The single most-discussed W140 issue: Mercedes used biodegradable insulation on engine wiring harnesses, and the marque-knowledgeable consensus is that every US W140 through the 1995 model year is affected, tied to LH-SFI injection. Sellers repeatedly (and sometimes wrongly) claim early 1992 cars are exempt; the counter-view is that undisturbed low-mileage cars simply haven't failed yet. …
Biodegradable wiring harness (all US cars through 1995)
Mercedes-Benz S-Class (W140) · mechanical