A professional conversion would have stickers to indicate.

r134a fittings resemble a the quick-disconnect fittings used for air compressors, though shorter. r12 fittings are threaded on the outside.

Frequently, the r12 -> r134a conversion fitting are screwed right onto the old r12 fittings. Perhaps they are still under there.

BTW, it is better to use a separate gauge set for each to avoid cross-contamination. Also, this is the legal way to do it. r134a sets are required to have 'closed' hose fittings that keep the gas in the hoses even when they are disconnected.

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1986 300E 5-Speed 240k mi.