Bad Expansion Valve - Cause of Intermittent AC - PeachParts Mercedes-Benz Forum

treiberg · #1 08-27-2010, 12:01 AM

I think I have isloated either a bad or plugged expansion valve.

Car (W124) has a history of intermittent AC (when it works, it is ice cold! and when it doesn't it is hot), so been barking up the wrong tree thinking my comrepssor was cutting out. Finally noticed that AC clutch always has the compressor engaged, so I got out the R12 manifold and took some readings:

-Static pressure on suction and discharge is around 125#. (ambient was around 100 degrees, so this is probably about right).

-Once the compressor engages I get a vacuum on the suction side! Discharge stays right at 125#.

So, after doing some reading on the internet, this looks like something is plugged, especially since I am seeing a vacuum on suction, and expansion valve is usually the culprit. So I start car back up and tap on the expansion valve and within seconds I start feeling ice cold air again (I had removed gauges, but it was pretty obvious what was going on).

So my thinking is that I either need to clean out or replace the expansion valve. Of course, this involves removing the refrigerant, valve work, flush, vacuum, and re-charge. I don't have all the equipment so will have to find a good AC shop.

Here are my questions:

-Why is this intermittent? Could the valve get plugged then unplug itself, then get plugged again, etc.

-Are there moving parts on the valve that can fail, or is it essentially just a restriction in the line?

-Since the cost of a new one is low, replacement seems the obvious choice. But if it is contamination, is there a chance that a flush could remove all the contaminants with necessitating removal/replacement of the expansion valve?

ekafant · #2 08-27-2010, 12:30 AM

I do not think that tapping would dislodge some sort of blockage. I believe it is just a sticking expansion valve. Your results with the low side in a vacuum definately show a stuck expansion valve. Fwiw, I lost all of my R12 when a brand new expansion valve stuck shut on my 300TD. Exact same symptoms. A flush would not be a bad idea, along with a new drier and replacing all o rings and fresh a/c oil.

treiberg · #3 08-27-2010, 12:42 AM

So I guess that means that the valve does have moving parts, and it is probably just sticking. For $26, a new expansion valve is the way to go, I am pretty sure.

What would a shop charge (assuming they can extract and reuse my refirgerant) just to avacuate, replace valve, flush, and recharge? Maybe 2-3 hours of labor? Wish I had a machine to do this myself!

vstech · #4 08-27-2010, 08:28 AM

is your car running original compressor and refrigerant? you can get a one time use reclaim cylinder at most supply houses, soak it in either dry ice, or put it in a bag, then put the bag in a bucket of salted ice. evacuate the cylinder, hook up gauges, let her rip the 12 out of your system, then open up the line, change the valve and put it back together, then evacuate your car, and put the 12 back in!
a shop is going to charge you upwards of 200.00 labor. and you won't know if it was done right.
now. the valve could be sticking because the compressor is worn and leaving bits of aluminum in the oil. this makes a flush the wise choice. you'll need the correct amount of mineral oil, and a reclaim system that can separate the oil from the refrigerant. then you will need to open all lines and replace the o-rings at each fitting. and that leaves the compressor with possible wear, so do you want to plan and replace the compressor too?

treiberg · #5 08-27-2010, 12:34 PM

vstech, I like your way of thinking with regards to DIY. I really don't like taking car anywhere where I am not sure of the quality of work, plus it costs money!

Car is running R12 and probably original compressor. It cools extremely well (when expansion valve is not sticking).

I might think about the single use cylinder idea...I suppose I would lose a little bit of refrigerant in the process. The AC is so cold when it works, that I tend to think the other system components are up to snuff, so no plans to replace compressor. If I suck out the refrigerant, does this mean I don't need to worry about adding any oil? That would make this job pretty easy. I have one of the cheapo vacuum units from Harbor Freight that hooks up to an air supply to create a vacuum.

I suppose there is no way cheap way to flush the system?

It is interesting how light of a tap on that valve will make the system cool again, I can barely just nudge it. My DIY mind is trying to think of a device that I could use from the cabin to give the valve a tap when it doesn't want to operate, I know this would be the ultimate redneck fix though!

vstech · #6 08-27-2010, 09:49 PM

SEARCH for my post about thayer's car... "compressor engagement device" pic is awesome!

treiberg · #7 09-14-2010, 04:50 PM

Put in my new expansion valve last weekend. Valve costs a whopping $26 but the labor to do the job cost me about 4 hours of my time (it's not worth much anyway). It was pretty tough because it is tucked up in that firewall. There are other threads out there on it. I did not remove the plastic trim. I did undo some of the vacuum lines that were in the way to allow more room for my hands. Used a socket and swivel to remove the 10mm nut, and the 3 mm hex key to remove two bolts that hold the valve in place. It looked like it had a bunch of scale or corrosion inside, but no evidence of metal shavings.

Recharged with DuraCool. Went with it because it is far cheaper than R12 and almost every post I've read speaks of success with the stuff. Get this, one can of it (only 6 oz) was all I put in and I am blowing nice cold air again. You only need about 30% as much of Duracool as you do of R12. Before charging, I vaccumed the system using the Harbor Freight vaccum that you hook to an air compressor, which worked very well.

Air works great now!

leathermang · #8 09-14-2010, 05:59 PM

Quote:

Originally Posted by treiberg

Put in my new expansion valve last weekend. Valve costs a whopping $26 but the labor to do the job cost me about 4 hours of my time (it's not worth much anyway). It was pretty tough because it is tucked up in that firewall. There are other threads out there on it. I did not remove the plastic trim. I did undo some of the vacuum lines that were in the way to allow more room for my hands. Used a socket and swivel to remove the 10mm nut, and the 3 mm hex key to remove two bolts that hold the valve in place. It looked like it had a bunch of scale or corrosion inside, but no evidence of metal shavings.

Recharged with DuraCool. Went with it because it is far cheaper than R12 and almost every post I've read speaks of success with the stuff. Get this, one can of it (only 6 oz) was all I put in and I am blowing nice cold air again. You only need about 30% as much of Duracool as you do of R12. Before charging, I vaccumed the system using the Harbor Freight vaccum that you hook to an air compressor, which worked very well.

Air works great now!

Penny wise and pound foolish on the Duracool and phoolish physics on the type vacuum used.
Suggest others search the archives for good info on both these subjects.

treiberg · #9 09-14-2010, 06:05 PM

Hey leathermang, you might be correct, but there are lots of positive testimonies on the Duracool (don't think I've seen a single negative from anyone who has actually tried it). And on the vacuum, if the device can generate a vacuum as well as a pump, then there shouldn't be any difference. Pulled a nice 25-30 Hg vaccum for about 30 minutes with that tool.

I'll report back in the next 6-12 months and let everyone know how things are going.

bobodaclown · #10 09-14-2010, 06:19 PM

Great job. Keep us updated!