2016ford transit connect ac only works at high rpm stationed

[Gmanfunk]

Hello all and thank you for your time to look at my question. I have a 2016 ford transit connect. Non turbo. The ac when starting at cold has a pressure of 150 high and 30 low. Car cools great. After 30 min or so. The gauge shows 100 over 150 and the clutch will not engage. If I rev the engine to about 2k the clutch comes on and ac cools but the new pressure is 250 high and 45 low. What could this be? Mechanic wants me to change compressor but I do not think that’s the problem. Thank you again for viewing my post.

[JohnHere]

I believe your vehicle uses a variable-displacement compressor and that the system might be undercharged. Begin by recovering/evacuating/recharging according to the manufacturer's specifications to ensure you have the correct amount of refrigerant in the system, and then repeat your performance testing.

Couple of questions: Does this vehicle use R-134a or R-1234yf refrigerant? And what is the approximate ambient temperature in your area?

[Tim]

vehicles without RHVAC 28.2 oz (0.8 kg)
vehicles with 3.5L EcoBoost RHVAC 41.92 oz (1.19 kg)
vehicles with RHVAC 38.8 oz (1.1 kg)

[Gmanfunk]

Thank you for the response. It is a 2.5 with rear temp control. It uses r134a and it’s about 85 outside.

[JohnHere]

Okay...let's begin with the recovery/evacuate/recharge service at a professional Mobile Vehicle Air Conditioning (MVAC) shop to ensure that the system is charged correctly. Pressures alone won't tell us that.

Often mentioned on this Forum, slight seasonal losses of refrigerant from the compressor shaft seal—a normal occurrence—often result in an A/C system acting up after several years due to an insufficient charge. The seal needs to leak a minute amount of oil and refrigerant to help it to do its job.

Has this vehicle been driven on roads on which ice-melting chemicals are used in winter? If so, it's also possible that a slight leak has developed in the long refrigerant lines leading to the rear A/C due to corrosion.