SPT secondary postion transducer. loss of SOK input to mlb3 board. MLB3 board supplies 24 volts to cartop SPT unit. the wiring of the unit then sends the 24 volts back to the mlb3 board to verify 24 volts available at cartop unit. fairly simple to diagnose problem. look at print, possible blown fuse, bad mlb3 board, traveling cable etc. for some reason SOK input is not correct.