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For the '81 300D:

I just bought a can of DIY R-134a... instructions say to connect to the "low pressure" service port. I believe this is the upper port located near the air filter, and *not* the port down by the alternator?

I've searched for a schematic of the a/c lines, no luck. I've never done one of these before so I'm ignorant of the positions of the ports. Thanks!~
 

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Typically, the high pressure port will have a red cap on it, and the low pressure port will have a blue cap.

Here is how the A/C works. This will help you identify the correct ports.

The compressor compresses low pressure gas refrigerant to high pressure gas refrigerant. High pressure gas move to the condenser, where it condenses to a liquid, releasing heat to the atmosphere. Then, liquid refrigerant moves to receiver dryer, then on to the expansion valve. From there it goes into the evaporator behind your dash, where it changes into gas, absorbing a lot of heat from the environment, and turning the evaporator coils ice cold. The low pressure gas refrigerant them moves back from the evaporator to the compressor where it is compressed to a high pressure gas and the cycle is repeated all over again.

So the low pressure fitting (the one you want to add the refrigerant to) is in line between the evaporator and the A/C compressor. Anything between the compressor and the condenser plus receiver/dryer will be the high pressure side.
 

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1959 220S cabriolet, 1983 240D original owner, 1999 E300 turbo diesel, 1988 560SL, 2003 SLK320
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This is probably a dumb question, but your car has been converted to R134a right?

Len
 
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