Got called to a little Yacht:

Unit description and application: Refrigerant R12 low temp freezer box with two holdover paltes

There are two refrigeration circuit for this unit. two separate condensers, one water cooled and the other air cooled. two different compressors one engine driven and the other compressor sites on a 1/3 hp hermetic compressor commercial condensing Tecumseh unit . two hold over plates are held in the freezer section. there is two TXV's on there respective line sets. from the txv refrigerant then passes to the first holdover then through the exit of the first hold over to the second hold over. I did not see any type of oil return between compressor (over course they are very different) i did not see liquid line solenoid.

When they have 115 volts on shore power then they would run the Tecumseh. (for what i think is one circuit of r12)
When the boat is off shore they use one of the two diesel engines to turn a belt/engine driven compressor (for the what i think is the second r12 circuit)

Interesting fact Tecumseh unit is controlled by a low pressure switch. But the other unit just has a high pressure differential switch and nothing else? i not sure how the second refrigerant circuit is maintaining box temp when out at see. any idea?

Question #1 are the two refrigeration circuits connected? (I did not see any evdnce that lead me to belive that but maybe im missing something)

Question #2. The Tecumseh condensing unit needs to be replace (small receiver leak and bad compressor)
The unit is R12.....so is the TXV. i really want to avoid touching the txv as it seems i will have to remove part of the counter in order to replace it. THAT BEING SAID I AGREE THAT I SHOULD CHANGE THE VALVE OVER TO THE NEXT REFRIGERANT. But i would like to know if i can use a R134a condensing unit and let it run on MP66. MP66 can work on many different oils. what are the problems might i run into if i do this?

try to keep in mind there is a very very limited amount of space around that txv thats why i ask. i know if i buy something r134a then i should really only put r134a.

Just posting for more information. At this point i will replace existing r12 with 404a and change the txv.

Thanks for any info