My opinion and question re: recirc / AC compressor
 

I've just searched the threads to determine if the automatic engagement of the A/C compressor when recirculate is engaged is normal or not. Unfortunately, it is normal. (so are fleas, if you're a dog.)

I object to this "feature" on a couple different fronts. One is the assumption that I'm too stupid to engage the A/C compressor myself if I experience interior fogging in recirculating mode. I'ver owned more cars with A/C than you can count on both hands, so I think I'm qualified to decide whether I need to run the compressor or not.

Secondly, the fact that the compressor is engaged without illuminating the indicator, whose sole purpose in life is to tell you if the compressor is running or not, is patently absurd. The only thing worse than an unwanted Nanny is one that lies to you.

There are several circumstances under which it might be desireable to recirculate the air without running the compressor, particularly when passing through a dusty construction zone or patch of polluted air, but GM has taken that choice away from us.

Traditionally, auto engagement of A/C compressor has been limited to defrost mode for obvious reasons, although most cars allow an override if desired. These are generally thermostatically controlled, so as to not engage the compressor when ambient temperatures are below a predetermined level.

My question relates to my winter usage of recirculating mode. I use it for preheating the car on a cold day. Setting recirc through the vents heats the interior dramatically faster than drawing in outside cold air. Has anyone determined if the auto A/C-recirc mode is defeated below a certain temperature?

I don't think it is. I have used my car last winter in subzero temps and as far as I can tell from memory:

1) The AC compressor is always on in defrost mode;

2) The AC compressor is always on in recirc mode;

3) Defrost & Recirc modes are mutually exclusive;


1) is the one really bugging me, as the windows defrost function is the one mostly needed in cold & damp mornings; as soon as the engine is warm, I'd get more heat from the system if the AC wasn't on. If you've ever driven in freezing rain, you know what I'm talking about...

So there's no temp threshold on the AC function, which would be nice, and on the other hand, there's a thermostat on the block heater so that it doesn't turn on above -18C, which I don't want/like.

Yves

Time to install a bypass switch / relay on the compressor clutch circuit, although I'm thinking the ECM might toss a code if it doesn't like what it sees from the high side A/C pressure sensor.

Why wouldn't you use defrost or bi-level modes for interior fogging? Recirculation is not on during these modes anyway and these are the modes that are designed to clear fogging.

I agree that the light should illuminate when the A/C is on period.

No reason you can't still use recirculate in this scenario. The A/C will come on and then go off when you turn recirculate off. No big deal and the amount of extra fuel used would be nil.

The only ricer I ever owned was a 1974 POS toyota pickup. When you turned your defrost on the A/C did not automatically come on. You had to manually push an A/C Button. I like GM's method which has been around since the 60's.

Believe it our not we sometimes get cold mornings and even days in Texas. I have never used recirculate to facilitate the heating of the vehicle. Once the coolant temperature gets to where it needs to be, it works great in all the vehicles I own and the hundreds I drive monthly at work.

Question: Do you have remote start?

No, ambient temperature has no affect on recirculation mode operation.

FYI: Here are the only times a the compressor shuts off besides turning off recirculate, turning off the A/C by pressing the button, turning the fan off or turning the vehicle off:

• Throttle position is 100 percent for 10 seconds.

• A/C high side pressure is more than 2951 kPa (428 psi) and will reengage once the pressure drops below 2068 kPa (300 psi).

• A/C high side pressure is less than 310 kPa (44 psi).

• Engine coolant temperature (ECT) is more than 125°C (257°F).

• Engine speed is less than 475 RPM.

• Engine speed is 6,100 or more RPM.

• ECM or PCM detects excessive torque load.

• ECM or PCM detects insufficient idle quality.

Huh? I thought the idea of the WOT shutoff was to help in highway passing. I dunno bout you guys, but I dont have my foot to the floor for 10sec too often.

I do. Defrost or bi level is much more efficient at defogging. I was going on the assumption that Chevy turns on the compressor on recirc to prevent interior fogging from occurring. In my experience, unless the car is full of people, it takes a fair amount of time for that to happen.


MY 04 GTO allows independent compressor control, but GM Holden is somewhat autonomous. I recall a 77 Monte Carlo I had that would not engage the compressor in defrost mode if the temperature was low. Bug or feature? Who knows. I had to dust off a few brain cells for that one. My 97 Probe is only a semi-ricer ;) and has no compressor switch, but rather what appears to be a simple thermostat in the compressor circuit. No BCM to my knowledge.

No remote start for me. I have a 5 speed.
In Cleveland temps in the teens are commonplace. I can assure you that preheating in recirc mode through the vents loosens up the ice on the side windows and heats the interior dramatically faster that continually drawing in 10 degree outside air.

I really appreciate the detailed information you posted. I'm guessing from your handle that you're a SM at a chevy dealer. Good deal. I have a buddy who's an SM in Houston and has been very helpful to the GTO community. I'll be ordering a Helms manual in the near future.

You're welcome.

I would like to clear up a few things about A/C recirculate mode.

This setting (Max A/C in other vehicles) is to expedite cooling of the interior space. The difference between Recirc. and outside air setting is similar to cooling your home with the windows open or closed. On recirc. mode 80-90% interior air is mixed with 10-20% outside fresh air to reduce the compressor load and speed up cooling.
An automotive A/C system is a variable capacity (tonnage) system. As engine speed increases, more refrigerant is circulated through the system while it removes heat. As the temperature of the air passing over the evaporator drops (as the interior is cooled down), the low side suction pressure/refrigerant temp. drops also. The A/C system has a low side pressure switch which turns off the compressor when it gets to around 35 to 29 psi. This is to prevent condensation from freezing on the surface of the evaporator. R-134A is 32 degrees at 27.8 psi.
Without this low pressure cutout, the interior and the evaporator would get too cold and freeze up.
So, the compressor does not stay on continously, unless the temperature is high enough. Recirculate mode is therefore more efficient for cooling and engine load. The higher the temp/pressure in the A/C system, the greater the load on the engine.

As for the affect on fuel mlieage, I drive with the A/C on recirc always and on my 80 mile round trip commute I average 30-32 mpg. On a recent evacutation trip of over 800 miles with a fully loaded vehicle, through hills, no cruise control and 40 miles of stop and go traffic my mileage was 32.7 avg.

Sorry if this was long winded, but I have noticed this question popping up frequently. And since I'm in the A/C field, I thought that I could shed some light on this subject.