7790 moves very absurd

[Simplr]

Just bought a 7790 just the other day......... engine runs fine and never drops from given Rpm.. The problem is that when I drive the thing,it begins to slow down to a stop and then kicks back in again......... It pretty much is a Simplicity 900 series with a basic Sundstrand hydraulic pump............ Just changed the hydraulic fluid with fresh hydraulic fluid.......still has same problem?!@#$% But when the tractor goes to stop, when I pull on the hydraulic deck lift handle, the thing then takes offimmediately, does it sound like a filter or something else to you guys? Just need some help fast.........

[KraigTM]

Is the pump belt slipping? Does the clutch pulley have full travel without rubbing or binding? Does the clutch pulley spring have proper tension?

[Sandy_Lake_Imp]

The fact that it takes off when the imp lift is activated makes it sound like a hydro rotating group problem to me. You could check the charge relief first.

[Al]

Hi, The implement lift valve is in series with the case of the hydro and uses the charge pump output for the case. This shared resource is the source of oil for the lift. It sounds like when it gets charge pressure the pump and motor seem to work, these 2 items sound OK to me. I suspect either the charge pump has excessive clearance or something may be afoul in the lift circuit and it is letting pressure leak to the return line. Operating the lever changes the fluid routing and may stop the leakage. This would drop the charge pressure in the case and the main pump would not function because there is low or no pressure forcing oil into the pump pistons on the intake stroke. Good luck, AL