Voltage is applied at all times to the horn relay through the 15A horn fuse in the underhood fuse block. The blackout (B/O) service drive switch is used to control the horn relay. Power is applied through OG (circuit 40) to the blackout (B/O) service drive switch, which is normally a closed switch. When the horn switch is depressed, the horn switch contacts close, providing a ground to the coil of the horn relay. When the coil of the relay is grounded, the relay energizes and the contacts close, applying battery voltage directly to the LH and RH horn. Because the horns are grounded at ground (G100) the horns will sound as long as the horn switch is depressed.

The blackout (B/O) service drive switch is used to disable the horn relay when placed in the blackout (B/O) mode. When the blackout (B/O) service drive switch is in the ON position, power to the horn relay is disrupted leaving the horns inoperative.