If both the speedometer and the odometer quit, check the sensor connection and wires at the differential.
If the odometer works when the speedo stops working, then it is most likely problems with the electrolytic capacitors on the circuit board of the speedometer. This is a common problem. Pull the speedometer from the instrument cluster, remove the needle and face plate to access the capacitors. There are two or three electrolytic capacitors that have a tendency to leak electrolyte (hard black deposit) onto the circuit board and short pins of the chips. Clean the board and replace faulty capacitors.
Visit the following link:
http://www.brickboard.com/FAQ/700-900/
Michael
Michael (User name 'Bolbo' at www.brickboard.com)
October 2008