I live in Palm Springs. It's very hot here during the summer; 105 to 118 everyday. Some interesting things that I have noticed about the ac system in my 83 300sd with new compressor, new expvalve, new mb condenser and r12. During the afternoon heat, if I get stuck at a red light and idle for more than 30 seconds I lose most of the system cooling. However if I ad some rpms up to about 1300, my cooling remains nice and cold. Also, when I drive through large mall parking lots at low speeds (5mph) if I drive in 3rd or 4th gear I lose most of the system cooling. However, if I drop it down to first gear, I get ice cold air, just like driving at speed. It seems to me that compresser speed-engine speed is the critical factor in system cooling (at least with r12) as opposed to condenser capacity-air flow.

Actually I think you are precisely describing a lack of airflow across the condensor...
That is the bottle neck or controlling factor in terms of cooling... you can have a very inefficient system and match it up to a GREAT airflow and condensor...and get plenty of cold air...
You might check the fins on both the condensor and the radiator...and see that your aux fan is coming on... if those things are ok...and you want to correct the situation you might install a high performance fan like the Spal....
Remember that engine speed also controls the fan pulling air through the condensor and radiator...

My aux fan works perfectly, comming on when the high-side pressure reaches the switch point. It is on all the time when the temp is over 100 degrees. I'm not sure about the air-flow bottleneck theory. If I increase rpm's just slightly the air flow does not increase very much at all. However, vent temps stay ice cold, compared to idle where the vent temps go to luke-warm (barely any cooling). Do you think that just a slight increase in air flow would make such a dramatic difference? New cars increase rpm's automatically when the compressor is switched on. My 300sd does not increase it rpms automatically, but it should.

the compressor running faster and the air flow across the condensor both help. it also helps that the fan pushes hot air out of the engine compartment.

youre both right.

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[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Fans and airflow are tricky deals...
I think you should clean and straighten your fins on your condensor and radiator.... and then see if that same slight increase in rpm's causes the same dramatic difference...
On the primary bottle neck being the air flow across the condensor.... Trust me on that... or check it out on any air conditioning site.... that is well established...
The problem is that in the present condition you can not isolate the compressor rpms as a variable...since increasing the engine rpm's also increases the fan on the front of the engine....

Replace Radiator Or Take It To Shop That Will Clean The Inside And Out Side. The Outside Is Fill With Dust And No Air Moves. Replace My Radiator And Top And Botton Water Hose .and Replace The Metal By Pass 5 In Line Or Clean It Top Front Of Engine.
Fill With 50/50 Mercedes Coolant And Last Fill The Top Hose At Radiator

He posted that he installed a new condensor......the radiator may still have some blockage due to bugs, dirt, dust bunnies etc. You can pull the radiator and have it professionally cleaned for about $50.00. Or just take it down to the local serve yourself car wash.

That would help the flow across both the condensor and the radiator.

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84 300DT Puke Yellow. Totalled after 438,000
84 300DT Orient Red. 169,000 (actual mileage may vary)
2002 Explorer EB (wife's)

the water from the back of the rad to get out the dirt on the front.

__________________
[SIGPIC] Diesel loving autocrossing grandpa Architect. 08 Dodge 3/4 ton with Cummins & six speed; I have had about 35 benzes. I have a 39 Studebaker Coupe Express pickup in which I have had installed a 617 turbo and a five speed manual.[SIGPIC]

..I also have a 427 Cobra replica with an aluminum chassis.

Air flow and engine speed have a lot to do with how cold the discharge air will be. Any air conditioner will put out warmer air when idling than when the car the is moving. If you read about how to charge an air conditioner, the engine runs at 2000 rpms with a fan blowing on the condensor to simulate driving at 40 mph.

Driving at 65 mph, my 84 300SD will blow 40 degree air with an outside temperature of 100 degrees. I suppose it would go lower, but the system prevents it from freezing the evaporator. On the other hand, when idling, the vent temperatures are much higher. With temperatures above 90 degrees, all I can say is that these cars cool down slowly, and the air conditioning works best at highway speeds. Below 90 degrees, the air conditioner works very nicely in stop and go traffic.