The A/C compressor on my 1993 240 takes about 20 minutes to come on. Once it comes on it runs fine and the air is cold. I checked the low side and it seems to be charged ok.

What could the issue be?

 

I believe the 93, being the only year w/R134a from the factory, has both low pressure and high pressure switches. They keep the compressor from running if detected pressures are too low or high. Might start w/ them.

 

Yes, there are two pressure switches. Likely, it is just the one that is right next to the Dryer can. If my compressor does not kick on (every time) I just touch that connection and it takes off immediately. The other one is located under the car, near the outlet of the radiator hose. Might want to clean the connections, or tighten up the prongs carefully.