Kubota ZD21 hydro rebuild

[fatjay]

Hopefully cracking the case open tonight. I have to take the fender off, fuel tank, left axle, and then I should be able to get to the hydro. Problem is, messick's is listing two models, C45702 and C45703, with very subtle differences. Any idea how I know which one I have? If it's a model year change, then probably the first one as it's a 2001, but I'm looking to be sure.

Also there's no listing of a gasket, only an oring. Is that right?

Left side only goes forward, no reverse, so hopefully there's not to much damage. I'm anxious to get this thing up and running.

 

[fatjay]

Got the fender and gas tank off. Overall it's coming apart pretty easily. Tomorrow I hope to have the axle and the hydro off, so I can split it open and see what went wrong.

 

[fatjay]

Got it all off and the hydro split. It seems like there was serious cavitation going on.

Notice one is worn more than the other:

And that is a pretty big groove.

 

[KWentling]

Can't help you with your questions, but I also have a ZD21 (2004) so I'm interested. How many hours on yours? I assume this was gradually getting weak in reverse before
it gave up entirely? To me that looks like metal rubbing on metal. How can you tell that it is cavitation damage? I have some hydraulic pump experience but have never been
into a hydrostatic transmission before. More pictures would be great!

 

[fatjay]

I bought the machine like that for $500, so if I invest a couple more bucks it's still well worth it.

About 2% of the time reverse would work fine, the rest it felt like it wanted to, but ultimately didn't. The issue comes from not enough of a hydraulic reservoir, coupled with driving sideways on hills, high side gets no oil. Metal on metal is completely fine, as long as it's completely submerged in hydraulic fluid. When air is introduced, the metal on metal action without lubrication wears down fast.

There is a patch kit for adding a reservoir to the zd21, which runs about $200. It plugs into the dipstick and adds an auxiliary tank in the wheel well, so when it wants to suck air, it instead sucks hydraulic fluid, thus preventing the cavitation.

I'll get more pictures when I get it back together. My hands were oily as **** when I took it apart, lucky I got the pictures I did.

 

[KWentling]

I was aware of the hydraulic reservoir/sidehill issue, and the additional reservoir mod. I asked the Kubota dealer who sold to me about that and he didn't think adding it was necessary. I do some barrow ditch mowing and I still wonder about it.

Any idea why only reverse was affected? I'm aware that those run "metal to metal", but that side obviously lost its oil film. Once the smooth surface was lost, no amount of oil will prevent the metal on metal contact, so the wear continues. It does look like the other side is in the early stages of the same problem.

Do you have the service manual for this? If so, is it detailed on this kind of repair? Kind of disappointed in the manual for my BX, so I haven't got the ZD manual.

 

[fatjay]

I have the service manual, it doesn't really go into details though.

Forward occasionally didn't work, but it was seldom. I actually considered leaving it as left no reverse, but I knew something was wrong and couldn't in good conscious leave it.

The motor is on a 45 degree angle, this was the high side that was damaged. Also the swash plate pivots in that same 45 degree angle. Reverse pivots it down, which would meant he hydraulic fluid would have to flow top side, where-as if it forward, the hydraulic fluid would flow in the bottom (of the top barrel). The pistons go into the barrel but are spring loaded. That means if the swash plate is down, it pushes half of hte pistons in and half out, making fluid flow in a forward direction. If the swash plate is up, it reverses the flow. I think.

Any way, it stands to reason that if you were mowing on a steep enough grade(the previous owner did, those hills were steep), you would cause the top barrel of the high side to run dry easily. Those barrels are moving at a good speed to generate that kind of torque, with regular mowing I'm pretty sure it wouldn't take long.

 

[KWentling]

Correct me I'm wrong, (I need to study up on how these work) The damaged side with the swash plate hydraulically drives the undamaged side, which drives the wheel?

I understand now with your explanation why the one side gets worn. Thanks for taking the time to share this.

 

[fatjay]

The swash plate is the high side. There's two pistons, high and low. The lower piston drives the driveshaft in either direction. and the output shaft attaches to the rear axel case, which has some more gears and stuff, before ultimately having another shaft that outputs to the wheel. The rear axle case steps the speed down dramatically, so the hydraulic motor has to be moving pretty fast, and if dry even for a short period, would pretty much instantly cause damage.

The added reservoir pipes in through the dipstick, allowing you to completely fill the case, instead of only up to the dipstick line. I'm fairly certain that my cap for the hydraulic fluid is faulty too, that thing is incredibly loose.

 

[fatjay]

Good news! Someone is able to get just the transmission cover! Bad news, it's $600. Still a lot easier than $1200 for a new transmission. I just need to sit in my garage and make sure that it's what I need.