Hydraulic Lawnmower Pump/Motor Unit Rebuild

My slightly pre-2010 Swisher 60" mower, with about 310 hours, has been experiencing poor performance from the hydraulic drive units, so I decided to do something about it.    The mower advertises, "Driven by Eaton" but I was unable to find any identifying marks on the unit's cast case, maybe for a reason, though internet research points to them in fact being made by Eaton.  My mower has a model 778014 pump and motor unit on the left hand side and a 778046 on the right, though both units appear to be identical once removed from the mower, only the removable fluid reservoir differentiates them (and which boss is tapped for the bypass valve lever).

RHS unit

LHS unit

The units had a few symptoms that there were problems.  The top speed had reduced over time.  The units would "load-up": after going on flat ground at constant speed, the mower would come to a stop with the power levers still on WFO, only to start going again after a second or two.  Lastly, the units didn't like to climb hills.  They'd completely stall and if the levers were left at WFO, they'd get some nasty power pulses, but if they were feathered a little, usually the mower would get up the hill.

I first looked up new units, just to see the cost, before I spent my time taking them apart and Sears had them for about $800...way too much for the ole mower.  The cheapest that I found was Amazon in the $500s, linked below.



$500 was still a bit rich so I pulled the two hunks of metal out of the mower, put them on the bench and started ripping them apart.  My first thoughts were that the fluid looked pretty nasty, very thick and always with a silver shine, not good, though it had been the same fluid that the machine had started life with.  I started on the wheel end, which exposed two planetary gear sets stacked on top of each other, probably making about a 10:1 reduction to the wheel, and a clutch system.

Planetary reduction stack and clutch

After that is the hydraulic motor, a radial piston type, bores arranged radially on a rotating "drum" with a ball acting as the piston in each bore that rides on an eccentric race thereby causing them to move in and out of the bore, which in the case of a motor, the pressurized fluid behind each ball with cause the drum to rotate.  http://en.wikipedia.org/wiki/Hydraulic_motor and an image search should be helpful.

Drum and "pistons"

The drum still in the case, the eccentricity can be seen at the bottom

The drum can be seen to the left still attached to the output

Next I pulled apart the pump side to find what appears to be exactly the same design as the motor, dimensionally.  Here of course, the drum is driven and the balls moving in and out of the bores is what pressurizes the fluid.  A big difference from the motor side was that the piston's race is movable, altering the eccentricity, which could reverse the suction and pressure lines, and makes for your reverse mechanism.

The pump drum in the center, surrounded by a movable race

That's about it, I disassembled everything, used a bunch of brake cleaner, and put it all back together.  I didn't replace any hard parts, other than having to cut a new gasket for the motor side, and did not find anything in the unit that gave me alarm.  I refilled each unit with ATF and actually Mopar ATF+3 at that because I already had some on the shelf, and stuffed them back into the mower.

So how is the mower working now?  I still have not done much besides some test cruises, but I'd say damn near brand new!  Hell yeah!  The top speed is back to normal, which I had a hard time remembering, the stalling problem is gone, and I haven't tried to climb any hill yet, but doing a zero radius turn on concrete, the reversing tire can be made to squeel!  All of the problems that I'd experienced with the units just came down to the fluid.