Why does the entire system need to be converted from R12 to R134a? Well? It comes down to if you want to do the conversion right, and have the system operate correctly. Or if you want to do it half-assed, and then who knows what’s going to happen?
Tester
This is not the proper way to do it, and will result in quick destruction of the A/C system. R-134a turns to an acid in the presence of the R-12 oil, and will eat out the system from the inside. Very expensive to fix.
The proper way is to open up the system, flush the lines and compressor, replace the accumulator/dryer, add the adapters, then evacuate and recharge. Then, it will last.
All I can tell you is that I knew a number of people that simply had the old system purged and refilled with R134 without problems and were happy with the results. Again, the chill wasn’t as chilly, but the systems were fine.
Nothing bad happened. They drove their merry ways.
AC systems are not safety systems. Failure of them to perform satisfactorally does not manifest itself as a dangerous situation or damage to any other system. You can try taking shortcuts and if they don’t work your loss is minimal.
I have converted my Mom’s '91 Escort - flush the system with a flush product, change the accumulator and if you want, take the fittings apart and clean and lub with 134 oil. Get a vaccuum done (I didn’t and the air makes it growl, but still cools OK) and fill er up with enough 134 oil(while the system is open) and 134a! 