37 (38) to 43 degrees at idle. it should constantly goes from low to high. Of course this is true if outside temperature is not abnormally high. i am not sure for the pressure however temperature and pressure are proportional according to gass equation p x V = R x m x T.

If fan speed is set to maximum then energy (coolness) drain from the evaporator is high and in a hot day you will have troubles to get the minimal of the above indicated temperatures. So, one has to use a common sense in these things ...

 

37 (38) to 43 degrees at idle. it should constantly goes from low to high. Of course this is true if outside temperature is not abnormally high. i am not sure for the pressure however temperature and pressure are proportional according to gass equation p x V = R x m x T.
If fan speed is set to maximum then energy (coolness) drain from the evaporator is high and in a hot day you will have troubles to get the minimal of the above indicated temperatures. So, one has to use a common sense in these things ...

yes exactly...after decades I go by common sense,
if not quite cold enough, add a little more...
but know when to leave good enough alone.
rule of thumb says 45 degrees below outside ambient..
it was 86, and it pulled 41, that's good enough...
ty, regards...

 

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evaporator temperature is not directly affected by outside temperature. Or better said, it should not be affected. it goes from 3 to 6 degrees (Celsius) and from 6 to 3 ... then again from 3 to 6, etc.

 

no...what I'm saying is, the rule of thumb of a good working AC system,
is the ability to pull down 45 degrees below outside ambient, in a car.
if it's 100 outside, it should be able to have 55 degrees at the vents.
the atypical "ice cold air"
on a house system, my HVAC friend tells me, 25 degrees below outside,
because a house is much larger, and more airspace to pull down
so if it's 100 outside, it should be able to do 75 inside
these are general rules of thumb, not hardfast
if we see that, we don't go digging around for problems...cuz it's working.
the AC won't dispense the same temp air regardless of outside ambient,
there are humidity, airspace factors, and what the inlet is set to affects it,
is it on recirculate, or pulling in 100 degree outside air to treat ? like that...

 

hmmm. cannot agree completely with your logic.

I will try once again . The temperature in the evaporator should cycle between 3 and 6 degrees Celsius. The outside temperature should not affect this process in practical conditions. It is only true that if outside there is 70 degrees celsius (abnormally hot), then evaporator temperature will most probably not achieve 3 degrees for most devices. Another thing which will help you to understand that your logic is not the best: when outside temperatures are low, let's say 5 degrees Celsius, the evaporator temperature will again be between 3 and 6 degrees. According to your logic it should be -20 degrees, which is of course not true.

if outside temperatures are extremely high, but real at same time, then again, a good AC system will have evaporator temperatures between 3 and 6 degrees Celsius. It will take some time that all vents in a car cool down, but after some time the air temperature out from vents will be exactly the same as if outside temperature is 20 degrees Celsius lower. Of course in practice situation is more complicated because different cars have different concept of HVAC. Our cars do not blow coldest air if not necessary. A lot of cars blow coldest air even if the preset temperature is only 2 degrees lower than outside temperature ... Etc.

Please, trust me and try to understand my general message above
Best, Samo

 

it's real world numbers...
today the ambient outside temp was 69 degrees F.
while driving, the evaporator temp on the readout was 36 degrees.
the ambient temp outside the lines, condenser, evaporator, drier, compressor,
affects the pressure inside.
if you see dots of water vapor on the lines,
they are also on the inside of the lines too, if there's any moisture in the system.
there are charts that state what pressure should be, per ambient temperature
we all know AC system is less efficient, the hotter the weather is.

 

we all know AC system is less efficient, the hotter the weather is.

Yes of course this is true. But:

The evaporator core temperature is designed to be between 3 to 6 degrees celsius no matter what outside temperature is and which temperature is preset on a AC control panel. When evaporator reaches 3 degrees, the compressor stops, when it is 6 degrees then compressor starts again. This is independent (not really, see below explanation) on outside temperature and that's why every AC compressor in the world has a clutch.

What happens in the cabin of the car is another story. Some AC systems are simply not enough powerful to cool down the car at high temperatures. This is of course more true at idle. Indeed, this means that such car will most probably not reach 3 degrees in the evaporator, it will stay at 6 or even higher. So the clutch in a compressor will never go off. BTW, if your car or any other car disengages the compressor, it means that evaporator temperature is 3 degrees and system is powerful enough.

the above table (your table) is related to gas equation which I stated above. higher the pressure of gas, higher its temperature at a given volume/mass (R is a gas constant). Nothing else. It does not tell you that higher the ambient temperature, lower the evaporator core. It only tells you that higher ambient temperature means more work for AC to maintain evaporator core between 3 - 6 degrees celsius. We actually do not discuss about the above chart. the gas equation and your chart is happening inside the AC lines ... lower efficiency is due to higher losses of energy ... higher pressure (temperature) means more "bumps" between molecules of gas, that's why "friction" is higher and consequently efficiency is lower. BUT all this does not affect the simple truth that evaporator core is designed to be between 3 and 6 degrees independent on outside temperature.

 

the reality of AC systems is, the hotter the day, the less cold the AC is out of the vents.
both home AC and car AC suffer this malady, even if new.
and take longer to pull down the compartment or room, the higher the ambient temp is.
with a car like MB, even moreso because the glass windows are heating the car,
while the the AC is trying to cool it. working against each other.
the systems will always dispense colder air on a 70 degree day, than a 120 degree day.
common sense. they must work harder when ambient temps are higher.
it's one of those dirty little secrets of AC systems,
the engineers and techs never admit to.
again, this is common sense.

 

all what you wrote is true.

my remark was related to (your) post #6 - first four lines, which are very discussable and not completely true; that's why I added some remarks .