Wayne Cook wrote:

On Mon, 27 Aug 2007 15:07:09 -0000, "Lloyd E. Sponenburgh"
lloydspinsidemindspring.com wrote:

Ignoramus19946 fired this volley in
m:

There is a local opportunity to buy a used, name brand tow behind
generator that is not working as follows: the engine (4 cyl. White
diesel) is running fine, the generator is not making any output.

I was gifted a nice but non-functional JD estate tractor. 32HP diesel
with all hydrostatic stuff.

The engine runs fine, although I think it's going to need an injector
pump rebuild... stumbles a bit at high speed.

BUT... although every hydraulic accessory on it works perfectly - lift,
blade PTO, accessory PTO, everything - the traction drive will turn the
wheels under no load, but won't move the tractor. You get a little minor
"oommph!" out of it when you attempt to drive, but it won't move in
either forward or reverse.

I noticed that someone had butchered the (very expensive) "neutral"
valves because they were leaking. Thinking this was the problem, I
removed both, replaced o-rings, etc, and _think_ I've made sure the
valves are in the "closed" position (basically welded shut).

Anyone seen this problem? It seems like the pressure is being bypassed
around the traction motor somehow... and the neutral valves still seem to
me like they'd be at the heart of it.

Very Very common problem unfortunately. The newer John Deere
hydrostats are bad this way. They don't have a oil filter and people
don't change the oil often enough to prevent this.

First off a lesson in hydrostatic drives.

First step is that the oil from the axle is picked up and pumped to
about 500 psi by the lift pump (a gearotor type pump on the older
units). This pump provides the power for the accessories and provides
oil to the main hydrostatic drive pump. (thus the reason you've got
power).

Next step the oil is pumped by a variable displacement pump. This is
likely where your problem is. The variable displacement pump looks
kind of like a the cylinder in a revolver. The cylinder revolves
against a valve plate on the front end. It has pistons that are lapped
to fit it that free float in the cylinder (the reason for the charge
pump is to push the pistons back). The back end of the pistons have
feet kind of like what's on the end of the screw of a C-clamp. These
feet ride on a "swash plate". The swash plate is controlled by the
"gear shift" so the when you're in neutral it's flat and the pistons
don't move. When you want to go forward you tilt the swash plate one
way and for reverse you tilt it the other way.

The usual failure mode of these pumps is that the face of the
cylinder and the valve plate get grooves cut in them from dirt in the
fluid. Once this happens the oil leaks past and the pump can't build
up any pressure.

From everything that I've heard there are no replacement parts
available for the newer John Deere pumps. I was able to get a new
valve plate and I lapped the face of the cylinder on my old (pre 1970)
John Deere hydrostatic mower. But that was about 18 years ago when I
did it. Since this is a metalworking group it's likely that you've got
the skills and equipment to repair the valve plate and cylinder (I
didn't have any machines except a drill press back when I repaired my
old mower).

From what you're saying I think that you're hydraulic motor is still
in good shape (common).

JD builds hydraulic systems without filters???!!! My opinion of JD just
fell precipitously