I have a 1992 Mercedes 400E with approx. 156K miles. We are getting ready for a long drive through Las Vegas to St.George, UT so I really need the AC working!!

Here are the symptoms that I have noticed:

Start the car cool, turn on Max. AC and fan and allow the car to run at idle. The compressor clutch is engaged and the compressor is turning.
Compressor seems to be slightly noisy (rattly) when it is engaged. It will run for approximately 15 minutes like this (ambient temp about 85F). Then it will start cycling and cycle fairly frequently (off about every minute).

The air blowing in the cabin at the beginning is only slightly cool, and the temperature does not drop much. The large diameter AC line under the hood is slightly cool to the touch, but definitely not cold.

After driving for a while, and checking again, I noticed that the compressor is off and not cycling. If I touch the large diameter AC line now, it is hot.

I checked the site glass, but it is hard to see anything clearly. It seems largely reflective. I will try cleaning it more.

The tap in the cool, large diameter line under the hood I believe is the low pressure line. Does that sound correct? (There is a larger diameter tap near the condenser at the front of the car).

I have a cheap pressure gauge that I got as part of a refrigerant refil kit. When I attached it to the low pressure side and tested it early after running the compressor, the lowest pressure I saw was about 45psi. When the compressor turned on, I could see this pressure go up quite a bit >60psi. From what I have been able to find, this sounds like too high a pressure in the system. The system has not been refilled for at least 2 years and maybe longer. Is there something else that can make the pressure too high?

I see the 2 auxilliary cooling fans in front of the radiator cycle on and run when the coolant temp reaches about 100C. They appear to be running on high at this point. I read some mention of a low speed fan run due to AC demands? Is this a major issue in terms of my lack of cooling? Particularly given that cooling is low even at highway speeds and low coolant temperature.

Thanks for your help with my AC problems. Think COOL!

Kim