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Scott, I've had some hydraulic problems too and maybe can offer some
thoughts.
Since I have panel mounted hydraulic gages for both up and down circuits,
you can imagine how much easier debug is. I've noticed that both factory
pressure switches have a more than adequate dead zone. The up pressure
switch shuts off the pump at about 1200 psi and kicks it back on at 800.
The down switch is about 700 and 500 psi respectively. With the spring
weather, I noticed that my down pressure (while on the ground) was bleeding
off more rapidly than during the winter causing the pump to come on every 5
minutes or so. This is bad, not just because of the nuisance but because
that pressure is bleeding across to the high pressure side and does not
usually dump back to the reservoir. On each burp, the pressure is bleeding
over to the up side until the pressure in both circuits is over 500 psi and
the pump no longer trips on. When you take off, you should still be able to
raise the gear because the hi pressure switch is looking for at least 800
psi before the pump is turned off. The trouble occurs when there is some
warming as the day gets hot and your pressure climbs. It can easily climb
over 800 and then your gear will not come up on take off.
Since I was getting bleed across from low side to high side, but not vice
versa (hours in the air without a burp) I figured and hoped that the problem
was not a cylinder nor the cross over valve, but something in the pump. The
pump was sent in, a valve was found to be reversed and corrected and sure
enough, the behavior changed. Now, the low side did not bleed down at all,
but the high side would slowly bleed down. That valve is suspect! A few
days later, I found that both the low side and the high side were bleeding
down more rapidly (burping every 5 minutes). The next preflight, when I
cycled the master, the pump would cycle every 5 seconds and the hydraulic
gage bounced around in saw tooth fashion. Fortunately, I glanced at the
cross over valve and saw that it was out of position. I had bumped it while
positioning and removing a tool bag in that foot well for a trip. When I
reset the handle, it was fine. My down circuit is rock solid and my up
circuit will bleed down causing a burp in 30 minutes or so. I can live with
that. I also noticed the same intercom noise one second before the pump
kicks on.
If you can get some temporary gages spliced into your system for debug, it
would be worth the trouble.
Ed de Chazal, LNC2
Rochester, Michigan
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Please send your photos and drawings to marvkaye@olsusa.com.
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