This is a request for several pieces of information regarding the testing and charging of the A/C system in our Mercedes, specifically the 1991 190#E 2.6, in my case. I suppose I am mainly asking LarryBible, since I’ve seen him posting concerning these matters the most. However, I know that there are several others with the knowledge and experience in this system and I’d welcome the instruction from anyone.

Preface: I am quite handy mechanically and on my car. I just don’t have much experience working with the A/C system. I did pass and receive my 609 certification and have purchased a manifold gauge. Depending on those readings, I will be purchasing R12 and lubricant/oil. My car has never had an A/C service in its 13 years of service (I bought it 3 years ago). The A/C has worked fine, but its cooling ability has diminished slightly each of the last 3 years. I am told that even a “non-leaking” system can lose and ounce or so of refrigerant each year due to hose porosity, etc., so I may be close to a pound low of R12 from the system capacity of 2.2 lbs.

Questions:

1. What should my high and low side pressures be? I am assuming, of course, that this is with the engine running – but is it with the A/C on or off? I assume, ON.
2. How do I know, depending on the readings, how much R12 will be needed?
3. How much lubricant/oil should be used (ratio) in relation to the R12?
4. Anything else you want / need to tell me.

Thank you

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1994 C280, dark green metallic
105k miles

1991 190E 2.6, Black
191,500 miles
(sold to another forum member)

2003 Chevy Tahoe LT, Redfire Metallic
105k miles

1989 Mustang GT Cobra Convertible
43k miles