Theory Of Operation; Adjustments; Trimming Characteristics - Mercury 40 Service Manual

Theory Of Operation

The Power Trim system consists of an electric motor,
pressurized fluid reservoir, pump and trim cylinder.
The remote control (or trim panel) is equipped with a
switch that is used for trimming the outboard "up" and
"down", and for tilting the outboard for shallow water
operation (at slow speed) or for "trailering". The out-
board can be trimmed "up" or "down" while engine is
under power or when engine is not running.

Adjustments

Trimming Characteristics

NOTE: Because varying hull designs react differently
in various degrees of rough water, it is recommended
to experiment with trim positions to determine wheth-
er trimming "up" or "down" will improve the ride in
rough water.
When trimming your outboard from a mid-trim posi-
tion (trim tab in neutral, straight fore-and-aft, posi-
tion), you can expect the following results:
TRIMMING OUTBOARD "UP" ("OUT")
WARNING
Excessive trim "out" may reduce the stability of
some high speed hulls. To correct instability at
high speed, reduce the power gradually and trim
the motor "In" slightly before resuming high
speed operation. (Rapid reduction in power will
cause a sudden change of steering torque and
may cause additional momentary boat instabili-
ty.)
Will lift bow of boat, generally increasing top
speed.
Transfers steering torque harder to left on instal-
lations below 23 in. transom height.
Increases clearance over submerged objects.
In excess, can cause porpoising and/or ventila-
tion.
In excess, can cause insufficient water supply to
water pump resulting in serious water pump and/
or powerhead overheating damage.
5D-4 - MID-SECTION
WARNING
Excessive engine trim angle will result in insuffi-
cient water supply to water pump causing water
pump and/or powerhead overheating damage.
Make sure that water level is above gear housing
water intake holes whenever engine is running.
Operating "Up" circuit will actuate the "up" relay (lo-
cated under engine cowl) and close the electric motor
circuit. The electric motor will drive the pump, thus
forcing automatic transmission fluid through internal
passageways into the "up" side of the trim cylinder.
The trim cylinder/trim rod will position the engine at
the desired trim angle within the 20_ maximum trim
range. The power trim system is designed so the en-
gine cannot be trimmed beyond the 20_ maximum
trim angle as long as engine RPM is above approxi-
mately 2000 RPM.
The engine can be raised beyond the 20_ maximum
trim angle for shallow water operation, etc., by keep-
ing the engine RPM below 2000 RPM. If engine RPM
increases above 2000 RPM, the thrust created by the
propeller (if deep enough in the water) will cause the
trim system to automatically lower the engine back to
the 20_ maximum trim angle.
TRIMMING OUTBOARD "DOWN" ("IN")
WARNING
Excessive speed at minimum trim "In" may
cause undesirable and/or unsafe steering condi-
tions. Each boat should be tested for handling
characteristics after any adjustment is made to
the tilt angle (tilt pin relocation).
Will help planing off, particularly with a heavy
load.
Usually improves ride in choppy water.
In excess, can cause boat to veer to the left or
right (bow steer).
Transfers steering torque harder to right (or less
to the left).
Improves planing speed acceleration (by moving
tilt pin one hole closer to transom).
Operating "Down" circuit will actuate the "down" relay
(located under engine cowl) and close the electric
motor circuit (motor will run in opposite direction of
the "Up" circuit). The electric motor will drive the
pump, thus forcing automatic transmission fluid
through internal passageways into the "down" side of
the trim cylinder. The trim rod will move the engine
downward to the desired angle.
90-852572R1 JANUARY 1998