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The o/d solenoid valve is on the driver's side, above the pan. Trace the wire down that side of the tranny.


...and I think you should hear a click from that solinoid when key on and switching the OD-off button. (when the engine is warm)

overdrive relay is a circuit board type control unit located near the ECU. It should also click, same as the solinoid. ( this is true of 1989. Trucks are different, so 1987 might be different)

also a switch in the shifter console.
also a coolant temperature sensor

1989 5 door montero 3.0 auto
stock original Japanese
about 200k miles