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LC20 Hydraulic Pump Wiring Considerations:



1.  Lancair wiring provides for two circuit breakers, 50 amp for the pump and

5 amp (or smaller) for the relays.  They also state that during any testing,

to pull both breakers.  Disengaging both is not always done -- "I forgot",

they are not co-located, one is a breaker and the other is a fuse or whatever.

If the main power bus is activated and only the pump breaker is opened, then

the up or down relay will be activated (depending on the position of the gear

switch) if the corresponding pressure switch is not sensing enough pressure

because the pressure has leaked down or the gear extension valve is open.  The

"gear transit" light will not be on because the power for it comes from power

being sent thru the relay contacts to the pump and that circuit was de-

activated by pulling the pump breaker.  This means the "intermittent use"

relay could be engaged for the length of time of the testing (in my case, 4

hours).  The  heat build-up in the relay can result in melting internal

coatings and causing the "selenoid" plunger to become stuck in the "picked"

position.  Removing power from this relay will not open the contacts and the

real excitement begins when the pump breaker is reset.  Of course, moving the

gear switch to the other position is only an attempt to run the motor in both

directions at once.  Yikes!



The solution (after replacing the screwed up relay) to prevent this is to

cascade the 5 amp relay power breaker (or fuse) off of the 50 amp pump breaker

so that when you pull the pump breaker, no power is available for the relays

either.



2. Locate the 50 amp pump breaker where it is easy to get at -- not on the

baggage bulkhead.  It may be desireable to stop the pump quickly whilst flying

solo and reaching behind is not safe (remember John Denver).  Since my battery

is behind the co-pilot seat, it was not much more wire length to put the pump

breaker just forward of the glove box cover, on the top of the between seat

console.  Because I have second-generation pressure switches, sometimes the

pump will run too long on a gear-up request and pulling the breaker for a few

seconds helps the pressure switch settle down.



3.  If the wire to the "gear transit" light were shorted, it would surely burn

before the 50 amp breaker popped.  One should, for safety's sake, put

isolation diodes at the indicate (I) position of the pump relays and an in-

line fuse on that wire.



Scott Krueger

N92EX