Scag Tiger Cub Hydro Pump Air Pocket?
- Thread starter ngrobb01
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Before taking the cover off put a mark on the housing on the side of the charge pump cover that has the square end on it and install in this way when putting back together. You are saying oil is foamy. Will it pull at all? Does sound like you have the charge pump cover on wrong. Try turning it 180 degree and try to purge it again. Don't just take the bolts out and turn it if you do you will destroy the spring on the check ball. You will have to lift the cover straight up and reninstall it.
Planning to do it tomorrow now (at work today). I’m fairly confident I have the charge pump on backwards on the left pump (which is the problem pump) after reviewing pictures. I’m optimistic right now that this is the reason I’m having trouble with it all.
Here is a pic after reinstallation. The indentation on the charge cover is opposite but the ports for each pump are oriented in the same space. To me that would say they’re opposite of what they should be, and the picture I have of the left pump prior to disassembly to replace seals shows that the indentation should be on the opposite side and both pumps would look identical once installed. I mistakenly took my “before” picture before pulling off the metal bar that supports the seat and holds the interlock switch for the seat. First time doing this, learned a lot already lol
ILENGINE
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I think we are on the correct track. Need to turn the pump cover around on that one pump so both cover indents face the same direction. The indent will face the same direction because the pumps themselves are flow opposites
I haven’t seen any type if diagram that shows the flow path of the hydro fluid? That might help me understand how/why it works the way it does for any future problems. The manuals seem to be lacking some good information from what I’ve seen
ILENGINE
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definitely lacks information as far as charge pump orientation. Just says to make the pump cover and the pump housing prior to disassembly so you get them reinstalled in the correct direction. Would be nice if they had a graph for the pump units. There is one for the integrated units for which way the cover goes.
Some information on hydrostat systems. The charge pump which is under the football cover pumps oil into the piston pump. That piston pump then pumps that oil to the wheel motor. The oil from the wheel motor gets returned to the piston pump and is then pumped back to the wheel motor. So Think of the hydro to wheel motor as a closed loop system which for the most part is. Now some oil gets lost in the piston pumping phase which flows into the pump housing and is returned to the oil tank. As the oil is lost from the piston pump the charge pump gets oil from the oil tank through the filter to replace what oil the piston pump is loosing. So if the system is lets say 90% efficient then 90% of the oil loops between the piston pump and the wheel motor and is unfiltered, and 10% gets lost and returned to the tank to be filtered.
So in the case of your hydro pumps the charge pump and top plate have the same oil flow pattern for both pumps, but your control lever on the outside of the pumps that controls direction work in opposite directions. Your right hydro when the stick is moved forward tips the piston pump swath plate down in the front, but the left lever moves the swath plate up in the front. Which in turn causes the oil to flow in opposite directions coming out of the pump units.
Some information on hydrostat systems. The charge pump which is under the football cover pumps oil into the piston pump. That piston pump then pumps that oil to the wheel motor. The oil from the wheel motor gets returned to the piston pump and is then pumped back to the wheel motor. So Think of the hydro to wheel motor as a closed loop system which for the most part is. Now some oil gets lost in the piston pumping phase which flows into the pump housing and is returned to the oil tank. As the oil is lost from the piston pump the charge pump gets oil from the oil tank through the filter to replace what oil the piston pump is loosing. So if the system is lets say 90% efficient then 90% of the oil loops between the piston pump and the wheel motor and is unfiltered, and 10% gets lost and returned to the tank to be filtered.
So in the case of your hydro pumps the charge pump and top plate have the same oil flow pattern for both pumps, but your control lever on the outside of the pumps that controls direction work in opposite directions. Your right hydro when the stick is moved forward tips the piston pump swath plate down in the front, but the left lever moves the swath plate up in the front. Which in turn causes the oil to flow in opposite directions coming out of the pump units.
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This is why the SM say to scribe index marks before removing the caps. Also while your are at removing the cap check that the charge spring and ball are in place. Plus these plastic balls do wear and need replacing during rebuilds.
Rotating the charge port 180 degrees on the left pump seems to be the fix. Both wheels are turning and the hydraulic oil is no longer frothy like there is entrapped air. I need to adjust the control arm linkage as the “neutral” position pushes the right wheel in reverse and the left arm forward now, but the steering feels much tighter and controlled than before. The pumps sound whinier than I remember. Would it be worth going through the purge process a few times just to make sure there’s not more air in the system somewhere? Both wheels turn even with the bypass valves open as it sits right now.