B9200 kubota problem pushing.

[New old tractor guy]

I’m new to the tractor world and just purchased a used B9200 kubota. Engine runs great, bucket works pretty well for a 20+ year tractor but the wheels lose power when I try to scoop a bucket of sand or dirt or anything really. Thought it may be fluid but they all look decent. It runs better after warming up but still the power to the wheels stops when pushing. The guy I bought it from told me it needed a high pressure pump or valve or something for other implements to operate. Any advice would be much appreciated. Thanks

 

[npalen]

What gear range are you using when trying to fill the bucket? You may be overpowering the hydrostat relief valve by not gearing "down".

 

[New old tractor guy]

I’ve tried all 3 gear speeds. If the throttle is all the way up it’s even worse. The middle gear and mid throttle seems to be the most effective combo

 

[MTGreen]

I would check your rear tire pressure. Some of the smaller tractor wheels don't get enough grip on the tire when the pressure is too low and they will spin but the tires won't and you can't see what's going on from the driver's seat

 

[LD1]

Have you ever ran a hydrostat?

They are backwards to conventional thinking. Meaning you have to let OFF the HST pedal for more power. IF its lugging/bogging down, and you are trying to give it MORE pedal thinking you are trying to give it MORE power, that is the opposite of what you need to do

 

[New old tractor guy]

Have you ever ran a hydrostat?

They are backwards to conventional thinking. Meaning you have to let OFF the HST pedal for more power. IF its lugging/bogging down, and you are trying to give it MORE pedal thinking you are trying to give it MORE power, that is the opposite of what you need to do

Yes I’ve run hydrostat. This is the first piece of equipment I’ve owned but probably a couple hundred hours over the years. It’s not bogging down so much as the wheels just stop spinning. When I let off it allows me to push forward again. Sometimes I have to back up a hair and then I can go forward again. It feels like something is disengaging for a moment. It doesn’t rev higher or make any unusual noises though like a clutch would. The bucket runs fine during this too. The previous owner told me it needs a high pressure valve but it was for other implements. I’m starting to think that high pressure valve does more than he thought?

 

[New old tractor guy]

I would check your rear tire pressure. Some of the smaller tractor wheels don't get enough grip on the tire when the pressure is too low and they will spin but the tires won't and you can't see what's going on from the driver's seat

The tire pressure is good. The tractor wheels just stop moving

 

[npalen]

A video of the above would be helpful.

 

[rScotty]

I wish I knew more about HST transmissions because it sure sounds to me like that is where the problem is. There is a high pressure pump in there, and it is a major component.

Lots of us old mechanics know standard gear transmissions quite well, but not much about HST even though they have become very popular. In spite of their complexity and requirement for nothing but the purest trans/hydraulic fluid, HSTs have been very reliable. Argueably more so than a clutch and manual setup.

But it sounds like yours either needs a replacement cable or needs an internal rebuild. Here is what happens normally: As you push down on the HST pedal you are allowing a "wobble plate" to tilt which in effect changes (raises) the gearing by allowing a high pressure pump to send more fluid to the drive motor. If that wobble plate moves - but the system doesn't have have the high pressure fluid available - then it is as though you shifted to a higher gear and let off on the power at the same time.

But it might not be the high pressure pump at all. It could just be the cable that moves the wobble plate. That cable goes from the HST pedal to where it wraps around a drum that turns a shaft that moves the wobble plate. I broke that cable on mine - which is how I know about it. A one hour fix. You can see the cable and see if it is doing its job by crawling under the tractor.

On most HST trannys there are ports on the transmission housing for measuring HST pressure. If it isn't the cable, then making pressure measurements is where I would start if it were mine, I would get a real good OEM shop manual for that model, make up some pressure guages (cheap to do) and make some HST pressure measurements. Of course a dealership mechanic might be able to do that too, but chances are they've never rebuilt a HST. Today they just replace them. And it may come to that as well.

Good luck,
rScotty

 

[MTGreen]

I wish I knew more about HST transmissions because it sure sounds to me like that is where the problem is. There is a high pressure pump in there, and it is a major component.

Lots of us old mechanics know standard gear transmissions quite well, but not much about HST even though they have become very popular. In spite of their complexity and requirement for nothing but the purest trans/hydraulic fluid, HSTs have been very reliable. Argueably more so than a clutch and manual setup.

But it sounds like yours either needs a replacement cable or needs an internal rebuild. Here is what happens normally: As you push down on the HST pedal you are allowing a "wobble plate" to tilt which in effect changes (raises) the gearing by allowing a high pressure pump to send more fluid to the drive motor. If that wobble plate moves - but the system doesn't have have the high pressure fluid available - then it is as though you shifted to a higher gear and let off on the power at the same time.

But it might not be the high pressure pump at all. It could just be the cable that moves the wobble plate. That cable goes from the HST pedal to where it wraps around a drum that turns a shaft that moves the wobble plate. I broke that cable on mine - which is how I know about it. A one hour fix. You can see the cable and see if it is doing its job by crawling under the tractor.

On most HST trannys there are ports on the transmission housing for measuring HST pressure. If it isn't the cable, then making pressure measurements is where I would start if it were mine, I would get a real good OEM shop manual for that model, make up some pressure guages (cheap to do) and make some HST pressure measurements. Of course a dealership mechanic might be able to do that too, but chances are they've never rebuilt a HST. Today they just replace them. And it may come to that as well.

Good luck,
rScotty

My understanding from tearing into them in the past is that the wobble plate is part of the high pressure drive pump. The wobble plate is a disc that has a number of pistons riding round and round on one face. When that wobble plate is flat and perpendicular to those pistons, no fluid goes in and out of the pistons and so no fluid goes to your hydraulic drive motors. This is the neutral position where the tractor doesn't move. When you move your treadle or pedal, it angles that wobble plate. The angle makes those pistons riding around and around on the face effectively move in and out as they go around which pumps fluid in and out. The greater the angle the greater the amount of fluid that goes in and out and the faster your motors move. When you push your reverse treadle or pedal, the wobble plate angles back the opposite direction which reverses flow from your pistons. So it is effectively a variable high pressure pump that goes to your hydraulic drive motors internally in the hydrostat.
Here's a short YouTube video showing visually how it works:

Here's an example torque curve for one of those hydraulic motors. The graph reads torque on the y-axis and rotational speed on the x-axis. You can see that at high RPM the torque drops very low and that there is an optimum torque output somewhere around 2/3-3/4 of the max speed give or take. So this is why backing off a bit when you need more torque makes sense

There are two independent pumps on a tractor type hydrostat that are both driven by the engine input: the one described above for the drive motor then a second one with a fixed wobble plate angle that provides high pressure fluid to your three-point, steering, auxiliaries, etc.

So from a less theoretical and more practical standpoint, it is possible to have one pump go bad without the other; as it is possible to have a linkage problem. The linkages can be lever or cable type but they are complicated mechanical assemblies. I would start by taking off whatever covers you need to to visualize that linkage assembly and with the tractor off manually actuate your treadle or pedals and look closely at that assembly.
If that assembly is functioning like it's supposed to, then start checking pressures with an oem manual as suggested above