Vehicle: 1985 K20 w/ 6.2 + TH400. Overview of what I have done so far: -> Replaced compressor (reman) -> Replaced Accumulator (new) -> Replaced orifice tube -> All new o-rings (greased and properly tightened) -> Pulled ~28hg for ~15 minutes -> Charged with R290 Works great. Ice cold - will run you out of the cabin. Issue is: the compressor cycles a LOT - like every 2-3 seconds when driving at 55mph. Now, before you get to saying "it needs more refrigerant" - read on. At aprox 1500 RPM around 75 degrees f ambient, I am holding a solid 38psi. I have tried less and I have tried more. The issue here is this: the damn gearing on this truck is so low that at 55mph, the poor detroit diesel is running upwards of 2500-3000 rpm. At such a high RPM, the compressor is working overtime - hence the rapid cycling. I can rev to 1800-2200 all day long and the compressor occasionally dis-engages/engages - as expected from a cycling clutch system, but I feel like this was a design flaw on GM's behalf. At highway speed, the RPM is simply too high to maintain a sensible compressor cycle. Of coarse, one fix is to compensate with more coolant - then I would be running way too high at idle. I could also completely by-pass the low-pressure switch, but not sure what this could lead to. I would not be afraid of running the low side at 5-10psi (hell, I have run PC coolers like this in the past) - I am just afraid of actually pulling a vacuum at some point and starving the compressor of oil... Perhaps this is fine? A cycle every 3-4 seconds is roughly 15-20 cycles a minute. At 2500 RPM, thats a lot of stress on any clutch I would think. My initial guess is that this is very hard on the clutch - am I wrong here? I have not tested it above 78 degrees f ambient yet - perhaps it will not cycle as bad once it warms up a bit more. The system had the same issue before I replaced everything and when it ran r134a. Thoughts? Surely I am not the only one who has an old 80's silverado w/ a diesel and the th400 - this has to be common. Should I not worry about it?