Twin Disc MG-5170DC, Service Manual, Page: 53 / 250

53
Twin Disc, Incorporated
Operation
Marine Transmission Service Manual #1020667
Mechanical Trolling Valve
This trolling valve is a variable orifice that controls the pressure in the rate-of-
rise chamber. The pressure in the rate-of-rise chamber determines the rate-
of-rise piston position, which ultimately controls the main and clutch pressures
for the transmission.
When the trolling valve lever is in the detent (non-trolling) position, the orifice in
the trolling valve is closed. Oil cannot exit from the rate-of-rise chamber through
the trolling valve’s orifice, and the rate-of-rise chamber is fully pressurized (the
ball and spring regulator is blocked when either clutch is engaged). This full
pressure causes the rate-of-rise piston to remain against its stop in the valve
body bore, and main oil pressure is not reduced.
Detent position Trolling mode
orifice opened
orifice closed
Figure 22. Mechanical Trolling Valve
Rotating the trolling valve lever into the trolling mode opens the variable orifice,
allowing some of the oil to escape from the rate-of-rise chamber to sump.
This reduces the oil pressure in the rate-of-rise chamber. Since oil is always
flowing into the rate-of-rise chamber through the orifice in the orifice plate, the
pressure in the rate-of-rise chamber is controlled by how much oil is allowed
to exit through the trolling valve’s variable orifice. The trolling valve lever
position determines the pressure in the rate-of-rise chamber, which determines
main oil pressure. Main oil pressure is always the same as the oil pressure in
the engaged clutch with this type of trolling valve.