Check and remove the starter traction relay as follows:
Remove the starter (see above).
Using additional heavy gauge wires, connect the starter housing to the negative battery terminal, and connect the positive battery terminal to terminal 50. If this moves the starter drive gear forward, but the starter motor does not rotate, then either the starter motor is faulty or the contacts of the traction relay are burnt.
If the starter gear does not move forward, then the traction relay has failed and must be replaced.
Unscrew the nut and disconnect the wire from the starter motor to the lower contact of the traction relay.
Unscrew the screws and remove the traction relay from the starter.