Where did I say I knew everything? What I do know is, once you cool the evaporator to the point where condensate freezes, you've reached the practical limit. There are ways to change this limit, but most are impractical. You could increase the ability of the system to move heat by increasing the size of the condenser and/or evaporator, increasing the airflow through the condenser and/or evaporator, using hot outside air to keep the evaporator from icing up instead of recirculated air (but that kinda defeats the purpose). Obviously, some methods are more practical than others. Short of re-engineering the system, improving MB A/C brings to mind silk purses and sows' ears.

so, in your opinion, MB air is as good as it's ever going to be? Yes you can run the evaporator below your "practical limit" as it is being done today. Yes doing that brings consequences and danger if it's not engineered while the car is being designed or operating in the proper heated environment.

My problem is I will have to develop a "false temperature reading" of the ets to get the performance that I want. There's the danger, but it can be made up. The hybrid system I'm talking about will get you cold fast, but it takes operator monitoring. If the evaporator "freezes" you will notice the drop in vent temp; you must manually shut off the AC. You cannot count on the TXV to control, you cannot count on the ets to control. The point is that on really hot days I know you can run a very low evap temp because of the extreme heat flow over the finns; i'm going to make an effort to do it. Perhaps put a switch in line to change ets resistance on days above 100. A complete worry free system? not at all. But I can manage it. On normal days switch off and run MB specs 2to4degreesC.

BTW the TXV can sense a warmer temp also when it "freezes" opening up even more; adding more refridgerant flow danger. The potential for x-tra refridgerant flow is the main reason for adding an accumulator. I understand others are not even putting in that extra safety measure.

With today's technology in the car and temps being registered, monitored, even can be displayed thru the radio; it would be very hard to convince me that outside temp does not adjust the evap temp sensor/switch in todays car with a default setting for protection in case the outside temp sensor fails. My car is an 86. I do not believe it can do all that.

Why is there a "dessert ets spec" on GM cars shipped to the dessert? I guess they run at 32f evaporator but feel special because their car is called dessert spec. NO! Perhaps they know the evaporator can operate below it's "practical limit" and perhaps they know it must run below its "practical limit" for effective cooling/comfort.

ps moneypit
 

this is a good debate, that goodness somebody has replied. I've been wanting someone to toss this over with. So let me say thanks!
No hard feelings eh?

Ones that are already built, yes. Most people complaining own older cars, so they may have already addressed this issue.
The practical limit remains the temperature at which water freezes on the evaporator. The thermistor measures temperature wherever it is. If the thermistor location requires it to be below freezing for the evaporator to be at this practical limit, it does not mean that the entire evaporator is held below freezing temperature.
No, you cannot. Any time the evaporator surface drops below the freezing point of water, it'll ice up. That the air moving through the evaporator is hotter merely means that it takes more cooling capacity to get the evaporator temperature to that point.
You are still going to find that you run up against limits. Without increasing the ability of the system to move heat, you will not increase the heat it is able to move.
Fixed-orifice systems typically have the receiver/dehydrator in the suction line after the evaporator so as to allow any refrigerant that is still in liquid form to boil off to vapor before reaching the compressor. Variable-orifice systems monitor the low-side pressure or temperature to control the rate of flow. I find it interesting to note that Ford and GM use fixed-orifice systems. Not that I think fixed-orifice is superior. I believe they use fixed because it is cheaper, and their systems are designed with cooling capacity that MB lacks.
Any 'climate control' system has to take ambient air temperature into consideration. Manual A/C does not. Cadillac has had the ability to display A/C parameters for almost 25 years now, although they used the climate control digital display and not the radio.
You continue to confuse thermistor temperature with evaporator temperature.

I don't anger easily, and never over internet stuff. Life's too short.

" Any time the evaporator surface drops below the freezing point of water, it'll ice up. That the air moving through the evaporator is hotter merely means that it takes more cooling capacity to get the evaporator temperature to that point"......Moneypit

I don't dispute that..absolutely correct. yes, i might be interchanging the wrong terms, but the picture is there. If there is a climate with null/low humidity, i think the "evaporator" temp could be lower than 32. But lets talk about vent temperature. I want the evaporator to get cold enough so that when the warm air passes over it; it's coming out the vents at 34 or 35 which is above freezing and is not 49. That is an optimized system. Not one built for all the masses.

