AC expansion valve - PeachParts Mercedes-Benz Forum

jobah · #1 07-29-2002, 01:52 PM

What are the symptoms of a bad expansion valve.

I suppose it would be real high pressure on the high side, and real low pressure on the low side, or, alternatively, relatively the same pressure on both sides.

Thanks

billrok · #2 07-29-2002, 05:05 PM

Why? What are you symptoms? Is the car not cooling? Compressor not running?? The expansion valve is not a part that goes "bad". It can clog but that is unusual unless you have suffered a total black death meltdown. Just wondering.

P.E.Haiges · #3 07-29-2002, 09:37 PM

I always wondered, just exactly what does the expansion valve do? I know it releases the freon into the evaporator, but is the expansion valve variable and if it is variable, what controls it?

Maybe someone here can enlighten me.

P EH

jobah · #4 07-29-2002, 09:53 PM

An expansion valve is a component that can over time deteriorate (it should be replaced when replacing a compressor). If it is stuck and cannot move to variably change the dimensions of the openings that pass through it, it can ultimately cause a catastrophic failure of the compressor.

In my case, I have tried several different refrigerants (BUT NOT 134a), and I have just rebuilt the compressor (parts purchased from acsource.com. I generally replace the expansion valve when the system is opened up (at $28.00) better safe.

I just checked pressures this evening and it appears that everything is working quite well with my latest repair (with the AC, however, it is generally short lived).

psfred · #5 07-29-2002, 11:03 PM

The expansion valve controls the temperature of the evaporator by controlling the amount of liquid freon passing through. When the evaporator is warn, the valve opens and allows more in, when it cools off it closes down and reduces the amount going through. It should hold the temp at about 35 degrees F and the pressure at 35 psi (R12) and about 27 psi (134a).

The most common mistake made in changing to R134a is to overcharge -- an R12 system must be charged to 85% (by weight) with R134a or the low side pressure will be too high, and it won't cool properly, especially at high speeds. A different expanstion valve may be needed to obtain a lower pressure, but shouldn't be needed if the old one is working.

I've converted several R12 systems now, including two MBs (115 chassis and 124 chassis). They all cool just fine, except the Volvo, and I think that is because I put too much freon in.

Properly done, R134a cools as well as R12, lubrication problems aside.

Peter

car54 · #6 07-30-2002, 12:00 AM

PSFred: you have a Volvo similar to mine. (83 760 GLE TD) I recently recharged my AC that hasnt worked since I bought the car(2 yrs). It so far holds a charge just fine, and the system cools like crazy. The only problem is the vent system. It seems stuck on defrost and floor. Every now and then it switches and blows air out the dash vents, but always goes back to floor and defrost. Does your car have the automatic climate control? If so, have you had to do any work to it or happen to have a repair manual on it? Maybe I can replace it with a manual unit from another car?

I have alot of questions about this car and havent found any real technical forums like mercedesshop. Maybe you can help or know of some forums..

#7 07-30-2002, 07:06 AM

psfred,

Your posts are always educational, well written and very much appreciated.

I do, however, want to point out that in Indiana, R134 may cool just as well, but on a 110 degree Texas day, you can definitely tell the difference. I have converted several cars to 134, and have actually converted one of them BACK to R12 later. My Vette converted extremely well, but it has a very small passenger compartment to cool, I think that helped. My '78 Ford truck with a York compressor is the one I reversed to R12 because the compressor just couldn't handle the pressure. The 123's really don't lend themselves well to the conversion, at least in the Texas heat, you can tell the difference.

The best I have achieved on a 123 is on my Euro that has a US installed, aftermarket a/c. My daughters 300D with R4 compressor really struggles on a hot day. I've tried about everything to try to improve it except add a parallel flow condensor.

Thanks for all the great posts,

lrg · #8 07-30-2002, 02:42 PM

PSFred,
Good description of what the expansion valve does. I would add one thing though. The reason the expansion valve is designed to hold the temperature at about 35 degrees is that if it allowed the temp to drop much further the evaporator would quickly freeze solid. As you can imagine, on even a slightly humid day you will get a great deal of condensation on the evaporator as it cools the air. If the temp of the evaporator drops to freezing the condensation freezes and you can no longer pass air through the coils. The expansion valve has a hollow tube that usually is placed right in back (downstream of the airflow) of the evaporator. As the temp drops toward freezing the expansion valve closes up a bit to let less coolant through and raise the temp slightly to prevent freezing. One item to check if you have replaced the expansion valve is to be sure the temp sensing tube is properly placed. If it is too far away from the evaporator you will likely get a freeze up and rapidly diminished cooling as less ands less air gets through the evaporator. Expansion valves are usually trouble free except when a compressor blows and debris has been scattered in the system. Then they should be replaced as a matter of course along with a good system flush.

