R107 - is it necessary to remove instrument panel to R/R expansion valve?

[nhdoc]

It looks like I have a clogged expansion valve on my ACC, the compressor works but the low side draws about 7 PSI of vacuum with a full charge (high side spikes after startup but settles at 130 or so). I replaced the receiver dryer hoping it was the point of constriction but now it does the same thing and when trying to blow air through the TXV line all I got was a small flow...looking at the CD it looks like MB says to remove the steering wheel and instrument panel to get to it...is there any way to get at it from below or is it absolutely necessary to remove those to get to it?

[KCM]

I suppose anything is possible, but it is much easier to remove the cluster and then you get a clear view of the valve. On the air bag equipped cars, you need to unhook the red connector under the floor mat in the passenger footwell (behind the black plastic cover), then unhook the battery cable. A torx socket is need to remove the air bag as well. Remember to mark the steering wheel in reference with the steering shaft mark in order to get the steering wheel in the same location, as it probably has no marks from the factory. After the steering wheel is off, it is a piece of cake to take the cluster out.

There is a filter in the inlet to the valve which is most likely plugged. Your valve is likely okay otherwise. It might be possible to get the inlet hose off from under the dash to get the screen out, but again it is a real pain to see much up there with the cluster installed.

[nhdoc]

Thanks...yeah my car has the airbag which is why I was reluctant to remove the wheel...oh well...life's an adventure!

Thanks for the tips on the valve itself too...the part is cheap enough that I may just replace it anyway but I'll see what I can see in the inlet.

[nhdoc]

By the way, is the screen at the inlet of the expansion valve removable for cleaning?

[TheV12pwr]

There is nothing wrong with your expansion valve!
Your high pressure is low, it should be about 200 psi and your low pressure is low ,it should be about 35 psi.
Use proper equitment to cnarge the A/C and always pull vacuum first,watch the high pressure,charge untill above 200-220 psi.
Let us know how it worked out!

[nhdoc]

Originally Posted by TheV12pwr

There is nothing wrong with your expansion valve!
Your high pressure is low, it should be about 200 psi and your low pressure is low ,it should be about 35 psi.
Use proper equitment to cnarge the A/C and always pull vacuum first,watch the high pressure,charge untill above 200-220 psi.
Let us know how it worked out!

Nope, there is nothing worng with the charge...the problem is a restriction in the system, which I thought might have been the receiver-drier but is in the expansion valve. There is a full charge now in it...I know this because I put it in, 2.2lbs of R12 (3 12 oz. cans). I evacuated the system after replacing the R-D and before I put the 3 cans in. The reason the high side is low is because the compressor is "starved" due to a clog (presumably at the valve). It can't build high pressure because there is nothing to pressurize coming into the suction side. The reason the high pressure spikes right after the compressor comes on is because then there is some gas on the low side (after both sides equalize) but once that is pumped through the compressor is pulling a vacuum and is starved. It actually is only pulling 7 PSI of vacuum...not 14, the vacuum side reads in inches of Mercury not PSI upon further examination...

[Richard Wooldridge]

Hi there,
It sounds like you put the right amount of freon in, but the pressure readings do indicate a lack of full charge. If you had a full charge in the system, the high side would read well over 200psi with a blockage in the expansion valve. I have seen them go over 250psi on r12 with a clogged expansion valve. When you were installing the r12 you should have observed that bubbles appeared in the sight glass, then either mostly or completely went away before the complete charge was installed. (with a full charge there will be some liquid r12 in the bottom of the receiver/drier).
My factory manual says a full charge for the earlier cars is 1kg, but it doesn't show charging data for the '86 and newer, just the mods on controls, etc. It does have some graphs that show the minimum pressure on the compressor low side should be at least 1.2 bar with a proper charge, and the minimum pressure on the high side should be at least 14 bar. So all indications are that you don't have enough freon in the system, regardless of the fact that you put 36 oz in, minus leakage and losses from bleeding hoses, etc.

[nhdoc]

The problem is that it doesn't matter how much freon you put into the system if it can't circulate due to a blockage the high pressure side will not build pressure. A pump with a blockage on its suction side will never build pressure on the outlet no matter how much fluid is in the system, trust me on this.

The fact that the low pressure side draws a vacuum is an indication of a blockage and if the compressor has no gas coming in the suction side to compress it will not raise the pressure on the high side.

There are some types of expansion valves that have a bypass that will allow freon to circulate past them and return to the compressor without going into the evaporator, but apparently this model does not and if the screen on the inlet is clogged there is simply no appreciable flow circulating through the system - that is the problem I believe.

I appreciate all the comments here but I know the car has a proper charge. There's a plate on it that says the correct charge is 2.2 lbs. I have even overcharged it in the past to test and the system reacts the same way. There is absolutely no cooling under any conditions and the pressures stay too low but when over charged the high side would quickly become over pressure (as there was then incompressible liquid filling the entire high side).

I've worked on car AC systems on and off for 25 years. If the system didn't have a blockage there would at least be some cooling even if the charge was a little low...which it isn't...in this case you need to know both the history and how to read the gauges...the answer isn't always more freon.

[KCM]

Originally Posted by nhdoc

By the way, is the screen at the inlet of the expansion valve removable for cleaning?

Yep, and available (though expensive) from Mercedes.

[nhdoc]

Hmmm...the EPC I have shows no separate part for the screen...you don't happen to have a part number, do you?

How expensive can it be??? the entire expansion valve can be bought for around $40 so I can't think the screen would be more than $10 maybe.

I've got a tank coming in to recover my R12 into and then I will tackle the TXV...I downloaded all the instructions from alldata on how to remove the airbag and instrument cluster...it does not look too bad. Once I get the tank in I am good to go with it...

[KCM]

108 835 02 47. Comes with the new valve. Last one I got from Benz was right around $10. Sounds like a lot to me for a small piece of screen. Guess I'm a cheapskate.

[nhdoc]

Originally Posted by KCM

108 835 02 47. Comes with the new valve. Last one I got from Benz was right around $10. Sounds like a lot to me for a small piece of screen. Guess I'm a cheapskate.

Thanks, I'll keep that number and when I get the old one out, if it's clogged, will just try a new screen before replacing the entire valve.

I look at it this way...it's all relative...if I can get my AC working again for $10 it is the bargain of the century and is about 1% of what the dealer would charge to repair it.

Thanks again for your help.

[nhdoc]

Just ordered the part from MB - $5 and change and it has to come from Germany so it takes a couple of weeks...still, I am sure it costs them more than $5 to handle it and get it here.

[ctaylor738]

The Torx bit is a T-27.

[nhdoc]

Originally Posted by ctaylor738

The Torx bit is a T-27.

Thanks...is that needed to remove the screen from the valve body?

Can the screen be easily removed without disconnecting the expansion valve from the evaporator?