In the days before electronic and digital speedometers, mechanical speedos were cable driven off the transmission. If your mechanical speedometer quit working, binds up occasionally, or makes a howling noise, the speedo cable may be the culprit. The only other major problem would be the nylon gears within the speedometer itself.
Specific Fitment:
1984-1986, 1988-1989 190D Manual
1987 190D Naturally Aspirated Manual
1984-1985 190E Manual
1986-1988, 1991-1993 190E 2.3 Manual
1987-1988, 1991-1993 190E 2.6 Automatic
1989-1990 190E Automatic