leathermang · #9 07-30-2002, 03:56 PM

Exactly where is that temp sensing tube ? what is it connected to ? Is that the little node on the copper disc on our (123mb) cars ? When the factory shop manual describes our combo valve as "Thermostatic valve with outer pressure compensation" where is that outer pressure being sensed ? Do any of these answers mean that the copper disc part of the combo valve should not be wrapped with insulation ? Thanks, Greg

lrg · #10 07-30-2002, 04:28 PM

Leathermang,
I've never fooled with the expansion valve on my 123 or 124 but I assume they are all pretty much the same. The expansion valve is located very close to the evaporator. On the units I've replaced the valve had a thin copper tube about the size of a pencil lead that was connected to the top of the valve. This tube was very easy to bend and when replacing the valve you first thread the end of the tube through a hole in the vent system and carefully fold it into a coil that sits right next to the evaporator. Generally you want to avoid having it touching anything that could throw off getting a good evaporator temp reading. The tube itself gets cold and the liquid or gas inside acts on the valve. Maybe there is a more modern electronic way it is done now but I've replaced two of these over the years and they were both the same. The insulation is usually used downstream of the expansion valve and may also be on the valve itself. The system starts getting cold right at the expansion valve, on some cars you can see the frost on the valve when it is working.

leathermang · #11 07-30-2002, 09:07 PM

Approximately how long was this tube you were dealing with ? Thanks, Greg

lrg · #12 07-30-2002, 09:56 PM

I imagine it varies depending upon valve and/or car but the ones I've seen are appx. 12-18 inches. Again I'm not sure of the exact setup on MBZ.

leathermang · #13 07-30-2002, 09:59 PM

Mine is only about 1 and a half inches long.. should I be suffering from 'temp tube envy' ?

P.E.Haiges · #14 07-30-2002, 10:44 PM

Psfred and Irg,

Thanks for the explanation of what the expansion valve does. I understand how the refrigeration cycle works but I didn't know what the function of the expansion valve is. Now I know.

P E H

psfred · #15 07-30-2002, 11:37 PM

Matt:

You may have a vacuum problem -- the climate control is a Frigidaire unit, same as the GM ones, so a GM guide will help. The usually default to defrost when there is no vac, so check vac lines from the engine compartment to dash. It is fairly easy to pull one loose, and I'm sure the rubber is shot on the connectors, just like MBs are.

You may also have a bad temperature sensor, so the system thinks it is alway cold in the car!

Note that you don't have an expansion valve, you have a CCOT (cycling clutch orifice tube) system. There is a fixed orifice at the evaporator in the pressure line, and there is an acuumulator on the low side, to which is attached a pressure switch. This switch opens as the pressure drops (the evaporator gets cold) and opens the clutch on the compressor. When the air is cool, the evaporator floods and the liquid freon accumulates in the accumulator, and the vapor is picked up from the top with a U tube. Ford uses a similar system. Biggest problem is clogged orifice tube screens, as there isn't a receiver/dryer/filter between the condensor and the evaporator.

Larry:

Most new systems with R134a have larger evaporators, and you can usually get one to fit your car. I don't know what the job is like in a W123, but I know my sister had a larger one put on their Volvo 240 when the moved to Florida -- the mechanic said "black car, black interior, Florida, you need more AC". You DO NOT want to put one in a W124 or W126 just to get more AC! On the W124 the book time is 16 hrs!

The difference between S Indiana and Texas is only lower heat (upper 90s this week) and higher humidity -- we will be getting late night fogs soon, and will have them periodically until late September when the air goes below dew point at night. Very sticky. Removing humidity uses a considerable amount of the cooling capacity, resulting in higher air temp. Probably makes us even, since my Volvo is black on black... bad choice, never again!

Re: Expansion valve thermo sensors -- the MB is internal, measures the temp of the gas stream coming out of the evaporator, I think. They don't have a bulb or "wire", a least. Very easy to change. Chrysler used to put the senor in a fitting in the low pressure line, and if you didn't have the conductive grease in it, the evaporator froze. MB has a temp switch to cut the compressor off if the evaporator gets too cold, and if it fails, it can mimic no cooling as the evaporator freezes up.