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  #1  
Old 05-22-2009, 06:28 AM
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A/C pressure switch

does anyone know if the pressure switch located on the receiver/drier is adjustable?
this is for a w123 240d

I will be running a HC mix refrigerant that runs at lower pressures than r12. the cutout pressure on the switch is just a little too high for what I will need. I would like to find an adjustable pressure switch so I don't have to just jump it.

The car currently has a universal receiver/drier with an aftermarket switch (stamped "made in Taiwan"). this switch has no adjustment screw. I will be replacing the receiver/drier with the correct parts.
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  #2  
Old 05-22-2009, 11:07 AM
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Changing the pressure of the cutout switch is going to accomplish nothing. It is NOT a cycling switch. It's in the high-side, so once the compressor first starts (IF it first starts), the pressure only goes up, so the low-pressure switch won't open unless you suddenly dump all of your refrigerant.
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Old 05-22-2009, 11:14 AM
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what kind of freon are you using. It should still run the same pressures, just a different volume of fluid.
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  #4  
Old 05-22-2009, 12:08 PM
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Quote:
Originally Posted by old_ View Post
the cutout pressure on the switch is just a little too high for what I will need.
What pressure do you have? And what do you think you need?

The OE switch closes as low as 10 psi. If you don't have that much static pressure in the system, there is no point in the compressor operating anyway.

Last edited by tangofox007; 05-22-2009 at 12:57 PM.
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Old 05-22-2009, 12:40 PM
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I agree..if you are having to jump that switch then something else is causing a problem...
... it will be over that minimum no matter what substance you use for refrigerant...
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Old 05-22-2009, 12:44 PM
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I think the stock low pressure switch is set higher than 10 PSI...based upon the one I just worked on it didn't close until the system exceeded 30 PSI. I guess if I were to be using a refrigerant which had a static pressure lower than that I'd just jumper the switch and not worry about it. If the A/C began to blow warm then simply avoid using it until you can recharge it or remove the jumper at that time.
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Old 05-22-2009, 12:54 PM
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Originally Posted by nhdoc View Post
I think the stock low pressure switch is set higher than 10 PSI...based upon the one I just worked on it didn't close until the system exceeded 30 PSI.
I checked the book for the W123 and the pressure varies substantially by model. Some W123 models are 10 psi, others show 29 psi.
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Old 05-22-2009, 02:47 PM
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the switch for my car does close at 29psi. I am not using freon, I will be using a isobutane/propane mix (yes i know its flamable, yes i know the system was designed for r12, blah, blah, blah) For this mix to work properly I will be charging to ~20-25psi on the low side.

Is the switch on the high side? If it is I guess it wouldn't matter because I would be easily exceeding 29psi.

Do you happen to know what w123 models have the 10psi switch?
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  #9  
Old 05-22-2009, 02:54 PM
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OK after looking at again I see that it is on the high pressure side. I should be able to use the stock switch with no problems.

thanks!
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  #10  
Old 05-22-2009, 04:10 PM
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Quote:
Originally Posted by old_ View Post
.

Do you happen to know what w123 models have the 10psi switch?
Later 300D's with the R-4 compressor.
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  #11  
Old 05-22-2009, 10:45 PM
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Originally Posted by old_ View Post
OK after looking at again I see that it is on the high pressure side. I should be able to use the stock switch with no problems.

thanks!
The problem is when the system is at equilibrium both the high side and low side are the static pressure of the refrigerant. Depending on the temperature and the refrigerant used it might not be high enough to close the switch, though I would think static pressure of any refrigerant at temps warm enough to need A/C would be at least 30 PSI so the switch should be closed.
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  #12  
Old 05-23-2009, 03:48 AM
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Quote:
Originally Posted by nhdoc View Post
The problem is when the system is at equilibrium both the high side and low side are the static pressure of the refrigerant. Depending on the temperature and the refrigerant used it might not be high enough to close the switch, though I would think static pressure of any refrigerant at temps warm enough to need A/C would be at least 30 PSI so the switch should be closed.
hmm I did not think about this. good point. I suppose I will have to just try it out.

I cant seem to find a switch with anything lower than 29psi cutoff.

I may have to just jump the thing. I'll just have to see what happens.

thanks for your help
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  #13  
Old 05-23-2009, 10:40 AM
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It should be easy enough to come up with a pressure-temperature chart for your refrigerant. But what refrigerant only has a 30psi vapor pressure at any reasonable temperature? I don't think it would be a refrigerant that is worth using.
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  #14  
Old 05-25-2009, 05:34 AM
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The pressure switch is not adjustable. The spec of the switches are as follows. This is from the Official MBZ w126 manual. It should be similar for w123.

Low pressure - OFF at 2.0 bar, ON at 2.6 bar
High side - OFF at 15.0 bar, ON at 20.0 bar.

The manual uses BAR. You can do your conversion to psi, pascal. There are a lot of experts around here on AC.
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