Dodge R1500 1999 Service Manual page 129

To prevent refrigerant cross-contamination, use following
methods to identify R-134a based systems and components.
Fittings & "O" Rings
All R-134a based A/C systems use 1/2" - 16 ACME threaded
fittings (identifiable by square threads) and quick-connect service
ports. See Fig. 1.
Fig. 1: Identifying R-134a Service Ports
Courtesy of chrysler corp.
Underhood A/C Specification Labels
Most R-134a based systems will be identified through the use