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  #1  
Old 05-14-2005, 11:34 PM
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one pound R12=how many pounds R134a?

I have an 87 300E and it normally takes 2.4 lbs of R12 to charge the AC in these cars. I have been told verbally by a good mechanic at an MB dealer to put 80% or about 2 lbs of R134a in. (My car has already been converted to R134a) Stu Ritter in the E Class Owner's Bible says R12 get 2.4 lbs and R134a get 2.2 lbs. So should I put 2.2 or 2 lbs in? Does it make much difference anyway? Is it safer to underfill than overfill?

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  #2  
Old 05-14-2005, 11:37 PM
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Yes it is safer to be under than overcharged. The standard rule of thumb is 80%, but I use the pressures to decide that last quarter pound.
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  #3  
Old 05-14-2005, 11:41 PM
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My pressures were 240-250 over 39
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Old 05-14-2005, 11:50 PM
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The reason I didn't state pressures is that they are so dynamic. They depend on numerous things: ambient temp, rpms, fan speed (both inside and outside). Your pressure sound about right depending....
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  #5  
Old 05-14-2005, 11:53 PM
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Also want to add that a tech helped me charge the system. The system seemed to perform better last year with 215/40 pressures. Outside temp about 80F.
So what is the algorithm on using pressures to optimize the system?
Thanks.
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  #6  
Old 05-15-2005, 02:24 AM
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AC refrigerant level

Did you fix the leak? Head pressure sounds a little high, but maybe it was hot that day or your condensor is dirty.

If the system had lost the charge completely--it MUST be vacuumed and the drier replaced or the compressor will soon fail. The lube oil for R-134 does not like even trace moisture or air. I always vacuum down to less than 100 microns (29.995" Hg vacuum) and hold that for 30 min to ensure all moisture removal.
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Old 05-15-2005, 01:31 PM
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It was down to about 1.08 lbs per the machine. Then a 15 min vac was done and then recharge. This was done at a dlr. Temp was 80F. Do not know exact location of leak. Saw some dye another time at the upper right connection of condensor. I will hose off condensor. So would more R134a decrease or increase head pressure?
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Old 07-19-2005, 09:03 PM
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wouldn't more = more on either side?

I just topped mine off; was at 25 psi, topped off to 40 psi, 91 deg ambient but this was at idle, not 1500 rpm, and the system had been running pretty warm....
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Old 07-20-2005, 09:19 AM
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The pressures will be SIGNIFICANTLY different between idle and 1500 RPM. You should let it stabilize at 1500 RPM for about five minutes. It is also best to use a HIGH VOLUME fan to simulate ram air while you are doing this. I have an old squirrel cage fan that moves LOTS of air.

There is no magic formulat to use on every car. Your best bet is to go by manufacturers charging instruction. Once converted, you are on your own and need to undercharge then add in small amounts and proceed cautiously. The high side pressure in a 134 converted system can "run away" easily.

Good luck,
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Old 07-20-2005, 09:55 AM
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LarryBible,
Thanks for the reply.
My vent temps are maybe 50 degrees, but that's after only a 5 minute drive.
Sounds like I need to get a set of proper guages and check the high side charge as well.
If I have to vent some, can I get the low side valve to safely vent some pressure by pushing on some part of it?
As you can tell, I'm new to this AC stuff, but willing to learn.
I'm just very glad I added straight R134, not some magic fix-it stuff.
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1973 Olds 88, 1972 MB 280SE, 1978 Datsun 280Z, 1971 Ford T-Bird, 1972 Olds 88, 1983 Nissan Sentra, 1985 Sentra, 1973 230.6, 1990 Acura Integra, 1991 Volvo 940GLE wagon, 1983 300SD, 1984 300SD, 1995 Subaru Legacy L wagon, 2002 Mountaineer, 1991 300TE wagon, 2008 Murano, 2007 R320CDI 4Matic 52K, some Hyundai, 2008 BMW 535xi wagon, all gone... currently
2007 Honda Odyssey Touring, 2014 E350 4matic
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  #11  
Old 07-20-2005, 11:46 AM
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Help me here. I'm still unsure what the best way to determine the exact point where enough refrigerant has been charged. I thought one way to determine proper charge (after vacuuming) was to charge until a steady vent temp and pressures are reached, then keep adding until the high side pressures just begin to climb. This would fill the accumulator and allow for any slow seepage without losing cooling performance any sooner than necessary.
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  #12  
Old 07-20-2005, 05:38 PM
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I have basically the same question...since I"m not evacuating I need to know that exact point where the system is happiest.

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Bob Roe
Lehigh Valley PA USA
1973 Olds 88, 1972 MB 280SE, 1978 Datsun 280Z, 1971 Ford T-Bird, 1972 Olds 88, 1983 Nissan Sentra, 1985 Sentra, 1973 230.6, 1990 Acura Integra, 1991 Volvo 940GLE wagon, 1983 300SD, 1984 300SD, 1995 Subaru Legacy L wagon, 2002 Mountaineer, 1991 300TE wagon, 2008 Murano, 2007 R320CDI 4Matic 52K, some Hyundai, 2008 BMW 535xi wagon, all gone... currently
2007 Honda Odyssey Touring, 2014 E350 4matic
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