I've replaced the expansion valve in my SD recently (the old one was leaking). The old valve was insulated by two pieces of foam, which promptly disintegrated during removal. I lack special AC insulating tape, so I cut out two pieces of packing foam to hug the expansion valve snuggly. I believe the valve itself is now well insulated, but I have doubts about the capillary tube on the left side of the valve. The old (stock) foam was barely covering it. Also, on both old and new valves the tube is bent in a small arc to run parallel to the left face of the bulb on the valve, without actually touching it.

My AC is not running particularly cold, and that got me thinking that perhaps there's something wrong with the way I installed the expansion valve. I have a fair amount of water condensation on the low side hose inside the engine compartment (the section between the firewall and the compressor), and the Mastercool book I got cites this (plus higher-than-normal low side pressure, which I also got) as a possible symptom of the expansion valve opening too wide. (I also hear periodic loud hissing sound from the area of the expansion valve -- no idea whether it's a normal mode of operation or a sign of something being wrong). On the other hand, I'd imagine that the low side hose will always be colder than the surrounding environment, so depending on the current dew point, seeing condensation on it wouldn't necessarily indicate a problem.

It makes sense: if the capillary tube is not sensing the temperature of the refrigerant at the evaporator outlet, but senses some higher temperature instead, it'll open too wide. I've searched. I found a post whether someone suggested that when installing the expansion valve, the capillary tube should be bent and taped to the valve outlet (the one going to the compressor). The suggestion makes perfect sense, but I'm puzzled over the fact that the stock valve is not installed this way, and the factory manual is silent on the issue of the capillary tube. Also, that tube is pretty short, and doesn't look very flexible, so it would be hard to actually tape it to the outlet hose. Should the tube be left in its original configuration, and simply be insulated as best possible (what temperature will it be tracking then?), or should it be bent to be closer to the outlet connection? How do others approach this?

Thanks

Yes, the bulb does need to be insulated.

Go by your local heating/ventilation supply house and tell them that you need some "Presstite" tape. It is not expensive at all and you can just press it in place and be done with it. Or if you can talk to a home refrigeration service guy, he'll probably just peel off a few inches or however much you need and give it to you.

Good luck,

Thanks Larry. The bulb and the capillary tube are already insulated with foam, but more insulation is never a bad idea. I bought some special AC insulation tape today, and will take sure that bulb is insulated well.

The question that bothers me the most is not so much the exact method of insulation, but rather the configuration of the capillary tube. Should it be left as is, i.e. curled parallel to the valve body without touching it, or should it be somehow bent in an attempt to touch the pipe going to the compressor?

What I don't understand is exactly what temperature the capillary tube is sensing in its default configuration. Suppose it's well insulated from the outside. What will the temperature be inside the insulation in the area of the tube? I'd imagine it's close to whatever the temperature of that portion of the expnasion valve body is. I don't see why that will necessarily be equal to the temperature of the refrigerant leaving the evaporator. The expansion valve is made from aluminum, meaning it conducts heat very well, and there's two streams of refrigerant going through it: hot liquid going in (on my car it's pretty hot, unfortunately) and somewhat cool gas going out. I don't know how overall temperature balance works out between those two.

I experienced the same problem on my 300D (123.133). A hissing noise after rebuilding the A/C, it turned out I had installed the expansion valve backwards, and the hissing noise was a leak.

Look at your old valve, it has two different sized ports. 5/8 discharge and 7/8 suction. Your discharge hoses screw into both 5/8 ports and the same for the 7/8 hoses. Notice if there are slight differences in the port design, the openings are the same thread, but there might be a check valve on one discharge opening but not the other. Same for the suction ports. I also think there is a slight difference in the seat, while the hose will thread properly, it may not fully seat, therefore leaking out your precious freon.

