Hey everyone,

I've heard some rumours that you shouldn't turn on your a/c while at high speed (say above 40 mph) because it damages the a/c unit. Only when you are at a compete stop of going slow (under 20 mph) Any believers? TIA.

 

I've always wondered how this works..

How does the A/C compressor turn on with the belt turning at such speeds?

Does the accessory belt only spin at one constant speed? Or is it linked to the engine?

There must be some clutch or something (what the hell is a 'clutch' anyway?) that activates the compressor because you can FEEL the pump turn on and off.

Why?

 

never heard of that...

 

I'm going with false. Don't have anything to back that up, but I just find it hard to believe that current A/C units would have the glaring flaw.

 

no, no. This has little to do with MPH, more to do with RPM's.

The A/C uses a clutch to engage. If you're sitting @ 5500 RPM you could damage your compressor, but not when driven normaly. Also, if you notice, the A/C will not blow as cold when you're hammering the gas. It actually disengages (I know Lexus cars to this and believe most 'modern' A/C units do this as well)

So, basically no worries, leave your air on when you need it, it will switch off as necessary!!

-Cip

 

i've never heard this and am going with false as well...it lowers your gas mileage, especially in city driving....so if anything, then low speeds is where you wouldn't want to use this...most a/c systems stop working because they need recharging or have a bad seal, not because of damage to the unit itself

 

Yes, this is definitely false.

Although the A/C turns off when under WOT throttle conditions for power purposes. You can actually feel the A/C get warmer and then cool once the load factor returns to below WOT. But you can't hurt your A/C by leaving it on when driving around, how would people who live in Fla or Az, etc. deal with summertime highway driving w/o it? Just doesn't make sense. People who buy these high end luxury cars would have a fit.

 

Since we are on the subject of A/C.....I'm gonna hi-jack a bit. Whats the theory behind shutting off your A/C prior to shutting off your car. Does this matter??? Does it hurt anything??? Does it come on immediately when you start the car???

 

I think that's one of the habits of the 'old timers' where anything that was 'on' when you tried to start the car would put a load on the battery. This is not the case with post -'80's cars

Specific to the A/C on our cars, the clutch will not engage if the car is not running.

 

My question was does it harm your a/c by turning it on while at a high speed (say you had your windows down and decided to turn your a/c on).

 

Like mentioned earlier....speed doesnt have anything to do with whether or not the compressor is going to be harmed. Its all about RPM's. You can be doing 90mph and pulling 3400 RPM's and you can be doing 30MPH and doing the same RPMs. Compressor pulley speed is going off the RPM's of the engine...not the speed of the car. Hope this answers your question....

Im sure the engine is designed to only allow the compressor to be engaged within a certain range of the RPM's and not to engage lets say...over 5000 RPM's. (just a guess)

Make sense???

 

ive noticed the car cranks a bit sluggish if the AC wasnt off before you started it. The starter is trying to crank the engine as well as the AC compressor. My guess it stresses the starter uneccessarily when the AC isnt off when cranking

 

What car?? I've had 3 Lexus cars, as well as about 20 others & never noticed the problem you post.

Just b/c the A/C button is on does not mean the clutch is engaged. It's not, but I'm all ears if you can prove me wrong

Please clarify what vehicle this affects.

 

There's actually a more modern reason for this too - when you leave the A/C on right up until shut off you let the moisture that's built up in the filter to sit on it, which can produce that kind of musky smell you sometimes get when you run the A/C a lot in the summer. If you turn it off a couple minutes before you shut the car off and run the fans on high it helps mitigate that. It's a symptom of poor design, but it seems to happen on a lot of new cars.

 

AC almost never gets turned on in my car. It seems to kill my power bands proportionately worse than when it is at sea level as opposed to above 7,000 feet (Mexico City).