Starting car with A/C on--bad practice or not?

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pbm

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With summer approaching I was wondering if any of the engineer types know if it is bad to start your car with the A/C already ON from when the car was turned off. My friend always starts his car with the A/C off but I usually forget to turn it off when I shut down and it starts as I start the car. Is this bad for the compressor etc.. Thanks.
 
On modern cars with fuel injection, the PCM won't engage the compressor clutch until the engine has started and the idle has settled. So, starting up with the AC switch turned on is really a non-issue since the PCM and not the switch is what will determine when the compressor will engage.
 
It is a good idea to turn off the AC a minute before shutting down though, even though the above statement is true regarding startup. It is good to let the system depress. a minute before shutting off the motor, helps let heat out of the system as well.
 
While the AC compressor isn't engaged, the blower motor is running. At least on my car. When you turn the key the blower kicks on, is off durning cranking, and then back on.

Maybe a little more load. But I can't see it making a huge difference.
 
Probably more important is to shut the A/C off a minute or two before shutting off the engine to allow pressures to stablilize and some of the condensation to drain off before powering down the car. I generally try to remember to shut off the A/C about 1/2 mile from home when returning. This also leaves the A/C off at start up. I like to run the car for 1/2 mile or so before switching on the A/C.

I have no proof of the goodness of these habits other than the fact that my '96 Volvo has not had an air conditioner evaporator failure yet when such failures are considered normal in the first five years for these vehicles.

It stands to reason that letting the engine stabilize on start up before dumping on the load of an A/C compressor is probably a good thing.

John
 
The main purpose of remote starters is to start your car with the AC or heat left on.
 
quote:

Originally posted by pbm:
With summer approaching I was wondering if any of the engineer types know if it is bad to start your car with the A/C already ON from when the car was turned off. My friend always starts his car with the A/C off but I usually forget to turn it off when I shut down and it starts as I start the car. Is this bad for the compressor etc.. Thanks.

I have been told its bad but always leave mine on and then turn it off after starting. I have 210k on the car and never an issue with AC.
 
There are probably some years/makes/models of cars where you should turn off the A/C before starting the car, but probably not many THESE days.

If you let your A/C run right up until the engine is turned off you get a pissy STANK in the evaporator.

I also agree it's best to shut it off a minute early to let the A/C stablilize, but from my point of view it's mostly a temperature thing, not a pressure thing.

This is why I don't like automatic climate control. When I had it I was always overriding the controls...
 
On users manual of '75 Oldsmobile, after describing the use of HVAC it gives advises for "seasonal adjustments". At least for GM it is ok to left the "climate control" untouched seasonally. Instructs furter which combinations to prefer in which season and "AC on" option is included for summers.

There are pressure release valves, for thermal stress there is already anough when engine bay is hot while running. They all should taken care of.

But me also shuts AC 1/2 miles before terminal. Bc it still can blow cold, and in case I have to maneuver, D to N to R shifts are smoother with lower rpms on my engine.
 
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