Progress has been made. The problem with the rhs pump was discovered, and remedied. However, after 20 mins out cutting the lawn - at long last - the main seal blew.

The pump has a main cylinder with five pistons - think of a revolver cylinder, with each chamber taking a spring, and a hollow piston with the closed end out. As the cylinder turns the pistons may get pushed into the cylinder forcing fluid toward the high pressure lines feeding the drive motor.

The pistons each make contact with the main bearing, which resides in the swashplate. The swashplate rotates along a center axis which coincides with the center line of the cylinder. When pushing the control arms forward, the swashplate rotates causing the pistons to be pushed into the cylinder as they climb the ramp, and fall back out (the spring pushes them out) as they reach the down side of the ramp. Imagine the six shooter with springs and pistions in the chambers. Push it down on the desk and the pistons sink into the chambers. Lean it to one side and some of the pistons are pushed into the chambers, while the others extend outward. As the pistons move upwards they push fluid up, and out. As they fall downward they allow return fluid to enter.

On the top of the cylinder is a plate with two crescent holes aligning with the two high pressure lines; one for forward, one for backward. This plate is pinned to the head of the pump and aligned on the axis of the swash plate, so that as the swashplate causes a ramp on one side, that side is the feed. As the plate rotates to the other side, the opposite ramp causes the other line to get the feed.

Now, spin the cylinder and you have the pistons rising and falling as they climb and descend the ramped swashplate. When the swashplate is flat there is no movement of the pistions, no fluid flow, and no drive motion.

The tollerances for the pistons to the cylinder are tight. It turns out that minute particles from the left hand pump main bearing retaining ring destruction did make their way to the right hand pump. When lodged between the pistons and the cylinder these particles prevent the pistons from moving freely. In fact, some got stuck in the compressed position. These pistons provide no pumping action. They get pushed up, and into the cylinder, but don't come back out so never push any fluid. In addition, if they move just a little, from fully compressed, to not quite fully compressed, they leave contact with the main bearing on the down-ramp side, and re-engage contact on the up-ramp side making a chattering sound. This sound will vary depending upon if the pistons are simply slow to respond - not pinned - and work somewhat at slow speed, but not at all at high speed.

Having thoroughly cleaned the pump internals, it functions as designed.


Lessons learned:

- when a pump fails, tear down, or replace, both


On to the main seal, and hopefully discovery of the root cause. The engine case is supposed to run with a slight vacuum. Should I be looking at the breather? Guess that's for another thread.

 

My Laser Z '08 just went out with 1,024 hrs. I thought I had an air leak. The right pump wasn't working. Then it switched to the left which started grinding. Dealer says left pump took out left wheel and will cost over $2K to fix with right side soon to go. So it is a parts mower I guess. Only few hrs on new deck spindles and a good 31 HP vanguard.

 

Thanks for sharing your problems and findings, Dave. I'm no mower hydraulic system expert, but I do get to get my hands dirty here and there when one of ours fails. I really do find hydraulic issues and solutions that others have/had very valuable, as obviously it allows me to use that information if one of ours should have the same issue.

I have taken apart standalone hydraulic pumps before and fixed them. Most of the cause is contamination, as you've pointed out. These are pretty fun, and aside from removing/installing them, I enjoy messing with these.

I've taken apart a few standalone wheel motors, but never had any luck getting them to work and not leak upon reassembly.

By far, my LEAST favorite thing to work on when it comes to mowers, is the all-in-one transaxles that seem to be on every single new zero-turn Exmark that we buy. I've pulled a few apart (luckily just to do a re-seal). Everything about them is a PITA. Removing them is a lot more difficult and time consuming (although not bad by any standards, just a lot longer and "dirtier" than just removing a wheel motor or pump). Also, to me, they are more intimidating due to the fact that you essentially have a wheel motor and a pump in one housing, so there's a lot more parts. Even with the full repair manual from Hydro-Gear, I still get intimidated.

I've found that my biggest mistake, probably due to the fact that I only pull a pump/transaxle apart once every few years, is that I forget to mark the charge pump cap orientation so I usually get irritated when the wheel won't move after reassembly. It's only after I hate myself for 10-20 minutes that I realize that I forgot to mark it, and probably put it on wrong.

 

ya these are the older separate pump from the wheel drive. They said they have issues getting them to reseal when repairing. Of course they could be pulling my finger to make a new sale and the pump is the only thing out. A salesman of another brand said they never see theirs go out. Ya right

 

That's crap, but hilarious and believable. I've probably had 10 of these apart and I've never had an issue. Obviously, not to say it can't happen... but maybe I've been lucky. One of the reasons that the separate pumps are easier to deal with (and reseal, for that matter...) for me at least, are because they have an actual gasket (o-ring) that goes between the pump housings. The all-in-one transaxles are assembled using RTV, which you have to do your best to remove all of without contaminating the unit. Every single one that I've done is a royal PITA to remove the RTV on. Also, I've never had a standalone pump not seal. Once you toss the gasket/o-ring on and mate both housings, tighten it up and it's good.