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  #1  
Old 06-01-2005, 11:47 PM
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A/C Low and High side pressures?

Ok, I admit I have never had any training or need to know about A/C systems. For some reason, all of the junk I owned in the past either (1) did not have or need A/C; or (2) never had any A/C problems.

[B]I have searched here, and I have searched the manual on CD and on Alldata and do not see recommended pressures. I have an old 1989 Haynes AC manaul that lists pressures and for Mercedes '81 to '86 shows low 22 and high 265. Correct?

At 20-22 low I am showing only 150 high and the vent air is at 80+/- on a mid-80s afternoon. The high side line is hot to touch at the condensor and the low side line is cool at the valve.

I feel like this is something I should know but don't. What are the logical next steps in getting this AC back in working condition, short of a blanket removal and replacement of the Receiver/drier, expansion valve, etc. Of course, if that is the next logical progression, I need to know that, too.

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  #2  
Old 06-02-2005, 04:15 AM
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Howdy,

If you go to the Dupont Suva website and download their pocket guide, there is a good assortment of p-t tables:

http://www.dupont.com/suva/na/usa/products/134a.html

Hope this helps,

Sholin
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  #3  
Old 06-02-2005, 06:34 AM
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add freon?

If this system hasn't been open, it sounds as if you need to add more freon. If it has been opened it sounds as if there is some air in the system. 150 isn't near high enough. If there is air in the system it will be hot on the high side and cool on the low but will not chill. If you are sure there is no air(caused by not purging gauge lines when hooking up gauge hoses; second caused not fully evacuating the system before charging it) then add freon. Also check your pressures with engine running at 1500 rpm.
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  #4  
Old 06-02-2005, 07:37 AM
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Which refrigerant are you using ?
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  #5  
Old 06-02-2005, 08:19 AM
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Have you checked the monovalve?

Danny
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  #6  
Old 06-02-2005, 09:39 AM
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You can't accurately charge a system by pressure alone. To wing it with an expansion valve system you charge by subcooling. Get the chart listed above and learn to do subcool and superheat calculations. You have to know where your refrigerant is and what it is doing. If you do not have liquid where you need liquid and vapor where you need vapor your system will not work well and you may damage it. The correct way to charge these systems is by weight but you can wing it if you do your homework.

Hello from the Mill Mountain Coffee and Tea shop in beautiful downtown Roanoke, VA. Will shortly be on the trail and out of contact.

Run some searches here for more comprehensive information on these systems.
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  #7  
Old 06-02-2005, 10:13 AM
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TwitchKitty is very correct.... ALSO.... don't be fooled into thinking the sight guage is the final word either... no absolute rules there with regards to charging either....
When trying to wing it successfully..... put a little in...and drive the car for a few days.. then add small amount more...and drive it again.... will work fine many ounces "low" WAY better than one iota overcharged....
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  #8  
Old 06-02-2005, 11:29 AM
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Quote:
Originally Posted by dannym
Have you checked the monovalve?

Danny
Monovalve is not a problem.
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  #9  
Old 06-02-2005, 12:48 PM
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Quote:
Originally Posted by leathermang
Which refrigerant are you using ?
Somehow, I knew YOU would be asking that question.

Long story short, I was ignorant last year when I bought the car and took it to get the AC recharged. Two shops (one of which I used to trust) recommended recharging with Freeze12. So I (gulp) had that done.

Now I have seen the light and have my 609 license and some guages. I am going to revert to R-12, by evacuating the system and replacing the Receiver/Drier. I think LarryBible has posted instructions for this reversion, but I can't seem to locate the post.

In the meantime, this season, I have encountered some problems:

1. No voltage to the low pressure switch. Replace E.T.R. switch. I am providing 12+ to the switch from the fuse box for testing. I am NOT jumping the low pressure switch--I am just replacing the source of the 12+ to the switch.

2. Leaking schrader valve on low pressure side. Will replace when reverting to R-12. Leak controlled by cap on valve at this time.

3. Not cooling. I wanted to attempt to diagnose before I do the reversion if an expansion valve, for example, needed replacing. I would have thought that the system would cool somewhat better than 80 at the vent even with the low pressures I listed. Maybe I should just proceed to evacuate the system, replace the R/Drier and the expansion valve, re-charge with R-12 and go from there.

Note that the compressor is not making noise, the belts are not slipping, and the clutch is engaging.
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  #10  
Old 06-02-2005, 02:21 PM
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I AM predictable...

but we would have to know to answer your pressure question...

That is probably your best course... I think the TXValve is only about $30...

You must have enough stuff in the system to be changing from liquid to gas AT the evaporator core... that may be the cause of your 80 degree vent temps ....but where you live I am really glad you are converting back to R12... I am convinced you ( and your system ) will be much happier that way...

So are you asking whether we think it is ok not to flush ? Is that the question ? and not to replace your orings...since if you flush you will have those joints open anyway ?
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  #11  
Old 06-02-2005, 02:30 PM
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Quote:
Originally Posted by leathermang
So are you asking whether we think it is ok not to flush ? Is that the question ? and not to replace your orings...since if you flush you will have those joints open anyway ?
OK, that can be my question. If oil was not changed when freeze12 was put in, would the system need to be flushed?

Thanks.
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  #12  
Old 06-02-2005, 02:42 PM
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LOL.... did you ask Me that with a straight face ?
The only way to do it right in my opinion... is to flush, replace the orings with new correct color ones, smear them with the correct color Nylog when reinstalling them... measure the correct amount of fresh clean oil into it at the right time.... weigh the correct amount of R12 into it....
After , of course, doing the very best evacuation job you can do....after checking for leaks with a vacuum AND with pressure ( if you have the equipment ) by pressurizing with dry nitrogen before committing your R12 to the system...
But of course I am an old fuddyduddy about this stuff... pretty much believing what I read in the three AC manuals ( including the MB one ) I have.... and adding up the numerous problems encountered by people who leave out one or more of the steps which may be required to get it Right/Cool for the LONG HAUL...
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  #13  
Old 06-02-2005, 02:54 PM
LarryBible
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Since you are changing refrigerants, as you should IMHO, you should flush the system entirely, replace the r/d, add the correct amount of mineral oil, evacuate and charge.

Good luck and see you next Thursday night,
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  #14  
Old 06-02-2005, 04:49 PM
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Ok, here is the plan:

1. remove remaining old freeze12;

2. replace expansion valve and consider replacing hoses; repair shrader valve core on low side; replace E.T.R. switch; replace 0-rings and lubricate with Nylog assembly lube;

2. flush entire system with "mineral spirits first and then follow that up with regular a/c system flush to get out the residue" and then "blow, blow, blow" (LarryBible--04/14/2005)

3. vacuum test; I don't have the equipment to pressure test with dry nitrogen and do not have an electronic leak detector; and

4. Charge with PROPANE -- sorry, couldn't resist that -- charge with correct amount of R-12. I don't need a 30lb supply -- is there a better source for small cans other than e-bay?

Now all I need is a few days off work.
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  #15  
Old 06-02-2005, 06:57 PM
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If you get a chance to look at the Factory AC manual for Mercedes... they suggest using the old TXValve and changing it to a ' tool' for the flush..
basically taking the innards out and then hooking the two hoses going forward from the passenger compartment....

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