My 85 300SD A/C worked fine until the low pressure switch kicked in. I've since gotten my Section 609 certificate, some equipment, and some R-12!

My question is . . . approximately how much R-12 need be missing prior to the low pressure switch kicking in in an otherwise fully functional system? In other words - assuming everything in the system is working (notwithstanding the possibility of a small leak) - how much R-12 would I need to add to the system once the low pressure switch kicks in to get the system back up to a full charge?

I realize that there are several variables (e.g. noncondensable gases) which may influence an accurate response, but what I'm looking for is an approximation based on experience or on whatever voodoo you might rely.

I want to know if I have enough R-12 on hand just to get me through the rest of the summer until I get my recovery/recycler going (that's another story) before I start poking holes in cans - so to speak.

Your best - educated - guess???

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Peach Parts W124.128 User Group.

80 280SL
85 300SD
87 300TD
92 300D 2.5 Turbo
92 300TE 4Matic

Go to www.google.com

put "air conditioning charging" or something similar in the search field.

Go to a few links and learn about what your doing.

This is from one of them:

http://www.refrigtech.com/charge.html

Danny

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1984 300SD Turbo Diesel 150,000 miles

OBK member #23

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If you have lost that much refrigerant you have to assume you have also lost a large percentage of your cold flow oil .... so you can not safely just add refrigerant.

If you don't find and fix your leak then you will just lose the R12 you have...and there is no way to tell how fast it will be lost since your leak rate may increase... particularly if it is a moving friction seal like like the compressor shaft...

And you are supposed to assume that non condensable gases have been introduced into the system containing moisture if you have a leak.... so your system is at risk for moisture damage already.

If you are going to try a half way measure for a period of time then you might just put enough in in one can increments to get it working.... overcome the low pressure switch in other words... and see how long that lasts... instead of shooting for a full charge... your system will work with much less than full charge....

But that does not take away the fact that without a full flush you have no idea how much oil you do or do not have....and overfilling of the oil will tax the system also....

Here were the readings before after approximately 10 months of A/C non-use:

High 20, Low 20, Condenser 90F degrees, Vent N/A (clutch not engaging)

I put in one 14 ounce R12 can and here is after:

High 160, Low 35, Condenser 92F degrees, Vent 78.

My read is that the system is still low and I could get the vent colder, but not so low as to prevent the low pressure switch from cutting off. I've got some more R12, but I don't think I'll push it any further until I can recover and recycle properly. I want to ensure I've got the right amount of oil and patch any leaks.

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Peach Parts W124.128 User Group.

80 280SL
85 300SD
87 300TD
92 300D 2.5 Turbo
92 300TE 4Matic

Hey,
I was at an Indy shop a few days ago bull****ting with the owner. We were discussing AC systems. My symptoms were exactly as you described. One of my problems that the compressor never shuts off even at low pressures. This is the same with 3 different switches. But that's another story.

He was mentioning some things to me that I never thought about. Like maybe a collapsed hose on the inside that you would never see.
You never know where there can be a restriction.

He seemed to think I was undercharged but after talking a while it seems that more than likely there's too much oil in the system. I'm allready at 210# head pressure at 95 deg. It does just fine until the hottest part of the day or if the interior gets too hot. The coldest the center vent gets is 50 deg. When it's really hot out it takes forever to get to 70 deg.
At this point I'm done playing around with it. I just want it to work. I told him I would be back next Monday so hopefully then it will be fixed!

Danny

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1984 300SD Turbo Diesel 150,000 miles

OBK member #23

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