The refrigeration oil used in an R-12 system is NOT compatible with that used in a R-134A system, if changing refrigerants, change the oil as well.
Good methods nc pearson, but with regards to the oil, the only problem with the mineral oil in the old R12 systems is that it is not miscible with 134a; it cannot be dissolved in the refrigerant. bigger systems rely on the oil being carried through the system with the refrigerant, but on little systems like these little or no oil moves out of the compressor, so miscibility is not required. That is why you can drop 134a into a system without having to switch to Polyol Ester oil. Also, while weighing the refrigerant in on a scale is technically correct, in practice it often isn't practical. A system can also be charged using suction pressure and temperature measurements. I rarely ever weigh a charge into a system that small, and even if I do, I usually end up adjusting the charge to get it running the way I like it.
Because the temperature and pressure characteristics of R12 and R134a are so similar, the two refrigerants are compatible with each other too. Although it is certainly not "best practice" blending the two will do no harm. In a pinch you could top up an R12 system with R134a if there was no means of evacuating available.
As a professional in HVAC I'm staying out of this one!
I too am an industry professional. In my 19 years in the industry I have worked on everything from custom made aquarium chillers to 100 ton blast freezers and refrigerated warehouses. At one point in my career I was installing custom refrigeration systems for the local Hunter dealer, as well as maintaining and installing refrigeration systems for a couple of charter companies. I have experience with dual voltage systems as well as engine drive systems. I have made custom heat exchangers and holding plates, and even made my own condensers. I have repaired dozens of systems, and converted plenty of them from 12 to 134a.
These days I mainly work on commercial and industrial refrigeration and HVAC systems, but I still do the occasional boat system for fellow club members.
So, I think I am qualified to speak on the subject....
OP has only made 3 posts to this thread; very little of what's being tossed around here really is going to help until he updates his progress. I did offer him an evaporator if he wants it.
I'm certainly not going to enter another qualification contest. (wizzing contest) Why I hold back on advice.
134 and mineral oil don't mix but the system will still work. how long and how well to be determined if and when it fails. (But it's already an old system, so why not?) There is a whole list of results but very little visual proof of what happens.
No one would pay me to put 134 into a system that has not been flushed and changed over. Even though it's done allot by DIYers. I would not do it either.. it's not worth the trouble. If someone asks me if it can be done I say "do what you want, you will anyway"
Cost to convert small vs new by a professional often makes sense to go new with labor rates being so high.
Even Danfoss says; replace the compressor.
12volt boat systems are small but very expensive compared to traditional 120-230 volt systems. Old vs new makes allot of sense for those that are not DIY types.
Of course I would expect a professional who is equipped with the appropriate equipment to properly evacuate a system. That is why they are paid the big bucks! My point is that just because a system has a leak does not mean that whole system needs to be ripped out and thrown in the garbage! In many cases a few hours of technician's time can have the system back up and running reliably. There are some DIY owners out there that also have the ability to effect repairs given adequate advice and access to a couple of specialty tools. Unfortunately many systems don't even have service ports to allow access to the system, making it even harder for DIY repairs.
It is true that many refrigeration technician's rates are high enough that it may not be worth while for the owner or the technician. Not many boat owners would want to pay the $120/hr my company charges, but there are guys out there that work for much less.
There are also some owners out there that want to fully understand all the systems on their boat so they can be as self-sufficient as possible. If you are cruising in remote areas and your refrigeration goes down, it would definitely be good to know what needs to be done and be able to do it. It helps to know what can and cannot be done. It is likely that buying a new kit system from your local Westmarine just isn't an option.