Hi, I got the 420 out this weekend, and put the gauges on: low side cycles smoothly at idle at about 10 second intervals between 35 and 40 PSI. Is this cycling caused by a control valve in the system? It has been converted to R134-A and works OK but not as great as I'd like. I installed a new compressor and dryer last year due to the seal leaking on the previous compressor.

Low pressure "cycling" between those pressures is normal. Controlled by the expansion valve.

Have you checked the fresh air door for proper operation?

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the fresh-air door does work and it does improve the performance.Also I overhauled the mono valve and, in economy mode the air is not being heated so i think that's OK as well.
Far as I'm concerned, the fresh air door can stay closed. The Silver Shadow I have is all recirculate if I understand the manual. It works fine and is a lot less complex system, but it would be since it was made in 1970. the good old days!
I know the condenser is a little small on the 420; I thought about a parallel-flow condenser which might help. Seems like there was a factory PF condenser on the later Diesel model which would fit perfectly, for only a lot of money.
I just wondered about the cycling; it looked so smooth and even that I figured it was evidence of a properly-functioning control. What about the pressure? About right? If the suction pressure were a little lower would the evaporator operate at a lower temp, therefore higher capacity? But then, how to get it lower: remove some charge, or? what is the limit for a converted system?
Best converted system I have seen is a 94 Suburban: ice cold and no problems since conversion about 5 years ago.

The early W126's need more air across the condenser for the system to cool better.

Running the aux fan at the higher speed can help.

Also make sure that the condenser is CLEAN, no pine straw or dirt between it & the radiator.

__________________
MERCEDES Benz Master Guild Technician (6 TIMES)
ASE Master Technician
Mercedes Benz Star Technician (2 times)
44 years foreign automotive repair
27 Years M.B. Shop foreman (dealer)
MB technical information Specialist (15 years)
190E 2.3 16V ITS SCCA race car (sold)
1986 190E 2.3 16V 2.5 (sold)
Retired Moderator