The factory A/C is universally described as marginal even when new, fine for the front seats but no match for American systems, and hopeless in extreme heat. Converting from R12 to R134a is common but reduces capacity further; knowledgeable owners note a larger evaporator expansion valve is needed, and some recommend modern retrofit kits that keep the original controls.
Air conditioning: weak when new, and the R12/R134a dilemma
Mercedes-Benz 250S-300SEL 6.3 (W108/W109) · ownership