Brake Interlock Electrical System Operation
(1) Wire 109-RD-6 from alternator circuit breaker. (2) Wire 101-RD from alternator circuit breaker. (3) Fuse. (4) Neutral start switch.
(5) Neutral start relay "P9". (6) Parking brake switch. (7) Brake relay "P6". (8) Brake relay "P7". (9) Axle interlock valve. (10) Drum
interlock valve. (11) Key start switch. (12) Wire 70-BU. (13) Wire 776-YL. (14) Wire 776-YL. (15) Wire B765-BR. (16) Main relay.
The key start switch must be in the ON position before the
circuit will operate. In the ON position, electrical power is
provided to parking brake switch (6) from main relay (16)
through parking brake fuse (3).
When parking brake switch (6) is placed in the ON position,
current flow to the solenoids on interlock valves (9) and (10) is
interrupted. This action blocks the oil flow to the parking brake
and the parking brake remains engaged. With the parking
brake switch in the ON position, a current path is created
between fuse (3) and wire (14). With current flowing in wire
(14), relay "P6'" (7) and relay "P7" (8) are energized. When
relay "P6" (7) is energized, the current flow to the axle interlock
valve (9) and drum interlock valve (10) is interrupted. De-
energizing brake interlock valves (9) and (10) has the effect of
applying the parking brake. The de-energized brake interlock
valves also prevent the propel pumps from upstroking.