A 1990 would have come originally with R12 which you can no longer buy. You can retrofit the system to R134A by changing the oil in the system and installing retrofit adapters. I have to warn that this change doesn't work out well with all cars as they were engineered for R12. When recharged, you use only 80% of the original R12 charge.
I would suggest Googling R12 retrofits and read up on the methods and pros and cons.
Wednesday, August 2nd, 2017 AT 10:56 AM
(Merged)