Anyone know what could cause the ac to have mediocre performance while idling at 500-600 rpm with high speed fans on, but performance increases when rpm are at 2000 while still idling with high speed fans on? It seems like I've been at this issue for a while with my 201. The compressor is a brand new Denso. No clogged condenser, no leaks. A/c actually preforms amazing, but if I sit idle for a while the vent temp will go from ice cold (40°F) to semi cold (~60°F). I am also running 4 electric fans, stock auxiliary fans and two 10 inch spal pullers, which I will eventually convert to one large puller.

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1990 190E 3.0L

What is the refrigerant you're running and what is the ambient temp? If you're on 134a, that wouldn't be too expected on a hot day (90+) due to its relatively poor performance in those conditions. When it's hot outside, 134a needs a higher mass-flow and a decent air flow over the condenser. With the fans you're running, the air-flow is probably ok, but the mass-flow is down due to the lower RPM of the compressor.

Have you thrown a gauge on the system to find out what the pressures are doing? Charge adequate? You could have a sticking expansion valve if it's otherwise been operating correctly, but suddenly doesn't. The high/low pressure differential can tell you a lot.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)

I don't know about your specific car, but most cars have a way of increasing the idle speed a bit when the A/C compressor turns on.

On my car there is a signal sent to the idle control module under the passenger floor front. Then that module adjusts the idle control valve.

On newer cars, a signal is sent to the engine computer from the A/C system. Then the engine computer adjusts idle up a bit.

Check your factory service manual A/C and idle control. See if it says what the idle should be normally. Then what it should be increased to with A/C on.

I am running r134a. Everything related to the a/c system had been replaced less than a year ago. Hi/lo side are within spec according to chart plus my ambient outside temp which is 90°+. My car does idle up when I turn on the a/c. How can I increase this "mass flow"? My airflow across the condenser from the fans is quite a bit with the 4 fans and the way I shrouded it so that there is virtually no air going around the radiator or condenser. I always do the comparison whenever I am in another car, for example, a friend's '99 E320 sitting in idle and the ac was ice cold. It seems that I have massive amounts of air flowing with the fans compared to the low speed fan of a newer maybe mid 90s car and my system is sub par. When driving it's a different story.

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1990 190E 3.0L

The only way to increase the mass-flow of the refrigerant is to increase the RPM of the compressor. That's why the cooling improves when you get the engine RPM up.

It sounds counterintuitive, but you may be overcharged, especially with all the extra airflow on the condenser. Connect your gauges, set the air on max cold, max fan, and leave all the doors open. Have someone hold the engine RPM at ~2000 RPM and watch your gauges.

Ideally you should be ~225PSI on the high side with all that airflow. If you're significantly higher than that, let a couple ounces out and see if things improve. If you're much below, you might be undercharged slightly. Suction side is usually around 25-35PSI with a proper charge.

If the pressures are WAY off, suspect the expansion valve.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)