<<I don't know if the problem is from a spool valve, this is the second pump I've had in my plane and both have done the same thing in warmer weather. And, of course, being in southern California it's usually always on the warm side.

I can see on the gear pressure gauge that as the heat goes up so does the pressure. It's gets to a point where the pressure opens both pressure switches, so when I switch to gear down there's no connection. One small tap on the dump valve and it releases the excess pressure on the low side and the pump lites up.>>

Randy,

The 'gear won't go down problem' is different than the 'gear won't go up' in a number of ways.  Many discussions prior to this thread have dealt with the gear not extending.  I thought we were discussing the gear not retracting, but your comments above refer to the gear not extending.  The system is not symmetrical so problems in one direction will not replicate in the other.  The spool valve only opens a low resistance path for the down circuit and gravity is a factor that affects the two directions differently.  In the down direction gravity actually hurts us and can be the cause of failures to extend.  Gravity acting on all the undercarriage acts as a bank of spring loaded piston pumps that can shove the spool valve back in the wrong direction just long enough to trip the low pressure switch thus shutting down the real pump.

 

In the case you just described with pressure in both circuits, is there pressure on both sides prior to selecting down or just after you select down.  The situation I described above occurs just after the gear is selected down.  Oftentimes it is perceived as just a pump hesitation and gear comes out on its own and occasionally everything goes quite.  As I recall there was some unique history to your pump(s).  With the current design (and the correct spool valve orientation) when the high side is pressurized it opens a low resistance path from the low side to the reservoir.  The low side simply cannot maintain pressure.  (see Parker schematic for details) This functionality of the spool valve is not needed to get the gear up and down, but it does allow retraction with less work on the part of the pump and more importantly for us it prevents both sides from pressurizing while the gear is retracted.

 

 

<<I don't understand this analysis.   If the system pressure is high enough to hold both pressure switches open, how would opening the dump valve release the excess pressure?>>

 

Bob,

In the failure to extend case with pressure on both sides, opening the dump valve allows gravity to relieve pressure by removing rod volume from the hydraulic circuit.  It wouldn't work if our cylinders needed to retract to get the gear out.  In that case we would need to open the stuck circuit to the atmosphere or reservoir

Chris