I am sure the controller is an old resistor type, I have never seen one in a truck before but I have had the cards in for repair (the PCB with the 2 relays on the right hand side of the photo) which is a timer card used to give a delay between speeds (gears). The fault is most likely to be either a wiring fault between the foot pedal and the timer card or a faulty timer card. unfortunately I don't have any wiring diagrams to help you check out the wiring.
Regards Shane