Hey G friends,

Cars I've owned previously (a while back now) always used to "cycle" the air conditioning compressor depending on how much cabin cooling was being requested. Our Gs (we have an '05 and an '06) seem to run the compressor constantly and use the heat to control the temp... Could this be true?

I understand why "defrost" needs both heat and cool together to quickly dry the air for removing condensation. But in the heat of summer it doesn't make sense to me to run hot and cold at the same time.

Lately I've been leaving the temp set to 60 degrees and just varying the fan speed as needed.

Tell me if I'm wrong about the system...
thanks

 

If you hit that little "Automatic" button, and set the cabin temperature to what you want, it'll turn the compressor off and dial back fan speed once the cabin reaches your desired temp.

Also, our cars run both the heat and the A/C due to the dual climate controls.

 

Normal operation in the summer allows the HVAC to control the fan speed and the temp. Usually you'll find the fan speed is what is controlled the most rather than adjusting the blend door. So if you set the car to 60 on a warm 90 degree day, the fan speed will be max. Over time, as cabin temp drops, the fan speed should dial back, but this is dependant on factors such as interior temp, sun load, etc.

If it was a humid 85 degree night, you'd probably see the auto control start to lower the fan speed a lot sooner. Bump the temp up to 70 degrees or so and you should really see the fan speed lower.

If you want to turn the A/C compressor off and just vent air, hit the A/C button and turn the light off


If it's just running max fan all the time and never adjusting, there might be a problem with one of the sensors. There is a way to "read the codes" through the HVAC display

http://www.nicoclub.com/FSM/G35/Seda..._Sedan/atc.pdf

 

OK, thanks. I think my error was that I got in the habit of running it in "manual" mode so I'm the one adjusting fan speed, turning off the compressor, etc. I'll try it in "auto" as it was designed to operate.. Duh!

 

In AUTO mode, you cna also turn off the A/C compressor. That way in the fall/winter when you are using the heat, you can use the automatic feature without needlessly running the A/C and wasting gas.

 

These cars don't run a standard AC compressor that can only work at full power, or not at all. They run a variable displacement compressor, so it can vary its cooling power depending on requirements, unlike other cars that switch the compressor on and off to regulate cooling.
Therefore whenever the AC button is on, the compressor is engaged.

 

I have a related question that doesn't seem to require its own thread.

Whenever I press the "auto" climate control button, the A/C light turns on and presumably the compressor starts... compressing. It does this even when its 30 degrees outside and I've got the interior temperature set to 70 degrees. I would think that the A/C wouldn't be necessary unless I've got the defroster turned on.

So why does the A/C turn on automatically? It seems like a complete waste of energy. I've been turning it off manually right after hitting the Auto button, but isn't it stressful to the A/C to be turned off immediately after being turned on?

It just seems like stupid behavior for the A/C to turn on automatically when the outside temperature is so low. Should I just leave it on or should I turn it off?

I'm not sure what the engineers were thinking when they designed the climate control system.

 

Well, i guess the thought is "Auto" is automatic control. So depending on the conditions, it would apply heat or A/C or defrost as needed, so the A/C compressor might be in some use.

The A/C compressos has variable displacement, so the load on the engine changed. Some ac compressors cycle on and off, the one on our G35 just varies how much it pumps.

 

Thanks for the reply. I wonder if variable displacement includes zero displacement.

 

Same here. This is the biggest gripe about the AC/heat system. Not only that, when you use heat and hit the recirculating button, the AC comes on and you cannot get rid of it unless you use outside air. So what happens when I follow a big time smoking diesel truck, I had to inhale the damn fume!!!

 

I've been in that scenario as well. Very aggravating. I don't know of any other car that behaves this way.

 

just run the a/c at all times.. it makes for a more comfortable air mix in the cabin even when it's freezing temps outside.
it's called automatic for a reason

 

comfortable air mix? huh?

 

less humidity, less over-powering heat
I find the balance of cold and hot air creates a comfortable cool or warm air, depending on the temperature desired and not on outside conditions

i.e if you want to sit in an oven during the summer for whatever reason, you can turn the temp up to 90, leave the a/c on and it will get hot in there.
the opposite goes for winter. just because it's freezing outside does not mean you'll want to sit in an oven.

try it once, if you don't like it you can always go back to turning the a/c off

 

If you turn on the AC when the outside temperature is much lower than the set temperature, it still runs the compressor, as it assumes you want to dry the air to prevent the windows fogging up.

As for the variable displacement compressor.. it has a minimum displacement, it can go to 0 without disengaging.