I also think the skinny 1 inch "tube" hanging off the side of the valve (not sure what the proper name is) should be pointed downward. Alan

Rurik, I think some confusion has crept into this process...
First , the bulb and tube on the 123 H type combination TX valve is supposed to be vented to the atmosphere... it has a little hole in the end... this provides closer reaction to regulating the temperature of the evaporator ( as sensed by the exiting gas ). Below freezing temperatures are what is being avoided... and that temperature varies some with barometric pressure and humidity... only at sea level on a ' standard ' ( 29.92) day does water freeze at 32 degrees...

The capillary tube which is supposed to be in contact with the evaporator is long and spindly... and I think has an electrical connection to a switch in the system.... and is different from what you are dealing with on the side of the 123 R12 Combination TX valve ( it is an expansion valve and suction throtteling valve together ).

I'm pretty sure my expansion valve is not installed backwards. The thin inlet hose (coming in from the receiver-dryer) is on the right side, and the thicker outlet hose (going to the compressor suction port) is on the left (looking from the driver seat position). This is the way my old valve was installed.

My system held 29" vacuum for 24 hours before I charged it, so I shouldn't have any really large leaks (although one never knows, the vacuum test is not definitive). The hissing noise I hear from the area of the valve is not constant, it's heard periodically when AC is operating. I can't say yet for sure that I'm not leaking any refrigerant, but it's been a few weeks, and my pressures are still high.

Right, there's the the ETR sensing tube, which goes inside the evaporator housing, and there's the short tube on the expansion valve -- I'm talking about the latter, see the picture.

Having thought about it, and having looked at the old valve, I'm now thinking that the right thing is to leave that tube in its default position, because that "mushroom" that the tube attaches to likely has that peculiar shape specifically so that the capillary tube can run parallel to its surface and sense its temperature. The mushroom looks to be hollow on the inside, so it must be filled with the vapor of the escaping refrigerant. It still feels like it would be more logical to tape that tube to the outlet hose, but whoever designed the valve had good reasons, I'm going going to argue with those, just trying to understand how it works.

Is there any field test to tell whether the expansion valve is opening too wide? I don't think it affect the overall AC performance that much, but every little bit helps in my situation (crappy condenser, large black car, Texas).

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I think tests on the TXvalve are of the ' bench' type... except for the usual symptoms listed like evaporator fins freezing up... a clear indication the TXvalve is not keeping the temps below freezing on the fins... one of its primary purposes.
Jbaj007 would be able to answer this stuff... he has taken the apart, modified them... all sorts of good stuff...

In general I think you are worrying about the wrong item in your system... but it won't hurt to know lots about it...

rurik,

You seem to have well understood the ETR capillary tube vs. the TXV capillary tube positions and functions. Leave the TXV tube in the default position. The TXV tube is a reservoir only for refrigerant (sealed separately from system)that expands or contracts depending on the temperature of the EXITING gas from the evaporator. It therefore "self regulates" it's opening.

The ETR is actually ONLY a switch (on-off); factory set at ~40ºF(+/- ~4ºF). It's purpose is to prevent evap freeze-up. It is adjustable, by means of a small screw that acts on a spring, that acts on a set of points, just like in an old fashioned relay. Adjustment is usually not needed, unless you are really fine-tuning the system, and is not recommended by MB, even for it's techs. You need to fully understand what you are doing.

The TXV, likewise, is adjustable, by varying the pressure on an internal spring by turning the brass plug at the end with a special tool (home-made). Again, usually not needed and not even an MB tech approved adjustment. When early conversions to R134a were made, some clever techs learned to turn it clockwise a bit to compensate for the R12 vs R134a density (molecular size). One would even crank it a bit more for a less dense refrigerant like HC.

The field test for the TXV means measuring the inlet and outlet temperature at the TXV with some very accurate thermocouples (what's called "Superheat"). Unless HVAC is a passion, don't go there.

Insulate the components as Larry said.

Post hi/lo pressures at 2000 rpm and ambient temp. Vent temp, also. R12 or conversion?

__________________
The Golden Rule

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

[QUOTE=rurik]I'm pretty sure my expansion valve is not installed backwards. The thin inlet hose (coming in from the receiver-dryer) is on the right side, and the thicker outlet hose (going to the compressor suction port) is on the left (looking from the driver seat position). This is the way my old valve was installed.

