Raymarine Autohelm 100 Install Manual page 39

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It is also necessary to ensure that the helm pump is fitted with reversing
check valves, otherwise the autopilot pump will drive the helm pump
(sometimes referred to as motoring the wheel) in preference to moving the
ram. Single helm pump systems without check valves should incorporate
a double pilot check valve and block (available as part Z068). This is shown
in the following illustration.
Notes:
1. A double pilot check valve may also be necessary on long tubing runs.
Tubing expansion may result in poor autopilot performance. The valve
should be installed close to the cylinder with the pump in between (as
shown).
2. If the vessel has two steering positions, check valves will already be
fitted to ensure independent operation of the two wheels.
Minimisation of hydraulic fluid loss during connection of the drive unit will
help to reduce the time and effort required later to bleed the system of
trapped air. Absolute cleanliness is essential since even the smallest
particle of foreign matter could interfere with the correct function of the
steering system precision check valves.
TYPE 100/300 Operation and Installation Handbook
D274-3