I've got one sucess story on my 300E. Vent temps would cycle between 49-54F regardless of outside temp - the ETS was turning the compressor off WAY too soon. I checked the ETS readings with a VOM and a digital thermometer (stuck both into a vent in my house A/C system) and found that the sensor was in spec, but on the far side (warm) of the allowable tolerance. (I think it was 17.5k at 15°C, or something - cant' recall exactly). I installed an 82k ohm resistor shunt, which pulled the ETS resistance readings to the other (cold) end of spec, actually a little outside of spec but not too far (dropped to 14.5k, I think). Now my vent temps cycle from 37-42F, which IMO is perfect.

My 300D is bone stock (no ETS modification) and cycles between 38-42F with R-12. My E500 is also stock but cycles between 42-45F with R-134a. Mercedes says the vent temps should never get below 5°C (41°F) but I believe that's too conservative. I don't think pushing the temps near freezing is wise, but getting the system to the high 30's (F) shouldn't be a problem. I'm going to test the resistance readings on the ETS in the diesel when I pull the dash next time, and I also want to install a resistor shunt (but about 100k) in the E500 to drop the temps a little.

If you're not getting low 40's vent temps, you're not getting proper A/C function.

:vbac47679

Mine is around 17.5k also
Now see, that's what I've been looking for. I had to go through all that to get this great information. My mod is more "mechanical" or "process oriented". ChE thinking sucumbs to EE.
Thanks, I like the 37-42 cycle.
re; ets That's what I meant when I said MB were conservative (maybe another thread). I needed this info badly. I'm not going to mess with superheat, accumulators, but no one would come forward. Moneypit and myself just having a good time debate.

Is a shunt series or parallel?

Parallel. I used 1/2 watt resistors from Radio Shack, bought a pack of 1M and 100k. A whopping dollar each, btw. :p

One 100k plus one 1M = approx 91k total
One 100k plus two 1M = approx 82k total

If you are at 49F vent temps now, and your ETS is on the far/warm side of spec like mine was, I'd try a parallel shunt in the 80-85k range (90k if you want to be a little conservative). I took a photo of the mod, soldered at the stock connector housing, but haven't uploaded it to my website yet. Stay tuned, I can probably do that soon... I also have the more complete original write-up from Stu Ritter or Richard Easley. I'll post that too.

(I'm still not sure why an "in spec" ETS gives such poor vent temps... my theory is that something in the climate control brain pushbutton unit is also on the far/warm side of spec, and the two tolerances add up to lousy vent temps! Not sure if I can prove that though. :cool: )

:o

thanks!
 

I figured it out after I took a look at the resistance closely, it had to be parallel..I would like to see it. After fab, isn't it just an easy splice between the thermistor lines?

Sure, if you could find a place like that. Florida is not like that. Maybe the middle of Death Valley. Any place else, the only time you'll see low enough humidity would be some place where it's already cold. Below 32F, humidity tends to get hard and fall on the ground.

Could be done...either by having the evaporator very close to the vents, or insulating the ducting between the evaporator and the vents. Even then, you'd have to make sure the air stays in contact with the evaporator long enough to drop to your desired temperature. GMs standard for A/C temperature is 40F lower than outside air. Make that happen, and you'll be quite happy, I'd bet.

By the way, you could just override the compressor controls to find out what the system is capable of, instead of messing with resistors and expansion valves.

See, there you go again. Take every single word out and want to dispute. A hard man. The above quote is quite informative. I wish you would have posted that much sooner and we could've discussed that instead of AC theory.

If you are looking for a load capacity test , just drop the sensor out from the housing and test vent temps..
You could also use this to calibrate where you want a shunt resistor value to get you to.
I would only use metal film, close tolerence resistor for the shunt. They are temp stable [ and 5 for a buck]

Here is a calc to save you some time

http://www.1728.com/resistrs.htm

Assuming you are talking the 2.3, I have specs of 10C at 18.3-21.5 Kohm
and 15C at 15.2-17.2K.
so, you may want see what end of the spread is in there now...you may get away with just another sensor on the other end of the allowable spread.

As stated, a 100K shunt is ballpark for approx. 3k change at these specs

Exactly. Pretty simple, really. The original guy who did this set up a remote connector so he could easily plug in different resistor 'modules' to adjust vent temps. I soldered the resistors directly at the connector of the ETS.

:cool: