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  #1  
Old 07-15-2008, 04:02 PM
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Join Date: Jul 2006
Location: Pittsburgh
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1997 E420 A/C Expansion Valve stuck open?

1997 E420 with around 125,000. Spring perches sagged around a year and a half ago, resulting in the radiator support scraping and breaking. After a lot of welding, I started to repair A/C problems which I believe were caused by the broken rad support. But after replacing almost everything, I still don't have A/C.

Last May, various diagnoses of leaking condenser, evaporator and compressor shaft seal. System ran well enough to get refrigerant with UV dye into it.

Started last August with a rebuilt compressor I bought on ebay, and had installed, along with a new receiver drier, by a local indy. Even though the original compressor had not failed, he flushed the system anyway. The A/C worked for about a day. The hub on the compressor was spinning, but it wasn't compressing. The indy removed the refrigerant, and said that about 25% of it had leaked.

Ever since then, I don't think the low side has been lower than the high side. In the fall I DIY'd a new condenser, and sent the rebuilt compressor back to the guy who rebuilt it, and he insists it works perfectly (and I don't doubt him). But even though the hub spins, I can add some refrigerant, but it doesn't compress. Without compression, I'm not even creating enough pressure to leak dye.

A few weeks ago I jumped 12 volts directly to the compressor, and kept putting refrigerant in. Both high and low gauges climbed together to about 70 PSI, but no compression, even with the hub spinning. The compressor guy told me it spins a little to circulate oil, but doesn't compress until it sees pressure on the low side (I guess relative to the high side). Suspecting the expansion valve might be stuck open, I also verified old UV dye in the evaporator drain hoses, and decided to bite the bullet.

Bought another drier, evaporator, expansion valve and drier-to-condenser hose (apparently a change MB has made) from Phil. Right now the new parts are in, the dashboard is sitting on the dining room table, but I still don't have A/C.

I harbor suspicions about all of the electrical components, so I am still jumping the compressor to the battery, with the Push Button Unit set to low and the EC light off. I only got a vacuum down to about minus 20 (which holds close to that overnight), so I suspect something may be loose somewhere, but I think I should be able to get enough refrigerant in there to run. But the high pressure reading climbs up right alongside the low, and even though the hub on the compressor spins, there is no compression.

At this point I have replaced all the major components, one of the four hoses and the low side port. I think that covers every single o-ring, unless I'm missing something.

The way I figure it, the high and low sides can only be the same if the expansion valve is stuck open, the compressor is an empty shell, or there is some short circuit or whatever in the manifold gauges. The latter two seem unlikely.

Could a brand new expansion valve be defective? Am I missing something else?

My A/C experience is limited, but I did buy the tools and succcessfully replaced the condenser and drier on the 1996 C280 last fall, so I'm not completely lost

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  #2  
Old 07-23-2008, 12:05 PM
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Update - charging wrong side of A/C?

Well, I looked for leaks and realized that the expansion valve was covered with dye, and there was fresh dye in the evaporator case drain (which I believe came from the expansion valve leak). I redid those seals, vacuumed down to minus 25 (held there overnight) and tried to recharge.

I got about a pound and a half in, and the hub was spinning on the compressor, but I never got cold air. I gave up, put most of the car back together, and drove to the local dealer.

The tech evacuated it and charged it up completely using the high side port. He said that if you try to charge on the low side port, as I had done, it somehow screws up the valves in the compressor. It blew cold air, I paid them $141, and drove home.

Of course it was blowing warm air by the time I got home. The hub spins, and both gauges read about 110psi. No obvious bad leaks, though there seems to be some dye around a crimp connection on the new hose, which I will take a closer look at.

After three weekends of playing around with this, I am at the point where I need the car, so I'm going to put the thing back together. If I do have a leak, I pray it's in the engine compartment and not the evaporator, because it will be months before I may have a chance to tackle taking the interior apart again. I am putting new MB spongy drain hoses on the evaporator case, because I feel that they will soak up dye and make an evaporator leak easier to diagnose in the future, compared to a rubber hose.

Phil tells me that there's no coverage for labor on these parts, which makes me wish that I had tested the evaporator somehow when I got it, rather than risk so much time on an unproven part.
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  #3  
Old 07-23-2008, 02:17 PM
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Charging as a liquid into the high side is preferred, but of course the compressor must not be running at the time. What he did is not only just fine, but is standard.

Charging a non-running system into the low side can cause too much fluid to accumulate in the inlet, slugging the compressor.

There is a test fitting for testing the evaporator for leaks. It bolts in place of the expansion valve and lets you pressurize the evaporator with nitrogen. The FSM shows the tool, its use and part number.

And get a better vacuum pump. Unless you're reporting to the tower, 25 inches is not nearly enough vacuum to boil the water out.
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  #4  
Old 07-26-2008, 06:13 PM
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Join Date: Apr 2008
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I made the mistake of charging the low side without the system running and now have issues with the compressor (i.e. econ light is always on and can't be switched off). Is there a way to remedy my charging mistake? Or is my compressor now shot? (note: I didn't have compressor problems before recharging)
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  #5  
Old 07-27-2008, 10:36 AM
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Check for codes in the pushbutton unit. That will tell you why the EC lamp is illuminated.
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  #6  
Old 07-27-2008, 11:35 AM
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Location: Gainesville FL
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If both sides are 110psi and the compressor is turning you have a bad compressor. .

The compressor is a variable displacement. The stroke of the bore and stroke is altered by varying the angle of the wobble plate. That is accomplished by a POA valve. The point being to limit compression to keep the low side from dropping below the point where water freezes.

The later variable disp compressors spin all the time and are controlled by a solenoidinto the control of the wobble plate. Your compressor should still use a clutch.
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  #7  
Old 07-27-2008, 04:36 PM
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Still not there...

I'm still baffled. I suspect that Stevbfl is probably right (that's why I returned the rebuilt compressor last fall), but the guy who rebuilt it claims to have done 400 (personally) without problems since then, and he had started and sold a huge company that does rebuilds, so he knows a lot more than I do. And, the dealer tech didn't point at the compressor when he had the chance.

I looked at it again today. My assumption was that the system worked for a few minutes earlier this week, and since almost every part has been replaced (by me, most recently), I must have done something wrong and caused a leak somewhere. I had washed the major areas with Engine Brite before I took it to the dealer, and hadn't seen anything too scary, but I figured I would just pour 134a in the high side, with the engine off, and then try to force a leak somewhere. Put about 30 ounces in. High side up to about 130. The hub spins, so the pressure sensor apparently doesn't think there's too much refrigerant in there.

The hub spins, but there doesn't seem to be compression (even though for a few seconds, I thought I heard something laboring, and the high side pressure did read about 5 psi higher than the low, which is progress). No change in temperature at the vent. No dye in the driver's side evaporator case drain. Nothing new at the expansion valve (as best I can see). Compressor and drier look good, as do the hoses. I partially disassembled the front to look at both sides of the condenser, and it looks ok.

If nothing's leaking, I must have about twice the amount of 134a in there as I should. When the tech recovered the 134a lat week, he said he got about .7 lb., which is much less than I thought I put in there.

Something I don't understand is this idea of charging the high side. I repaired a 96 C280, with an identical Nippondenso 7SB16 compressor last year, and charging on the low side with the system running worked out just fine.

Last weekend, suspecting the compressor, I decided to swap the original back in. Unfortunately, I had pulled the control valve from it and I didn't reinstall it successfully ( didn't have the right replacement seals, so I reused the originals, and I didn't know how to adjust the screw - if anyone can tell me how to do this, I'm all ears). The system held vacuum for a while, but when I tried to do a partial charge the control valve leaked, and I didn't get anywhere.

I hate the idea of having the refrigerant recaptured again, and trying another compressor. Should I take it back to the dealer - is there an obvious next step for the tech there to try next? How would they diagnose a bad compressor?
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  #8  
Old 07-27-2008, 05:42 PM
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You said the car worked after the dealer tech recharged it. So he didn't see what you are seeing now.

If the control valve works occasionally the compressor will work (occasionally). If you spin it and the pressures are equal it ain't working
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  #9  
Old 07-29-2008, 10:35 AM
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Regrouping - how to install and adjust compressor control valve?

Well, the A/C has to go on the backburner for awhile, because the Idle Speed Control Motor is rearing its ugly head again, and I have to keep this car on the road.

The technical issue is almost certainly the control valve as stevebfl states. I went back through my notes and I had added almost a full charge (on the low side, with no suction to help) to the point where I had 105 psi on both sides, with the hub spinning, and no compression. The dealer tech put a full charge in, it worked for awhile, but after it stopped I found that the pressures were almost down to where I had them. So the compressor only compresses when the system is completely charged, but I must have a leak somewhere. The thing isn't intermittent - It works until it stops, and that's it.

I have spent a huge amount of money and labor trying to get a $200 rebuilt compressor to work. If I had worked on this myself from the beginning, I would have found the three leaks in the condenser and replaced it. The sytem would have run a long time, I think, before the minor leaks in the compressor and evaporator would have stopped it. Even if I had let the dealer install a new compressor for $1,600 last year, it would have worked for awhile until it had leaked too much. And then the dealer would have looked harder and realized the condenser was the main problem, and then he would have charged me another $600 or so to replace it and I probably would have been set.

But that's water under the bridge. In the car is a rebuilt compressor, with the suspect control valve. I returned it to the guy who rebuilt it last year. It works in his machine shop, so he won't replace it. He insists that I have an electrical problem and is if no help to me. And he may not be rebuilding or testing his compressors properly, so I wouldn't want another compressor from him if he offered it to me. It's 1.6 hours plus $141 recharge per roundtrip on these compressors, and I've been on 4 roundtrips so far.

I also have the original compressor, which would be in working order (not counting the leak at the shaft seal) if I hadn't screwed around with the control valve. By the way, both of these compressors' clutches draw around 4 amps (figured I'd compare everything).

Am I close to having two good compressors? Can I get a shaft seal for my old (Nippondenso 7SB16C) compressor? Can these control valves be repaired or adjusted somehow? If not, who can I trust on a rebuilt, or new compressor? I really want to get the thing working as cheaply as possible, so that I can find any leaks.
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  #10  
Old 07-29-2008, 11:06 AM
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I've never tried to get a control valve, but they are available for some cars. You'll be better off talking to a AC place than a M-B place for this sort of thing.

You can also most likely get the shaft seal, although it will probably require special tools to replace.
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  #11  
Old 07-29-2008, 11:43 AM
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The only rebuilts I have ever trusted are manufacturer rebuilt. In this case that means Denso. I can buy new units for 20% more than a Denso rebuilt so I don't bother with that rebuilt. By the end of the job the small difference in initial cost makes less than a 10% difference in the job. Small price difference to go with new instead of rebuilt.
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  #12  
Old 08-30-2010, 07:37 AM
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Location: Pittsburgh
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New compressor fixed it

Believe it or not, I didn't get back to this problem until about a month and a half ago. I bought an aftermarket compressor, and it solved the problem. I believe that the rebuilt compressor worked perfectly with a full charge, but stopped after any minor amount of leakage. And because it wasn't compressing, it stopped leaking, and therefore I couldn't find the leak.

The only oil and dye that was visible was right at the compressor connections, which was there when I put them on. So either they hadn't been tightened enough each time this compressor was installed, or the compressor itself leaked right at that spot.

I'm sure that the guy who rebuilt it tested it at full capacity, and didn't run it long enough for it to stop working. And he tested it again after I returned it and still insisted that it worked.

The reason I stopped working on the A/C on this car is that it developed an ETA problem. I went through several reman ETA's before giving up and going with a new Bosch.

I think that some people who rebuild parts are over confident in themselves, and don't do the necessary testing before selling their parts.
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  #13  
Old 08-30-2010, 12:29 PM
david s poole
 
Join Date: Mar 2005
Location: dallas
Posts: 1,822
glad to hear that you got it fixed.the valve plate in the old compressor must have been faulty [usually is when compressor won't pump]

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European Performance
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"Fortune favors the prepared mind"
1987 Mercedes Benz 420SEL
1988 Mercedes Benz 300TE (With new evaporator)
2000 Mercedes Benz C280
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