Folks,
  In the heat of helping to solve a serious design problem tempers seemed
to have flared. I hope we can continue to have an open and civil discussion.

Issues that need to be discussed are FAILURE modes.

  For example, I did not take Randy's previous comments to indicate that he
saw 2000 PSI, but that without careful concern this could happen.

  The problem is when a significant volume of hydraulic fluid is warmed it
expands and increases the pressure in the sealed lines. This pressure is
causing the pressure switches to open. If the pressure is NOT released and
the pump is engaged by by-passing pressure switches the pressures can get
even higher. It's true that the pumps have internal relief valve. What's the
guarantee that the relief valve seals when the pressure is reduced? I do not
think any of us know! This is not a NORMAL operation mode. These pumps were
designed for marine use and NOT aviation use, as such should be operated in
a very conservative manner. (Not to mention high current loads when the pump
is engaged with a significant pressure drop across the pump.)

Wolfgang,
  Can you explain (or provide a circuit diagram) of your solution to this
problem. If this is indeed a good design many will gladly pay for a canned
solution. Others may choose to implement this them selves. In any case we
can all evaluate the system characteristics and potential failure modes.

Randy,
   As I am working on a IV-P I have not followed the discussions closely as
our hydraulic system operates differently. What is your solution? Do you vent the return line to the reservoir, bypassing the shuttle/check
valve?

I think if we can all stick to design objectives, system design, and failure
analysis we can all move forward.

I personally would never put a "BLACK" box into my aircraft that was
designed to change the behavior of some component in aircraft unless I could
assess the impact of that component on overall system operation.

I too am an EE ... there are many solid state devices used in engine
compartments of automotive and trucking environments that provide much
better reliability than relays, with flexible current limiting (absolute.
thermal buildup, stalled motor startup surge ...) capabilities.

r.t.s.