying to start the motor. Like FFDP mentioned, it's computer controlled so it won't do that anymore.
05-04-2015
Model T. LOL
What is the myth? Never heard of it.
When are you supposed turn it on? After 25 seconds of driving? LOL
05-05-2015
Yeah, never heard that one before either... Other than being one more thing to power up on initial start up, it doesn't hurt anything.
05-05-2015
I've been around cars my entire life and have never heard this myth. Apparently no one else has heard it but the OP, so apparently most people don't know the myth.
It sounds to me like someone wanted to sound smart and fed this line to the OP who didn't know any better himself, so he didn't know it was BS.
05-05-2015
I've heard it before, but I think it was only on the older carb'd stuff.
GMPP, Votland, Addco
05-05-2015
I have heard this before on old cars and newer computer controlled cars.
In olden times you manually selected the blend door with the selector that drove a cable. Same thing with the AC, Max, Vent, Bi-level, Defrost and Heat. This selector also moved the door via a cable as well, but dded switching functions to turn the compressor in the AC,Max, Bi-level and Defrost selections. Most of the cars I dealt with had a vacuum switch or other means to keep the compressor of until the engine was running. I once had a friend that couldn't start his car and we diagnosed the problem to a defective switch to turn on the clutch, it was stuck closed, so he was trying to start the car it didn't' have enough power to turn the engine and compressor over at the same time. Turned the AC to the vent or off and it started fine. We figured out the problem, but we never fixed it just had him turn it off to start the car, so he got in a habit of doing that even in other cars.
On later model cars I was always told never leave them in the AC position to turn them completely off before shutting off the ignition. I always thought this was foolish and besides I did a ton of remote start cars that you left it on to cool off the car (it's the opposite for you Yankee's). I being the person I am question everything and every time I asked someone why they thought it would mess up the AC they couldn't give me an answer. The most common response was because my _______ the auto mechanic said not to. Well I know several Master level mechanics and asked them and they said it was all a bunch of pop rocks and soda. Until I spoke with a good friend of mine that mentioned that he had been seeing problems with GM cars that the Blend door (the one for vent/floor/defrost) actuator would fail, and seemed to have them fail every few years and couldn't figure out why. After I started thinking about it I noticed on my Park Ave that I could hear the motor hunting and would notice that the wrong selection was made like I was half defrost and half vent even though I was selected to mid level. I also noticed that It would select the floor and vent even in the same position. This was after leaving it on AC and restarting the car later. I found that the gear in the actuator motor had broken (split) and replaced the actuator several years later it did it again.
Old cars no big deal, newer computer controlled cars, I think the jury is still out. My theory is that when you start your car and the system does not know the position of the doors (like after a battery change or loss of electrical power) the control module wiill find home by moving the actuators and monitoring them until it finds the correct position. Now some systems I have seen with a Pot that is attached to give feedback other systems rely only on a stepper motor to drive the door. If the controller doesn't know where it is it just steps until it reaches an endpoint and it may torque the gear and over time cause them to break (along with the normal wear and tear and just being plastic).
Anyway I have always heard this but never headed it.
Jeff
07-16-2015
The ac works when the compressor is running. Run the compressor - get ac
now for heat, that's coming from the heat produced by the engine. Turning the heat in right away before the car warms up - means cold air for a bit.
08-03-2015
Uhh guys I have a 1991 sunbird and you can't have the ac on when your trying to start it. It messes up the computer and runs at a high idle but thats it. And I think it was just my car so myth busted
Originally Posted by Bio248
Originally Posted by Kilroy
08-03-2015
You must've scrolled through 8 pages to find this thread again lol.
08-03-2015
Naw I just look at what you were posting and saw this garbage thats all
Originally Posted by Bio248
Originally Posted by Kilroy
08-04-2015
Cool, thanks.
08-04-2015
The ac idle speed is higher to run the compressor
08-04-2015
Rev the car up to 2000rpm and then turn the A/C on, then you won't get that drop in RPM lol.
08-04-2015
Some types of A/C systems (especially the one in your house) work in a way where when you turn off the compressor, the freon freezes to a solid when it decompresses. This can be a bad situation when you try to run a compressor. Most home units will automatically stay off for 3-4 minutes to let the freon re-equilibrate.
Automotive systems use a different type of freon have different system parameters so this doesn't happen. You can cycle your A/C with no worries, many cars (computers) do this to maintain cab temperature instead of through blower speed alone.
If automotive systems used the same setup as home systems this could potentially be an issue, but don't worry about it.
On the other hand, I have noticed that occasionally my car doesn't like to start after an hour or so of sitting and I find that turning off the A/C is enough to let the engine keep running. I've heard many people blame this on a dirty MAF/intake but mine is very clean and I'm not sure where to go next but I'm a mile away on a tangent and I don't want to hijack the thread.
-BC
08-04-2015
That's not the reason at all, freon doesn't freeze at any temps encountered in the system. The reason that home AC's that are newer is that during a brownout or a short loss of power the compressor stops and has to start against a much higher load than they are designed for, this causes the motor to stall and burn up or if you were lucky pop a breaker. Car AC is engine driven and not engaged when the engine starts so if you start it against a large load the compressor isn't damaged. It's the electric motor that is the weak link in home AC.
The only reasonable reason not to start your car with the AC on would be what I listed above, and that is just a theory based on experiences of myself and others. I have always left my AC on when I start my car, during the summer sometimes it never turns off.
Jeff
08-04-2015
Both sides should equalize when the compressor is off
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