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Sorry if its a funny question. This is my first car so please forgive me if this is stupid question.

Just want to know is it nessecarry to switch on & off the manual ac everytime the vehicle is switched on and off or is it okay to operate it with the key,That is let it be switched on always so it starts & switches off whenever the vehilce is started ?

 What is the right practice that wouldnt harm the vehicle ?

  • The right practice to not harm the compressor is letting the compressor engage and run for some minutes every week throughout the whole year. That way, the compressor's shaft seal and the o-rings of the system stay lubed up and the normal gas leaks (remember, we're talking of o-rings and seals, not welded pipes like with fridges and home AC) slow down substantially. Also, have the system leak-checked, evacuated, vacuumed and then regassed with the proper refrigerant recycling station every 3 years. – Al_ Jun 29 '18 at 11:24
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If you operate the AC just with the ignition key condensation might accumulate in the vents, because the ventilation stops immediately after the AC is turned off. That is why some car manuals (mine does) recommend turning off the AC some time before you turn off the engine, such that the ventilation will continue running after the AC is turned off and the condensation is dried up before you park the car.

If it is really hot outside this has the additional benefit that the temperature shock if you leave the car is not as big.

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  • Great answer. In fact, it's the only reason, at least with modern cars. Electronic control units take care of whenever the compressor should be engaged and whenever it should be disengaged and power/unpower the AC clutch relay accordingly. The OP only has to worry about regularly letting the compressor run some minutes every week so that the seals and o rings won't dry up, and having the AC system serviced by a licensed professional once every 3 years. Also, some compressors are in fact clutchless and always running with the engine on, whether the AC is on or not. – Al_ Jun 29 '18 at 10:55
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I leave mine set to on all the time - in fact that is what is advised in the owner's manual for my car, it won't hurt anything.

If you want to switch it off, then that won't hurt either...

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  • So,How about I keep the ac switched on and switch on and off the blower whenever required ?.Is it a good practice ?. – Darious Stone Jun 30 '18 at 3:15
  • @OP: you'll find you can't do that. That is, the blower may stay on on minimum speed even when you bring it to the 0 position as long as AC is on, unless you play with relays (my car does that), or as soon as you turn the blower off so does AC. If your compressor is clutchless, it will immediately default to 0% flow whenever the blower is turned off (and is designed with that in mind too). This is because a frozen evaporator (caused by a live evaporator with no air blowing over it) is a host to many problems for a compressor (too much cycling or too low lubrication flow rate for example). – Al_ Jul 2 '18 at 20:41
  • For example, if it's a TXV system, the TXV valve might do 2 things when the evaporator freezes up: either let some liquid refrigerant through (if it reaches the compressor, it can either deprive it of oil or slug the reed valves pretty good) or completely close (in that case, lubricant flow to the compressor will abruptly stop and the lube remaining in the compressor will soon migrate to the receiver). If there's a low pressure switch on the suction line (variable displacement comps don't come with them), the compressor will start cycling very frequently and it will wear much faster. – Al_ Jul 2 '18 at 20:49

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