Putting R134a Refrigerant In A 1991 W126 350SDL--Use A Different Expansion Valve?
 

I am working on the air conditioning in my roommate's 1991 W126 350SDL. It has a factory-looking parallel-flow condenser in it, and will get a brand-new compressor labeled for use with R134a, as well as all new hoses and drier.

Will the stock R12 expansion valve work with the R134a? Or is there a better expansion valve to use? I thought I read somewhere that R12 and R134a expansion valves are different.

Thanks. I'm hoping to install all the parts this weekend and have it ready to be charged at a shop.

The part number for the expansion valve in this particular model is 126 830 03 84, however, it has been replaced with part number 126 830 02 84. When I looked up both of these part numbers, I see several examples that read R134a on them. Interesting.

I wonder if all the new replacement expansion valves being made are for R134 now. Or maybe it's just indicating that it will ALSO work for R134a along with R12. I'm still wondering if the original expansion valve in the car is R134a compatible or if we should order a new one because apparently they are R134a-ready.

Another anecdote is that all new replacement hoses offered by any brand (genuine MB or URO) are all barrier/R134a compatible.

Since you have to remove the expansion valve anyway to flush the evaporator you can install a new R134a tx valve.

Don't buy URO.

-J

Flush the evaporator? NO! Not more work!

For what it's worth, the car was running R134a for years but recently it leaked out.

YES flush the evaporator. You're going through all this other work, unless you flush everything out getting the oil level correct is impossible.

That TX valve is a PITA but don't run 90% of the race and give up.

-J

How does one flush an evaporator?

So just how good a friend is this guy ?

there are several threads.. some currently running talking about flushing the evaporator... also important to clean the outside fins of the evaporator...

You can get a can of A/C flush from most auto parts stores. It is best to remove the carpet around where the connections are for the evaporator, so no oil/flush solvent will get on it. I usually put plastic and then cardboard down so if any splashes occur, it won't get all over the place. With the expansion valve off, squirt some solvent in one fitting of the evaporator, and then blow compressed air (as dry as possible) into it. Be sure to put a rag on the other fitting so the oil/solvent vapor does not spray all over. It also helps to put a piece of rubber hose over the fitting and into some kind of container to catch the oil/solvent. Do this till the fluid coming out runs clear. I usually reverse the process through the other fitting, so it is flushed both ways. Although it is best to use a dry gas for flushing, most people don't have it available. If you do, follow up with the dry gas at the end of the flushing process to get any moisture left behind by the shop air. The vacuuming process should take out any moisture left behind. Once the flushing is complete, you can hook up the new TXV using refrigerant oil on the Orings. You should flush the condenser as well if you can. Being a parallel flow, it will be harder to flush than a serpentine type condenser, but it should be done. With the hoses being new, evaporator and condenser flushed, you will be able to get the oil amount just right with no guess work. Be sure to check for leaks with pressure, not vacuum (this has been said many times here, and is the best way to find leaks). Be sure to remove the caps and connect the hoses to the drier THE VERY LAST THING.

One small tip is to use nylog blue (?) instead of oil on the o rings. Its a reaaally thick oil that is supposed to help form a better seal. It is a bit tough to find, and make sure you get the right color for r134a. This is not mandatory but it may help prevent leaks.

The fittings on the evaporator are called A/C BEADLOCK FITTINGS. On the evaporator side are sizes #8 & 10, on the other side of the TXV are #6 & 10.

This is what the fittings look like on the evaporator and hose ends (ignore the furrule)
AC A C Beadlock Fittings Crimp on Male O Ring Straight 10 Nut 10 Hose 0092 | eBay

If you can find mating connectors to the fittings on the evaporator, then it makes flushing easy. I have not been able to find them (not even sure what they are called or if they are available) and am in the process of cutting up old TXV's to make adapters for pressure testing and flushing the evaporator (if it pressure tests good).

I found that using a small soda bottle with the top cut off and a paper towel stuffed in it worked well for capturing the flush liquid. The flush I used came in an aerosol can with a short hose and a rubber nozzle.

I also had better luck getting the evaporator clean by filling the evaporator as completely as possible, letting it soak for a while, then blowing it out. Try to find a air nozzle with a rubber tip, then regulator your compressed air down as low as you can go. Blow it out several times, each time until it stops draining, increasing the PSI by 20-30 each time until you hit the max rating of the nozzle. This helps avoid spraying flush/oil everywhere.

I ended up using 4 cans of flush to do my entire system, though I had some nasty clogs (presumably from stop-leak). If you have a parallel flow condenser (I think you mentioned that earlier) it should not be flushed, and probably should be replaced, especially if you find a lot of gunk in the evaporator. For a clean system, especially if you're not flushing the condenser, 1 or 2 cans is probably more than adequate.

' cutting ' up the old TXV's ( or taking out the innards and being sure there are two independent routs for the in and out flush fluid.) is the specified recommendation in the MB FSM AC manual .

This is a great idea, because then the flushing can be done with the hoses on and the mess can be in the engine bay, not the interior of the car.

Isn't that oriented more so that you can use the dealership liquid refrigerant flushing equipment? I'm not saying it's a bad idea, just that for DIY'ers it might be unnecessary, since we're likely using an alternative method of flushing.

I assumed it had to do with actually being able to connect SAFELY to the evaporator... do not want a nice non leak evaporator to have to be replaced due to screwing up the connector threads when flushing...

it means you can use the hose connections which the car has... hopefully BACK OUT INTO THE ENGINE compartment area... for positioning your catch ' basin ' for the used flush....
Not sure what ' alternative method ' of flushing you are describing... you used a liquid flush and blew it out with air... (?) so the access to the input hose, and the place the used flush comes out would seem to be convenient if out in the engine area or the floor or whatever ????