It is possible to service it to some extent, depending on the problem.
Sounds like you may just need to lubricate the gears. However, you might have some broken teeth gumming up the works and requires some more intense service


You can remove your instrument cluster and open up the case to access the speedo and the odometers. The cable to switch the trip can be disconnected (small parts warning) and you can lubricate the setup. If I recall correctly, the trip is connected by a rather long gear from with plastic teeth from the regular odometer. I think the trip itself is a metal gear set. Mine would not reset either until I switched it out with a spare gear unit that I happened to have. I It also had a sheared off plastic odometer gear (which apparently happened around the 380,000 mile mark.. back in the 1980s).

Even repaired, my gears do not *completely* reset. I crank it around and for some reason the 1/10th number never resets. Works fine otherwise.

Incidently, my 220 is also in grey primer at the moment...
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1967 P220 (Someday my daily driver...), 1971 P1800 (Parts Car), 1972 145S (1993-1997), 1977 245 DL (1993-1999), 1983 245 (1997-2001), 1986 745 GLE (1997-2000), 1990 740 GL (2003-4), 1995 945 (2004-Present)....