Two speeds are fairly straightforward to troubleshoot. Without the engine running and the ignition switch on, move the two speed switch and listen for the motor on the rear axle. Right after you switch from high to low (or vice versa), you should hear the motor run for a couple seconds and then quit. If you don't hear anything, the first place I would start is the switch. If you unplug the wires going to the switch, you should have one wire with 12 volts. If you have 12 volts to the switch, plug the wires back into the switch and see if there is voltage leaving the switch. Flip the switch back and forth to see if voltage goes from one outbound wire to the other. IF all this checks, the switch is good. Next verify that each wire going to the two speed itself is getting fed when the switch is actuated.
Next step is to remove the cover on the two speed motor. Inside you will see a screw driven by the motor. The motor turns the screw which shifts the axle by moving the lever attached to the 'nut' on the threaded shaft. At the top and bottom of the threaded shaft is a 'knife switch' which opens the circuit when the shift lever has moved the correct distance when shifting. On the back side of the 'nut' is a nylon tip that sticks out to actuate the knife switch. Sometimes the tip breaks off and will not allow the motor to operate. For example, when the switch is in the down position, the knife switch opens one set of contacts, shutting the motor off. When the motor calls to run in the other direction, another set of contacts allows the motor to reverse. Many times these switch contacts fail. Parts for these units are available from medium and heavy duty truck parts houses.
Anybody else jump in with anything I missed!
