If the parking brake switch is moved to the OFF position and
the propel control lever is out of the NEUTRAL position, the
parking brake will remain engaged. When the propel control
lever is out of the neutral position, neutral start switch (4) is
open and relay "P9" (5) is de-energized. When relay "P9" (5)
is de-energized, current from fuse (3) goes through wire (12)
and relay "P7" (8). When the parking brake is ON, relay "P7"
(8) is energized in order to create a current path from wire (12)
to wire (13). Current flow through relay "P9" (5) and wires (12)
and (13) sustains relay "P7 (8) and relay "P6" (7) in an
energized state even if power to wire (14) is removed by
moving the parking brake switch to OFF. This keeps the
parking brake engaged and the propel pumps neutralized if the
parking brake switch is moved to the OFF position and the
propel control lever is out of the NEUTRAL position.
If the propel control lever is moved to the NEUTRAL position,
the neutral star switch closes and relay "P9" (5) is energized.
When relay "P9" (5) is energized, the current path to wire (12)
is disrupted. When no current is flowing through wire (12),
relay "P7" (8) and relay "P6" (7) will de-energize when the
parking brake switch (6) is moved to the OFF position. When
relay "P6" (7) de-energizes, interlock valves (9) and (10)
energize in order to release the parking brake. The propel
pumps are no longer prevented from upstroking when the
interlock valves are energized.
Brake Interlock Valve
SMCS Code: 4282
(1) Brake interlock valves.
Brake interlock valve (1) prevents the pump swashplate from
moving out of neutral when the parking brake is applied. The
drum propel pump and the axle propel pump both have a brake
The brake interlock valve has a solenoid on one end in order to
move the valve spool. There is a spring on the other end in
order to position the valve spool when the solenoid is not
When the brake interlock valve is in the BRAKE ON position,
the brake interlock solenoid is not energized. In this position,
the brake interlock valve shifts and the brake lines and the
directional control valve are open to the hydraulic oil tank.
When the brake lines are open to the hydraulic tank, the spring
applied brakes are engaged. When the directional control
valve is open to the hydraulic tank, the servo valve shifts to the
neutral position. This causes the swashplate to move to the
When the brake interlock valve is in the BRAKE OFF position,
the brake interlock solenoid is energized. In this position, the
brake interlock valve shifts and the charge oil flows across the
valve and to the brake lines and the directional control valve.
The charge oil pressure overcomes the force of the springs
and the brakes are released. When the charge oil becomes
available for the directional control valve, any movement of the
propel control lever causes the servo valve to shift the
Brake Interlock Valve
(2) Solenoid valve. (3) Charge pressure port. (4) Brake port.
(5) Tank port.
When solenoid valve (2) is not energized, brake port (4) is
common to tank port (5). The charge pressure port (3) is
blocked. This prevents the charge pressure from releasing the
When solenoid valve (2) is energized, the spool that is inside of
the valve moves. This allows charge oil into the circuit and the