What year did R-134a become the only refrigerant used in AC systems American Vehicles?

R-134a did not become the *only* refrigerant used in American vehicle AC systems in a single year. The transition from R-12 to R-134a was gradual, starting in the early 1990s and taking several years to complete as manufacturers phased out R-12 and redesigned systems for R-134a. There wasn't a specific cutoff year where only R-134a was used.