Converted AC Pressure
 

So I have a 300D that had the AC system converted from the r12 to the r134 and was told by my old mechanic to never charge it fully due to the conversion. does anyone know what that pressure should be in the converted system?

Normally you charge R134 3/4 weight-wise relative to R12. Pressure should be similar to R12

R134a performs so poorly in the W124 to begin with when the temperatures heat up that not putting enough in decreases performance even more.

Pressures will never be the same as R12, and that is half the problem because the additional pressures heat up the condenser and then the system can't shed the heat regardless of the fan set up. It'll only perform half decent on the freeway, and below 90F. Anything above that, especially at idle, and expect 50-60F coming from the vents. At idle, in 110F, mine is hopeless...75F coming out the vents. On the freeway it'll keep around low 50's, but unless its an hour drive, the car never cools down. Turn the fan down low and you immediately realize it did nothing to cool the cabin.

Climate control functions and pressures should be very close to the same if the conversion was done properly with no traces of mineral oil left in the systems. The fill quantity of refrigerant for a w123 300D would be 2.64lbs (10% less than R12). Also the compressor should be "healthy" and a new dryer & switches installed. I run a factory R4 compressor that has worked flawlessly up to temperatures of 110deg for three years without any faults. I'm not sure if you have a W123 or a W124 300D so I uploaded both the factory pressure chart for w123 R4 & a generic pressure chart that would apply to the W124 Nippondenso compressor.
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I hope this is what you were looking for!