I live in hot, humid NE Louisiana and I drive a 1986 300sdl. My A/C compressor cycles on and off depending on what temp it is outside and what temp I want it inside the car. It will stay on almost all of the time on a 93 degree day with 60 percent humidity outside. My electric cooling fan will cycle from low speed to high speed when I am in the drive thru at McD's when it is really hot out. I can tell when it does this because of the extra electrical load on the car when it goes from low to high speed.
On a cool morning, 72 degrees outside with 70 percent humidity, the compressor cycles on and off. I have converted it to R134A 5 years ago, and have to top off the freon yearly. When I say top it off I mean 3-4 ounces. This is the original compressor on the car. The car now has 110,000 miles on it.
I wonder if you have a dirty expansion valve. My 85 380se acted this way after I put a new compressor and drier and expansion valve on it 3 years ago. The compressor stayed on all of the time and it never really got cold in the car. I had to evacuate the system and clean out the lines. I had trash in the lines from the old compressor seizing, and they got caught in the expansion valve. What a pain it was to get to the expansion valve too. Once the system was cleaned again, no more problems, and that compressor cycles just like it used to.

By the way, at WOT when you press the accelerator all the way to the floor, there is a compressor cut out switch. It is a microswitch that the wide open throttle hits in the engine compartment. Follow your linkage and you will see the microswitch. With the car idling and the A/C on, use your finger to close the microswitch and the compressor will click off if everything else is working correctly on your car.

John

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John
1986 300sdl
1985 380se