I fixed 2 last year, an 87 and an 88, that had good gears and lost the drive motor. One came that way- the motor was pulled off the circuit board.

If the gear is fine, replace either the whole speedo head or disassemble and resolder the blue and black wires to the circuit board. The odo drive motor board is actually the connector on the back of the cluster; the blue and black wire bring the signal to the coil for the speedo needle, so you know it's getting the pulses from the rear end.

I swapped my circuit board because I wanted the mileage to be close to correct and the only units I had were way off. Swapping the whole speedo head or even cluster (check years, you want an 86-88 unit) is simpler.
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