Per--



Chris Zavaston is the expert on these old-style pumps so he can correct me if I'm wrong. When the gear retract, more fluid is returning to the reservoir than is being pumped to the hydraulic cylinders because of the volume taken up by the cylinder rods. On the old style pumps without the "back pressure circuit", the only way that excess fluid can return to the reservoir is through the low-pressure relief valve, usually set at ~800PSI (5516 KPa). Therefore, when the gear are retracting, the hydraulic cylinders see ~1200 PSI (8274 (KPa) on the "UP" side and ~800 PSI on the "DOWN" side. In other words, the down side is working against the up side so the high pressure builds up enough to open the "UP" pressure switch until the "DOWN" pressure bleeds off, at which time the pump starts up again and the cycle continues.



I have an old-style pump and it does the same thing...sometimes. It seems to work fine when the fluid is cold, but hesitates when the fluid warms up. I think that increasing the setpoint of the high pressure switch will help, but you have to be careful not to set it higher than the high pressure relief valve or the pump will never go off.



Hopefully Chris Zavaston will add his recommendations here.



--John



On 7/10/2014 3:43 PM, Per Kristensen wrote:

Yes sir ..starts very short cycling every 10 - 15 second when gear are retract,  just after takeoff. It goes on in about 15 minutes, then it stops and everything's OK..