[hatzie]

Compressors can blow out on R134 conversions when the system isn't flushed, vacuumed down, completely charged with refrigerant and filled with Ester oil.

This is how I refit an empty R12 system no matter what refrigerant I use.

Make sure the hoses get swapped to barrier hose. You don't have to change out the hard lines. Remove the crimp collars and crimp on new barrier hose with new collars. An AC shop can do this or splurge $$$ on a Mastercool AC Crimp tool.

Cut off the AC muffler (if there is one) and braze on a muffler bypass tube.

Make sure the o-rings and gaskets get swapped to R134 compatible.

Install R134 fittings and a retrofit label.

Flush the lines, Condenser (If I'm not installing a new high efficiency parallel flow unit), and Evaporator to get rid of any oil and contamination in the system and blow out with compressed air.

Replace the Fixed Orifice Valve with a Variable Orifice Valve (VOV).

Get fresh sealed oil matched to your refrigerant choice. PAG and Ester oils used with R134 (Ester on conversions). Mineral oils are used with R12 and hydrocarbon refrigerants like R290 (propane), R600 (isobutane), and hydrocarbon blends like Envirosafe.

Completely drain the unknown pedigree oil from the new compressor and fill it with a charge of sealed oil in the amount the service manual calls for. That way you know exactly what you've got in the system. No guessing. PAG, Ester, and Mineral AC oils absorb moisture fairly fast. I'd fill with oil just before you install the drier and close the system to minimize water absorption. You can buy oil with UV Dye premixed to avoid overfilling.

Unpack and install a new sealed accumulator/drier dead last. Doing this as the last step in closing the system minimizes the moisture the desiccant pak absorbs.

I hook up my gauges and vac the system all the way down immediately to check for leaks and cut down on the amount of moisture the desiccant pack in the accumulator/drier and the compressor oil absorb from any air left in the system. Alternatively you could sweep the system with several Nitrogen, CO2, or Argon/CO2 charges. Some folks use propane to sweep but it's technically a refrigerant so the EPA could fine you some ungodly sum of money for sweeping with it.

I pressure test, after the first successful vac, with just enough Argon/CO2 from my MIG welding tank to close the Low Pressure Cutoff switch and shoot some UV dye into the system. Run the compressor for a few minutes and check for leaks with a blacklight.

When I'm sure I've got a leak free system I Vac down again for at least 6-8 hours, close the vac/charge valve or disconnect the gauges completely, and leave overnight or longer. If it's still at full vac, after 12+ hours or so, I fill with a refrigerant charge till it gives me good cooling without frost and high head pressure.

A swap done like this usually lasts for years.