It's impossible to properly install the suction line into the discharge port of the valve and the discharge line into the suction port of the valve.
What I meant is this:
Look at the suction port and the discharge port. Flip the valve over and look at the same ports on the opposite side. Are there any differences in the seats, internal components that are present on one side, not the other?
The tube that faces towards the left should be facing downward, at least this was the case on my 300D 123.133 I orginally installed it facing up, and the valve leaked over time, hence the hissing noise.
I found that there were suttle differences between the openings as described above that caused a leak on my system. Do you have the correct black a/c tape to insulate the valve? I have a roll here, would be glad to send a piece in the mail.
Just my experience. Alan

That is something I'll try to figure out. My expansion valve is new, and even though there's nothing stamped on the valve itself as to whether it's calibrated for R134a or R12, I'm pretty sure it's calibrated for R134a. That's the refrigerant I'm using now, but my plan is to switch to R12 if R134a doesn't cut it, and thus far it's not cutting it. From what I've read, nothing particularly bad would happen if a R134a valve is used in an R12 system, some efficiency loss would be there.

Well, the car is sitting now with the central console and a few other parts pulled, because my ACC wasn't working (all air is blown via defroster vents, none via the central vents), which was exacerbating further the not-so-cold AC problem. So I can't easily take the measurements right now. The ACC subsystem itself is back to life now, but there's some other vacuum issues that I need to figure out. Right now I have a choice of having working ACC or being able to shutoff the engine with a key, but not both... Something's wrong in the shutoff circuit, and it's not the shutoff valve.

Instead of worrying about the 15% you lose from having R134a in your system.... I suggest you concentrate on the side of the equation which will allow you to compensate for that most effectively ....
That is the efficiency of your Condensor.... and the AMOUNT of air you blow across it...
That would point to a P-Flow condensor and a high CFM fan...
both have great discussions posted..... search for Spal for the fan name and use Gregs as the poster for the P condensor search... but spell out "P" correctly...which I can't do this early in the morning....
They are forecasting a record 98 for Austin today...

edit.... there is a special tool shown for holding the TXvalve.... sorts looks like a hand in a mit at the end of a handle... for those who have experienced leaks at those junctions... it is in a bad place to get to....
Even on junctions which do not have orings.... nylog should be put on all threads...

Was this problem present when you replaced the expansion valve? If not, before you go and dig a deeper hole check a few things.
Do you have a MityVac? It's a must when checking for vacuum leaks. Do the doors, trunk, fuel lid lock/unlock properly with the key? It sounds like you are guessing at the problem. A leaking door actuator, or cracked vacuum line/hose under the hood will cause the above mentioned problems. Usually start under the hood at the yellow check valve, pull a vacuum and go from there.
You need a vacuum diagram, I'm sure someone on this list could e-mail you the PDF file from the Mercedes CD for 126 cars. I only have the 123 CD. Then we can go about helping you out. Alan

Is this a 126 with the 617 or 603 engine? It's been a while, I think the 126.120 had the 617 and the 126.134 had the 603

Yeah, I know, condensor is the part that bottlenecks everything else. I'm contemplating installing a parallel flow one (have seen many of those threads), but the need for custom-made hoses and new mount holes is not particularly appealing to me. First, I want to see how well or poorly the stock system performs once it's actually operating properly, which is something I haven't experienced yet. I've spent a good portion of the day yesterday working my condensor over with one of those special combs, it looks better now (was a complete disaster before that), although despite all that combing there's no denying that this condensor's glory days are in the past. I'll also look into rigging the aux fan to run at high rpm whenever the AC compressor clutch is engaged.

I've actually seen 100F in the forecast for Austin today, and yesterday made me believe it.

Yes, I think that the specs listed by some who have put in the other condensor were 123 owners....
Have you called Trey at Carlisle Auto Air in San Antonio ? I think they also carry carry the Spal fan...