R12 Refrigerant Capacity
 

I just got through replacing the compressor and condenser on my wife's 1981 300SD (126 body) The sticker on the radiator shell says the capacity is 1.6 Kg or 3.5 lbs. I generally fill until I get no bubbles in the sight glass. I typically get to the no-bubbles point at 2.5 lbs. I am using R12. The compressor is a GM R4.

My other MB is a 1982 300SD and it doesn't have a sticker on its radiator shell.

What is the correct fill amount for R12 refrigerant for this car?

What did the compressor and condenser run roughly?? I still have 4 cans of R4 that I paid about $25 each for a couple of years ago; is that still a market price?

Witn txv system, the reciever holds a reserve of liquid refrigerant waiting to enter the evaporator. Because the phase change (liquid to gas) at the expansion valve is what provides most of the cooling, you dont want to allow refrigerant to enter the evaporator as a gas. That's why bubbles are bad. But what you don't know is what happens when the system is stressed. The mfr has supposedly done the math and the testing to know how much is required under all conditions. So its best to go with that recommendation, even if it means there is some liquid refrigerant backed up in the condenser.

TE:

To answer your question, ask yourself if the chassis (hence the evaporator) is the same for both cars, if the compressor is the same, if the condenser is the same, if the receiver is the same. If the answer is yes to all, then you may very reasonably conclude that the refrigerant capacity is the same also.

My '85 SD is spec'd at 2.9lbs last I looked.

I think I have answered my original question. The sticker on the radiator shell stated what the "maximum" charge should be, not the typical charge. Expansion valves can handle more than the optimal charge. I even checked the Mercedes manual and they reiterated to charge until there is an absence of bubbles in the sight glass.

I bought a refrigerant scale in 2002 and since then I have always measured the weight of the Freon added and recorded that amount in my maintenance log. Most of the time, the amount added on a complete fill was 2.5 lbs for either of my 300SDs.

I guess the 3.5 lbs figure was Mercedes' idea of adding more Freon so that the car would go longer before it starts exhibiting problems with cooling. Remember, the refrigerant sticker was printed in 1981 when R12 was not considered dangerous to the atmosphere and it cost only about $.99 a pound.