Mercury 75 Service Manual page 373

Down Mode
When the trim switch is activated in the down posi-
tion, the electric motor (c) will rotate the pump (j) in
the opposite direction. With the pump gears rotating
backwards, the flow of oil is reversed. Oil is drawn
through the filter (g), through the down circuit suction
port (k) and into the oil pump (j). The pump forces
pressurized oil into the down passages, oil will slide
the shuttle valve (m) into the up circuit pressure oper-
ated valve (n). The shuttle valve will mechanically
open the up circuit pressure operated valve and allow
oil, from the up cavity of the trim cylinder (w), to return
into the oil pump. This returning oil, from the up cav-
ity, will supply the oil required for the down circuit. The
oil is blocked from returning into the reservoir by the
ball (i) inside the up circuit suction port. Oil, under
pressure, opens the down pressure operated valve
(f) and enters the down passages inside of the man-
ifold (q). The manifold passage connects into the trim
cylinder passage leading to the top of the cylinder.
The cavity, inside the cylinder, above the shock pis-
ton (u) is the down cavity. As the down cavity fills with
oil, the piston rod (a) retracts into the cylinder, lower-
ing the outboard motor. Oil from the up cavity exits
the cylinder and is drawn back into the pump through
the open up pressure operated valve (n). When the
piston rod reached full travel, the oil pressure inside
the down circuit will rise until the down pressure relief
valve (e) opens, bypassing oil back into the reservoir.
When the trim button is released, and the oil pump
stops supplying pressure, both of the pressure oper-
ated valves (f & n) will close and; if open, the down
pressure regulating valve (e) will close. The closed
valves will lock the fluid on either side of the shock
piston (u) & memory piston (t), holding the outboard
motor in position.
90-830234R3 DECEMBER 1997 MID-SECTION - 5C-13