i would check the continuity across the contacts with a vom
if the speed solenoid is pulling in but the motor doesn't engage the solenoid may not be connecting internally. Also if there are speed solenoids there should be resistor coils on a board or mounted near the solenoids, check those resistor coils and connections.
Check the control wiring and also the power wiring for bad connections and corrosion etc.
Having a serial number would be alot more helpful since there is a transistor model too, not sure but this doesn't sound like a transistor control truck.