Does the odometer continue to work? If so, the problem is internal to the speedo, not wiring or sensor at the differential.

There are electrolytic capacitors on the speedo circuit board that leak electrolyte (black gunk that hardens). Replacing these capacitors and cleaning the board (especially between pins of nearby components) will typically fix the problem.

The common culprits are the following three capacitors but check all electrolytic capacitors:

10 micro Farads 25V
6.8 micro Farads 25V
22 micro Farads 50V

If you do replace them, make sure the capacitance of the replacements is the same as the originals and the voltage is equal or greater than the original.

Getting a speedo from the junk yard is a crap shoot. Since this is a common problem, the speedo either won't work, or will eventually fail like the original.

Michael
Melbourne, Fla

1985 740 460k
1988 780 Coupe V6 190k
1992 745 200k