Take the carbon brushes out, unplug the steer motor, undo the 2 x long bolts & lift the motor housing of the rest of the steer motor, blow out all the black carbon dust from the housing & also the motor armature with an airline.

Check the steer motor hasnt shorted out internally.

Why was the tacho changed?

Did the steer motor fault started immediately after the tacho was replaced?

Has the wiring to the tacho been checked in case its been trapped / shorted?

You havent untwisted the tacho wires?