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  #1  
Old 07-26-2003, 09:26 PM
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300E A/C- Odd pressures

1986 300E.

Ambient= 90 degrees

System pressure was low. Lowside 10, Highside 90.

Added r-12 until sight glass was clear (28oz.) Highside read around 180 at that point. Paused to let system equalize...

Now, Lowside is -22psi. That's right, NEGATIVE 22.
Highside is 140. Compressor kicks in fine, but even when I shut the compressor off, the pressures stay the same for some time. After about 20 minutes, they do start to equalize (low=+10, High=130).

Ok, so I guess something must be clogged, right?
Where should I start?
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  #2  
Old 07-26-2003, 10:53 PM
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It sounds like the expansion valve is stuck shut.
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  #3  
Old 07-26-2003, 11:31 PM
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Clogged, stuck, or full of ice from air in the system.

Any time the system leaks down, air and water will diffuse in throught the leak while the freon is going out -- I know it doesn't sound possible, and for really big leaks it's not that much, but the air and water will diffuse through the boundry layer along the metal or rubber of the fitting (or whatever else) that is leaking.

Always a bad idea just to blow more freon into a low system without attempting to locate the leak.

Peter
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  #4  
Old 07-27-2003, 10:22 AM
LarryBible
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Yes, a definite indication of restriction and as Fred said it is most likely the expansion valve.

If it is not the expansion valve and an obstruction in a line somewhere, you can find it by feeling of the line temp. At the point of restriction it will be hot before the restriction and very cold right after the restriction.

Hope this helps,
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Old 07-28-2003, 10:31 AM
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Thanks for the responses.
Nice trick with the temps, Larry.
I'll try that.

Say, will it harm the compressor(or anything else) to operate it for very long in this state? I mean just for testing purposes (I unplugged the pressure switch to stop general operation).

Does this expansion valve have moving parts, or is it just a fixed orifice? Forgive my ignorance. I have more experience with r134a systems, and so far they have all had a simple fixed orifice design.


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  #6  
Old 07-28-2003, 01:16 PM
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The diaphragm on the end of the valve is actuated by the small line that senses temp. That small line is filled with mercury. If the mercury leaks out the valve will close up tight. This is not an uncommon failure and is often the case when pressures like yours are seen.

Hope this helps,
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  #7  
Old 07-28-2003, 07:41 PM
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The expansion valve controls the flow of liquid freon into the evaporator. In some systems this is the only freeze-up control, with the valve completely closed at 33F. while in others it never completely closes and there is a thermostat somewhere that shuts the compressor off.

It is open wider at higher temps, so that the air coming out is a contant temp. Also, it will never allow enough freon through to get liquid going into the compressor.

The CCOT type designs flood the evaporator, with an accumlator on the low pressure side to trap the liquid freon (and the compressor will tolerate small amounts of liquid, too), with a pressure sensor on the accumulator to prevent freeze-up.

Not too hard to change the expansion valve on the W124, on the W126 it's under the dash, more of a PITA.

I would be shocked if that capillary contains mercury -- I'd guess a viscous silicone fluid of some sort. Mercury would corrode out of any copper containg metal pretty fast (and eats most others, too, on top of being toxic).

Peter
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1972 220D ?? miles
1988 300E 200,012
1987 300D Turbo killed 9/25/07, 275,000 miles
1985 Volvo 740 GLE Turobodiesel 218,000
1972 280 SE 4.5 165, 000 - It runs!
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Old 07-29-2003, 08:52 AM
LarryBible
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Maybe it isn't mercury but some other fluid. I read mercury on an a/c sight some time ago, but that doesn't mean that whoever wrote it knew what they were talking about.

Regardless of the fluid, my point was that this is a relatively common failure and when it occurs, the expansion valve closes completely.

Have a great day,
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  #9  
Old 07-29-2003, 09:31 AM
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Larry.. Do any other types of failures occur with the expansion valve??? Other than sticking shut that is??

I'm still chasing a problem with my system not really getting that cold. When I hade everything apart, I did not replace the expansion valve.. I'm kind of running out of things to chase..
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  #10  
Old 07-29-2003, 10:57 AM
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Without really knowing, I'm imagining it could stick open, as well as closed.
From Peter's description of operation, this could cause a freezup condition.
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