On a search of valves , I have found that an 'external compensated valve' does have a pressure port that feeds evap outlet pressure back to the underside of the disc of the TEV.
They are commonly used on larger evaps where the outlet pressure drop can be large compared to the inlet .
By adding this extra control, it insures better metering and a full capacity from evap.. but it is just a compensator and these valves still have the standard heat sensing cap tube ..
There is quite a bit of info using Google...

The brass "screw" is to adjust the superheat. 4-16*F. of temperature increase past vaporization; insures that no liquid is returned to the compressor and "slugs" it. There must be some translation glitch German-English, because the H valve in the MB is an internally equalized not externally equalized. By the way, don't mess with the brass screw unless you have all the test equip., including thermocouples at condensor outlet to measure "subcool" and some other parameters. Makes little difference, anyway, since valve will compensate. The H valve is indirectly temperature compensating, but is really just doing this with pressure (directly related at this section of the P/T curve for MVAC refrigerants). The only direct connection to the evap. temp. is by the capillary tube from the separate evap. temp. reg. switch which is there to prevent evap. freezing. That is factory set at 0*C. (is adjustable, but don't). Thermal tape over the diaphram and pigtail should have nil effect one way or the other. MB uses a clamshell insulator over the x-valve, but i can see little purpose (maybe to reduce H2O condensate?)

__________________
The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)

Jbajj007, WOW, what a great answer....That puts all of the evidence together with no loose pigtails... Arthur , PSFred and Irg,good answers also.... thanks for hanging in here ...

Now the next big QUESTION... What would need to be done to convert this system to orifice tube type? What could be used from this system,,, and what would need to be added... ?

Parts of the reason I ask this are that most cars have changed to this system.... and that an expansion valve is usually designed for the 134 instead of using a 12 one.... and they are not available for our cars as far as I can determine.. so at some point 12 may not be available and it seems prudent down here to look at upgrading the entire cooling process. It should not require changing the compressor, and may even be easier on them...and it would allow the use of the new varible orifice smart valves.... which might really help down here in the heat and humidity..

Thanks, Greg

You'd have to essentially "gut" the system. Orafice tube systems use accumulators, not receiver/dryers (low side of system vs. high side with several other differences) and must have different control circuits (several) since a FOT or CCOT lacks the "self-regulation" of the TVX (exp. valve). Variable orafice tubes are available aftermarket with mixed reviews. I've heard of some "conversions" using R/D AND accumulators, but these guys are waaaaay out there.


Edit: Philosophically; since our diesels will outlast all the cars of the R-12 era, the available R-12 will continue to drop in price as demand for other vehicles wanes (they'll be junked) and make conversion moot. R-12 is a better refrigerant except for cost (and POSSIBLY ozone depletion).

__________________
The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)

I am certainly sticking with r12 as long as I can...and was really thinking of keeping the r12 when changing to the orifice tube....my ford truck has that combo....of course, if push came to shove, I have four under dash add on ac's on my shelf... and lots of compressors... about 8 at last count...we sorta fixate on AC down here, it really is necessary in order to be civilized when one arrives at one's destination.... LOL... Thanks for all the good specific and overview info... Greg

jbaj007:

How did you adjust the screw on the evap temp sensor? AC upgrades and improvements thread a while back mentioned that you adjusted it? Please correct me if I'm wrong.


Thanks


Steve

In the second picture here: Please tell me what the e.t.r. swich looks like!
the evap temp sensor is shown. There is a spring you can make out in the left view. There is a screw that adjusts the tension on that spring. Unfortunately, I can't remember exactly which screw without having the switch in my hand. It is not that difficult to remove and becomes semi-obvious when the switch is in your hand. I believe the screw is accessed from the bottom side (when the switch is in the car) through a slot (hole). I think conterclockwise was a lower cut-out temp. I apologize for not remembering more clearly. I don't think it was under the plastic cover. It is actually more obvious than this rambling would indicate , once you've got it out.

Once I got the hang of it, I could adjust it in the car (standing on my head ) with a "feel" for the screw head and SMALL screwdriver.

__________________
The Golden Rule

1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)

Steve, Why are you thinking you might need to adjust your expansion valve?
That is about dead last in the ' usual suspects' of things needing correcting when an AC system in our cars is not performing up to snuff. And you are risking your Compressor if you don't have the equipment which Jbaj007 mentioned earlier in the thread.....

At this point - the basics will be checked - especially pressures - high and low.

Thanks


Steve