What can cause the pressures in the system to have normal low pressure (36 psi) but only 140 high pressure? Outlet air is 55 degrees, the high side is hot and the low side is cold and sweating, with sight glass minimal bubbles. System has 4 lbs of r-134a and no leaks. I am concerned about a possible weak compressor as it was changed to a Nippon Denso about a year ago from the GM A6 that was original. Ideas?

Weak compressor is one possibility, but so is an expansion valve stuck open.

Roger on that - we isolated each valve by squeezing the liquid line closed on each evaporator and it had no effect on the high side pressure. Is there a possibility a compressor is too small on a large btu system?

A compressor too small could be an issue. 55F may be a bit warm (theoretically - it's a large volume of air to cool). About a 20-30F drop from ambient across the evaporator is average. Unless you have prior readings to compare to, your system may be OK.

I'd double check the cross-reference of the Nippondenso compressor vs. OEM.

Inlet pressure is about right, so any issue is with output - if the expansion valves are working right. So, I'd still not discard suspicion of the expansion valve. Actually, it's doubtful you could pinch those lines enough to restrict flow more than the expansion valve orifice.

Unfortunately, the only way I know of to prove expansion valve performance is to replace it (or them. In you case, one could be working and the other not; there would be a temp difference at the valves if one is stuck.

Th main unit is new, I would suspect the Acme bedroom unit as I did find the R-12 type O rings in the connections after I paid the shop to install all new crimped fitting hoses. I read a report from Prevost that installed flow solenoids in the liquid lines to activate any of the three units they install in their coaches. If I have to evacuate the system I might consider trying that. Have you ever seen those types of controls?

My Ford van (1T, LWB, light custom) has that for the rar air (turning on the fan opens the valve), but the rear air isn't stock Ford.

It makes sense to shut refrig flow off, as without the evaporator fan running, the expansion valve works hard.

If the extra evaporator was added without upgrading the compressor, then the compressor is too small.

I take your reply to mean I could have lower high side pressure with all other parts working if the compressor is unable to provide sufficient volume to the system to fully pressurize the evaporators and lines. I have about 20,000 btu in the evaporators and 45,000 in the condenser. Sounds like a larger compressor and solenoid for the rear unit are needed. Thanks for your help. I always get better help here than the basic web crawl.

I think you've nailed it - the compressor just can't pump the volume needed to supply the evaporators.

High intake pressure and low output pressure usually signify a compressor not moving enough fluid - or that what's moving is "escaping" through too much orifice aperture.

However, I'd not install a solenoid valve in the rear - it will just result in the compressor cycling more often. Run both A/Cs. I should have mentioned, I disconnected the solenoid valve in my Ford van.

Does the AC work. I thought 140 was fine. That's about all I run.

BTW mine is an R12 system running envirosafe.