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  #1  
Old 06-19-2004, 04:48 PM
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A/C Help -- Charge Question

1980 300SD: Had bad freon leak. Still using R-12. Replaced high and low hoses, seals, o-rings, receiver/dryer, have begged, borrowed, and stolen four 12-ounce cans of R-12, and two 4-ounce cans of oil charge, and now ready to pull a vacuum and recharge. How much oil? How much freon? What should high and low pressure readings be at 1500 rpm engine speed? A/C outlet temp?

Temps in northeast Arkansas in low nineties with humidity near that, too. Really miserable, and only want to do this once. Any help will be appreciated.

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  #2  
Old 06-19-2004, 06:21 PM
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Hopefully Larry Bible can help you with the rest, but my book shows: Oil Amount for 1978 -1980 300SD = 5.8oz, Refrigerant 2.66lbs, Low Pressure 22, High 205 (@80deg. ambient temp.) This data is from a Haynes Automotive Heating and A/C Book. Hopefully you have manifold gauges because anything short of using them is winging it and unsafe in my opinion...PM Larry Bible and cross your fingers that he has time to help.
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Last edited by bjcsc; 06-19-2004 at 06:46 PM.
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  #3  
Old 06-19-2004, 08:33 PM
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If you flushed the condensor and evaporator you would need 6-8 oz. I have read in places they (116) takes 8 and other info says 6.

Having too much oil will hurt cooling.

I added 6 to mine after doing a compete flush and rebuild in April.

My red factory a/c info tag in front of the radiator says 2.2 lbs of R-12.

I can get around 42 degree vent temps so far. I was a little low and added some today so we will see if vent temps improve any.

After clearing up the site glass, my low pressure was 25 and high was about 200. They held real steady after the condensor fan kicked in. This is at 1000 rpm.

Good luck and a cold A/c is REAL NICE!
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1980 300SD Manganese Brown Metallic Clearcoat 248k
1979 300SD White 148 k
1984 300D Silver project car for daughter
1982 300D wrecked
1980 300SD 310K not running
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  #4  
Old 06-20-2004, 03:43 AM
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Thanks, J.D.; That Helps

I hope to get this thing up and running on Tuesday, and will use your figures. By the way, I knew a J.D. Blann, from Nettleton I think, at ASU in the mid-60's. That you?
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  #5  
Old 06-20-2004, 09:56 AM
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My '79 SD has been converted to r-134a. If I flushed it and then pulled a good vacuum on it, could I improve my cooling very much to make it worthwhile? What parts would have to be replaced?
Thanks
David
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  #6  
Old 06-20-2004, 10:35 AM
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there is always oil in the system. (unless you flushed)
usually we replace 1 oz for each hose and 2 ozs for rec/drier.
in your case this totals to 4 ozs.
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  #7  
Old 06-20-2004, 11:31 AM
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The A/C in my car really stinks. Not literally. If it is 85 outside, you need to have the vents blowing directly at you to be able to keep the windows up, and that's moving down the road. You get restless sitting at a light, and this is after being topped off and it's still holding.
Thanks
David
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  #8  
Old 06-20-2004, 01:19 PM
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Quote:
Originally posted by WANT '71 280SEL
The A/C in my car really stinks. Not literally. If it is 85 outside, you need to have the vents blowing directly at you to be able to keep the windows up, and that's moving down the road. You get restless sitting at a light, and this is after being topped off and it's still holding.
Thanks
David
check pressures

check the evap temp..

too much refrigerant can cause this..
Run 134 a little lower than specified...

check for air breach in evap box
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79 240D my current toy
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240D sold
250SE sold
220D sold
280C sold
280S sold
300D (2) sold
300CD sold
300DT sold
300SD sold
380SL sold
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  #9  
Old 06-20-2004, 01:24 PM
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David,

If you want to simply perform an evacuate/recharge on your 300SD, I would suggest at least replacing the receiver/drier and switches, of course since you will be emptying the system you might consider going ahead with a new expansion valve as well. Anyhow, if your car has 134a in it, you will only recharge the system with 80% of the original R-12 capacity. By a rule of thumb, 2 ounces of oil should be added anytime a recharge is done but it really depends on how much you recover from your car's A/C system on the recovery cycle with the machine. I would set the vacuum to run for at least 1/2 hour and make sure it holds at least 30 in/Hg for no less than 10 minutes AFTER the vacuum operation is complete. Add the oil while system is under vacuum and then begin charge operation. As for which oil to use, we have had excellent results with Castrol PAG 46 which is a synthetic compressor oil and already has UV tracer dye in it. Roughly $8 for an 8 ounce bottle.
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  #10  
Old 06-20-2004, 01:59 PM
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My low pressure is at 35 idle and about 27 at 2500 rpms. I knew about the 80% thing, but we weren't sure exactly how much was in there to begin with. What do you guys think of my readings? Should I convert back?
Thanks
David
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  #11  
Old 06-20-2004, 02:00 PM
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How can I check the evaporator temp? My r/d is original, or it appears to be. It has all the paint flaking off and the sight glass is dark yellow and nothing can be seen inside of it.
Thanks
David
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  #12  
Old 06-20-2004, 03:11 PM
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Abner,

Not me. I have always lived in south Arkansas. Went to school at SAU Magnolia.

Back to 116 Mecedes:

WANT '71 280SEL,

I did the recirulate mod on both my 116 cars to have 100% recirulate only when the compressor is on. This should help some.
I didn't do a before and after so I can't say exactly how much it helps.

Here is what you do:

Remove the rh lower panel then the air duct. Look for a pink vacuum pod. It will be the outer most pot next the kick panel.

Remove the vacuum line and plug it. This will keep that pod from keeping the flap open to 20% fresh.
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1980 300SD Manganese Brown Metallic Clearcoat 248k
1979 300SD White 148 k
1984 300D Silver project car for daughter
1982 300D wrecked
1980 300SD 310K not running
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  #13  
Old 06-20-2004, 03:37 PM
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WANT '71 280SEL,

I use a meat thermometer I got at a truck stop to check vent temps. Look in a Pilot or Loves in the trucker parts shelves. They are a few bucks and work great. It has a clip like on a ball point pen and a small dial that indicated the temp. Really helps to determine what you AC is doing.

I can't help much on the pressures for 134a. The original dryers may not work as well wth 134a. If it is indeed an "original" R-12 type.

Good Luck to all!
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1980 300SD Manganese Brown Metallic Clearcoat 248k
1979 300SD White 148 k
1984 300D Silver project car for daughter
1982 300D wrecked
1980 300SD 310K not running
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  #14  
Old 06-20-2004, 04:33 PM
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So the air my car is cooling is 20% fresh, as in the ouside air. Then the remaining 80% is air in the cabin that is re-cooled? I'd like to have something like newer cars with recirc. The pod line that is being closed, what does it control? Howe does it recirculate air only when the compressor is on?
Thanks
David
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  #15  
Old 06-20-2004, 04:55 PM
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A lot of references and material has already been posted on this forum... a search can really be helpful....

for instance:

http://www.peachparts.com/shopforum/showthread.php?threadid=72821&highlight=thermometer+pocket

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