I have a 2002 r53 MCS with an a/c system that is not cooling well. It has good high/low pressures when I check with a manifold, but the air out the vents is not cold. I noticed that the electric fan cycles on and off when the a/c is running; should the fan run constantly with the a/c activated?

 

Fan - pretty much yes in heavy traffic - on the highway probably not, but it's hard to tell.
I have an early '02 R53 with manual heat & A/C controls. I find it difficult to get the distribution right but once I do, it's not too bad - just not the greatest.
Some folks say it's better on recirc - I haven't tried.

 

If the AC is on, tee fan should run on low. Can you tell if it's going from high to low, or low to off? If it turns off, likely your compressor is shot. If that's the case, time for a new compressor (first confirm your low speed fan is working.)

 

I can check the fan. I think the compressor is okay because I had a pressure differential; 45 on the low side and 225 on the high side, on a 90 degree day with humidity.