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  #1  
Old 07-09-2018, 02:50 PM
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Bad new ac low pressure cuttoff switch?

My ac has been working wonderfully after a bit of RnR including a new drier and low pressure switch, temp switch, r4 compressor, expansion valve.

Anyway, today ac started blowing warm all of the sudden after being on for around 15 minutes of light highway driving. I turned it off and waited until I got home. After some troubleshooting and checking pressure (low side was like 25 high side around 230-250 maybe) I ran across a theory that my low pressure switch could be bad and had allowed the compressor to stay on too long, causing the expansion valve to freeze. When I drove car for another errand about 45 min later the ac was blowing cool again.

Does this seem like that kind of low pressure cuttoff problem? I havenít checked if MV even makes one but perhaps these new switches are of suspect quality.
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Old 07-09-2018, 07:53 PM
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There is no low pressure cutoff. The switch on the receiver/dryer prevents the compressor engaging if refrigerant pressure is too low, but there's nothing on the low side.

If you still had the pressures you say above after the system had been shut off for a while, you have a clog in the system somewhere. You don't specify if the pressures above are standing or with the system running. If you didn't do a thorough job of pulling a DEEP vacuum to dry the system out or if you used the wrong oil for the refrigerant you filled up with, you can have problems. If the system was not thoroughly flushed prior to installing new components, you may have a piece of debris lodged in the orifice of the expansion valve.

What year is this? Model? This stuff needs to be listed, Ms. Cleo doesn't work here. If you have a 1985 or later vehicle, the Klima relay can cause problems with intermittent operation.
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  #3  
Old 07-09-2018, 08:53 PM
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?! Ms Cleo is gone ?! .

Oh, no ! .

I thought the new, red AC pressure switch was both high and low cut off , non ? .
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Old 07-09-2018, 09:12 PM
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Quote:
Originally Posted by vwnate1 View Post
I thought the new, red AC pressure switch was both high and low cut off , non ? .
It is on the HIGH side of the system. If the low side goes low, it does nothing to cut off the compressor, only if the pressure on the high side is low enough to open the switch (low standing pressure from a refrigerant leak).
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  #5  
Old 07-09-2018, 09:23 PM
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Thanx ~

Obviously my knowledge of AC is way out of date, I stopped doing AC repairs in 1985......
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1982 240D creampuff 370,000 miles
1978 300CD back from the dead&1980 300CD ~ SOLD
1984 300CD KEEPER ! 428,XXX miles
1984 Euro 300TD Fully optioned SWMBO's
1974 350SLC 4 speed stickshift SOLD & missed
Krazy Kommie Ural Motos (3)
BMW Moto R60/6 Barn Find, 8,000miles
1959 VW #113 Deuxe Beetle, 36hp engine, stock
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  #6  
Old 07-09-2018, 10:38 PM
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Some vehicles DO have a low-pressure switch on the suction side, usually vehicles with fixed orifice metering devices, that's how the compressor cycles. The Mercedes system uses the dual pressure switch to lock the system out on low charge and cut the compressor off on high head. Cycling is done based on cabin temp and/or evaporator temp switch (freeze 'stat).
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  #7  
Old 07-10-2018, 09:19 AM
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Oops, sorry, 1982 300D.

Yes I pulled a deep vacuum for 45 minutes after installing drier and before charging. I am using ester oil becaus I plan on putting r12 in and wanted an oil that would work with both r134 and r12.

The numbers I mentioned were while running/on. It did take a couple minutes for the high and low side pressures to equalize out after switching off.

The switch I was talking about is called (on Pelican) the ďAC Pressure Switch On Receiver DrierĒ

But maybe my problem is the freezing and related to the Evaporator temp switch? Does that require discharging the system to swap out? Can I get to it under dash/bglove box or is that a dash out kind of job? (If that is even the problem).
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Old 07-10-2018, 09:34 AM
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The Evap temp switch should cut the compressor off when the evaporator temperature gets close to freezing. If it has failed or is way out of calibration, the evaporator can freeze up on a humid day. When that happens, the system will blow warm or if it's frozen bad enough, won't blow at all.

