Mercury 225 EFI, Service Manual

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POWER TRIM - DESIGN II
Page 5B-14 90-884294 OCTOBER 2001
Tilt Circuit
In the up mode, as the trim rams (1) reach the limit of their travel, the mechanical check
valve (2) of the trim relief valve, in the port trim ram piston (3), contacts the cylinder cap.
The “pin” contact with the cover mechanically opens the shut off valve, allowing the trim
relief valve to bypass oil and perform the following functions.
•
Trim Limit
While the engine is running under thrust (at high engine RPM), the high pressure devel-
ops below the pistons. The high pressure will open the check ball on the bottom of the trim
relief valve (850-1150 psi), allowing oil to flow through the port trim ram piston. If the oper-
ator continues to depress the “up” button, the up pressure will not be sufficient to over-
come the propeller thrust, so the trim range is limited to the length of the trim rams. When
the engine thrust falls (low engine rpms), the check ball in the trim relief valve closes, al-
lowing oil flow to extend the tilt cylinder ram into the tilt range.
•
Over Trim At High Thrust
As the operator increases the engine rpms when the engine is raised beyond the trim
range, the pressure below the pistons begin to rise. When the pressure is sufficient, the
high pressure will open the check ball on the bottom of the trim relief valve (850-1150 psi),
allowing oil to flow through the port trim ram piston. Oil will continue to flow through the
valve until either the engine contacts the trim rams and the mechanical shut off valve
closes or the engine rpm’s are reduced.
•
Maximum Up Pressure Reduction
As the tilt ram extends to its limit, the up pressure below the pistons will increase and open
the trim relief check valve to relief the up pressure. If the up button is not released, the
up flow will continue to dump over relief causing the electric motor to heat up. The thermal
overload switch inside the motor will open, stopping the motor.