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

POWER TRIM

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 outboard 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 whether trimming "up" or
"down" will improve the ride in rough water.
When trimming your outboard from a mid-trim position (trim tab in neutral, straight fore-and-
aft, position), you can expect the following results:
TRIMMING OUTBOARD "UP" ("OUT")
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 insta-
bility.)
Will lift bow of boat, generally increasing top speed.
Transfers steering torque harder to left on installations below 23 in. transom height.
Increases clearance over submerged objects.
In excess, can cause porpoising and/or ventilation.
In excess, can cause insufficient water supply to water pump resulting in serious water
pump and/or powerhead overheating damage.
Excessive engine trim angle will result in insufficient 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 (located under engine cowl) and close the
electric motor circuit. The electric motor will drive the pump, thus forcing automatic transmis-
sion 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 engine cannot be trimmed
beyond the 20_ maximum trim angle as long as engine rpm is above approximately 2000
rpm.
The engine can be raised beyond the 20_ maximum trim angle for shallow water operation,
etc., by keeping 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.
Page 5C-6
WARNING
WARNING
90-828631R3 MARCH 1999