Yamaha F150A Service Manual page 67

Stationary condition
When the PTT switch is not pushed, the gear pump assembly will not pump the PTT fluid, the up-
main valve and down-main valve will remain closed, and the PTT fluid in the system will remain at a
constant pressure. As a result, the tilt ram and trim rams maintain their positions until the PTT switch
is pushed and the PTT fluid flows through the system again.
Shock-absorbing function
If the lower case strikes an obstacle while the boat is moving, a force causing the tilt ram "1" to
extend will be applied suddenly. Because this force will cause the PTT fluid pressure in the tilt cylin-
der upper chamber "a" to increase, the tilt piston absorber valves "2" will open so that PTT fluid
enters the space between the tilt piston "3" and the free piston "4" to decrease the PTT fluid pres-
sure. As a result, the damping effect of the tilt piston absorber valves "2" protects the PTT unit from
damage before the tilt piston "3" strikes the top of the tilt cylinder. After the collision, a force is gener-
ated as the outboard motor returns to its original position due to the weight of the outboard motor
and the thrust of the propeller. The PTT fluid enters the tilt cylinder upper chamber "a" through the
shock return valve "5" in the tilt piston "3". The tilt ram "1" stops when it contacts the free piston "4".
1
a
A. Fluid flow when the outboard motor
strikes an obstacle.
B. Fluid flow when the outboard motor
returns to the original position.
A
2
1
a
5
3
2
4
2-22
B
PTT unit
0
1
2
3
4
5
6
7
8
9
3
4
10
A