I have my Dad's '90 Eldorado that he converted to 134 many years ago. It had the typical poor cooling so well documented on these cars due to the ECM controlling the Compressor cycling. The threads on this topic are very old in this forum, & I want to pass on what I did this summer to get this system producing cold air that would satisfy just about anyone. I started with a functioning system. Did the basics, Evacuate refrigerant, replaced the GM White orifice with a Blue .067 tube, replaced the accumulator, & "O" rings. I vacuumed the system and charged it with 32 oz of R134a. It calls for 38 oz of R12.
The part that makes this system give you cold air is in placing a resistor in parallell on the Low Pressure sensor connector. In reading the old threads here, a 2K , & 55K resistors are mentioned in 2 different posts. No one has explained just what the resistor accomplishes, but here is what I found by trial & error. First, the resistor changes what the ECM reads from the low sensor & when it cycles the compressor on & off. With 134a you need the compressor to run longer & pull the evaporator pressure lower than what the system is set to to do with R12. The 2K resistor eliminated cycling, & iced the evaporator in no time. No resistor at all did not run the compressor long enough & it was off too long resulting in poor vent temps. I went out & purchased a 500 ohm, 22,000 ohm & 47,000 ohm resistors. The second try was with the 500 ohm, & it gave the same results as the 2K. I switched to the 47K resistor & got decent results. The system ran the compressor longer & gave cold air; however this resistor also seemed to raise the cut in point because the cold air was basically lost before the compressor was cycled back on. This combination did give much improved cooling over no resistor. So I then replaced the 47K with the 22K. This 22K resistor I feel is really spot on for great vent temps,& proper cycling with the 134a in this Cadillac.
We have had a very hot July & August here with 90* & 70 + dew points pretty common. I'm driving this car daily & am very satisfied with how it cools now. High blower is giving 48 - 50* air on these days, & it cools the car quickly. Low blower can drop to around 38*, but typically is in the 42-44* range.
Conversions on these cars has probably become few & far between, but there are likely some of us out there still trying for great cooling in these older Cadillac's. I would guess this should work on any 86 to 93 Cadillac except the Brougham. I think it's important to do the basics on the conversion & not overlook a good job converting. To place the resistors on the low pressure connector, just un plug it, insert each wire from the resistor in each terminal on the plug & plug it back on with the resistor body over one of the tabs. Just push it back on & your done.