What refrigerant are you running? That isn't specified anywhere in this thread. If you're running 134a, your suction pressure is low. Make sure when you're measuring pressures that you have all windows open and the system defaulted to max fan speed. Low airflow from the blower being on low or the evaporator being dirty/frozen will cause low suction pressures. If you know the coil isn't frozen and you have good airflow across the coil, you have a restriction in the refrigerant system somewhere.
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  #9  
Old 07-10-2018, 10:46 AM
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134 now, plan to change to r12 soon - wanted to work out system kinks before blowing my supply. Where abouts is the ETS located?
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  #10  
Old 07-10-2018, 11:53 AM
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The ETS should be in the plenum where the evaporator coil is. Search the forum for your specific year model, I believe up until 1985 or so they were a mechanical switch with a sensing bulb poked into the evaporator box. If you haven't done so already, consider cleaning the evaporator coil. It is likely a horror show if it's never been cleaned!

A dirty evaporator will cause poor cooling performance and encourages frost buildup from the evaporator flooding. If it floods all the way back to the compressor you can destroy it if the pistons inhale a slug of liquid refrigerant, so it's worth diagnosing!
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  #11  
Old 07-10-2018, 12:40 PM
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My compressor is the wrong type anyway, wrong clocking. So itís bound to be destroyed anyway. But if the ETS is the prob and it requires pulling dash Iím donít anyway.

Such a sad end to so much work. All because I wanted to stick with r12 and no shop around me would touch it. Curses.
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  #12  
Old 07-10-2018, 01:17 PM
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Pretty lousy attitude...

The R4 is lubricated by oil vapor not by a sump, so not sure how you figure it's the "wrong clocking". A lot of people talk out their backsides about things and when it goes on the Internet, it gets mistaken for "gospel".

The ETS should be gettable from under the dash, same with cleaning the evaporator. Cleaning the evaporator is done by going in through the blower housing. Requires contortions to do, but certainly doable without pulling the dash.

R12 is nice, but if done properly, R134a will blow just as cold. Just takes an extra minute or two to pull the cabin down to temp. Is that really such a big deal? Not really. You've already invested this much time, effort, and money into the project, why not continue on and finish it and enjoy the cold air?
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  #13  
Old 07-10-2018, 02:52 PM
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Yes I am discouraged, that is evident. So much work doing all this to potentially see it explode (maybe literally?)

I'll give it a look see, and possibly change out the ETS. Its just hard to ignore the folks (in another thread here) who are referencing youtube videos - and are telling me that the compressor clock position also means an oil hole/port is in wrong position. I suppose anybody could be right or wrong on the internet - but I come here to get smart folks' opinions - it's difficult to know how to pick and choose from them. Bottom line is I know I have the wrong compressor installed so it's hard to be too optimistic about it working correctly. I'm certainly trying.

I have a supply of r12, so it seemed prudent to use in order to get the maximum originally designed cooling - otherwise I've been happy with the cooling from the r134.
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  #14  
Old 07-10-2018, 03:05 PM
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Sometimes you need to walk away and come back with a fresh head. Don't give up and don't give into Internet "experts" either. When you have a sump-less compressor, the "clocking" isn't that important. Having the correct charge of oil in the system is. The R4 isn't a great design to begin with. You already have it installed, run it until it craters. When it does, clean out the condenser and replace the filter/dryer and go again having learned a bit more (including whether or not your system leaks).

If the bearings were starving for oil, I'd expect you to already have problems. Since you aren't having issues with the compressor, leave sleeping dogs lie.

Set the fan to Max, open all the doors, set the temp wheel to Max-Cold and check your pressures and report back. How was the R134a charged originally? By weight or by pressure? If by weight, how much % of the original R12 charge was added?
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  #15  
Old 07-10-2018, 04:52 PM
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Thanks for the words of encouragement.

My system is not leaking to my knowledge. I pressure tested it (before I replaced anything) and it held. In the course of my rnr i replaced every oring. I think itís pretty tight - I felt good about that aspect.

I charged by weight. And I think I did about 80% of the r12 weight. I had notes at the time and followed the advice on the amount relative to r12.

I put in ester oil - somewhere around 4 ounces in compressor and 4 in the new drier.

I feel like the compressor is making a bit of a moaning noise when engaged (not exactly grinding but perhaps a little ?) - but since I havenít had an operational system before I donít know whatís normal. Compressor is also pretty hot to the touch after running for a bit - is that normal?
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