The key switch has three functions - in the off position it connects terminal E to F to which no wires are connected - in the on position it connects and maintains a connection to terminals C and D - when turned to the start position, it connects terminals A to B, it maintains this connection as long as the return spring is kept under tension by the hand of the operator. The switch may fail in many ways, but in the off position, there should be no power to anything except the power steering fuse, the red/black wire at the key switch, the EFI fuse, the chassis relay and the fuel pump relay. By the wiring diagram; the only time there should be power to the speedo is when the key switch in on (connecting the red/blk wire to the red/wht wire) - red/wht supplies power to the speedo. If the switch fails to break the circuit positively, then momentary or maintained energization of the speedo is possible. It's also possible for the connection to be so fragile that it can make connection, heat up, break connection and repeat the cycle (similar to a circuit breaker) until the battery is discharged.
I suspect the key switch needs replaced.
Shop Owner and Mechanic with over 50 years experience