89SP speedometer malfunction

[fred89sp]

The speedometer in my 89SP has quit. A couple of weeks ago it would
bounce widely between 0 and 35 at speeds up to 35 but ran fine over
35. Now it stays at 0.

Last year I installed the correct new rear drums and recently had the
underside checked out as part of my spring lube and chassis
inspection. At the time the speedo was erratic. I was told the
sensor appeared to be in place with wires attached.

Any suggestions on where to start?