THIS PROCEDURE MUST BE PERFORMED BY A QUALIFIED TECHNICIAN.
THIS PROCEDURE MUST BE PERFORMED BY A QUALIFIED TECHNICIAN.
The base I/O relay board is utilized within the system to provide power to the processor control board and the internal controls of all circuit boards in the system. Input power is applied to the L1 and L2 240 and L1 and L2 120 terminals on the base I/O relay board. This board has two fuses and they operate as follows:
The 1/4 amp fuse (Part # 1024036) provides power to all of the circuit boards in the system. Line voltage power is current limited through this fuse. If this fuse is open, the system's display does not illuminate. A high voltage surge, incorrect input voltage wiring, a problem with the base I/O relay board, processor control board, or interface cable can cause the fuse to open.
The 5 amp fuse (Part # 1024037) on the base I/O relay board protects the variable voltage (triac) outputs for blower and damper control. These outputs pass voltage from the L1 terminal to the blower and damper output terminals. If this fuse is open, the most probable causes would include a shorted blower, shorted blower wiring, a shorted limit switch, or a failed triac on the base I/O relay board.