Noisy HST with power loss.

pc1383 · Feb 17, 2017

My ST30 (30hp UK model with front bucket and rear 3 point lift) used to have plenty of power and I could buzz round the roads and fields in 3rd gear without any problems. The only time it "moaned" at me was if I tried to lift something too heavy with the front bucket.
Now it "moans" all the time and I need 1st gear to get up even the slightest incline. It will only do 2nd gear on a downhill slope and power feels like only 5-10hp instead of 30hp.
The engine runs fine and has had a full oil and filter service. Both the bucket and 3 point lift seem fine, (other than a slight judder when lifting for the first couple of minutes of running in the morning).
I was told that it could be a failed front gearbox seal, where the drive shaft enters the gearbox. This would suck air in and cause the low power and "moaning" noise.
(This does make sense as air in hydraulics always causes some power loss).
I split the tractor, changed the hydraulic oil and replaced the seal but no joy.
I have since added Lucas Hydraulic Oil Booster and Stop Leak, without any obvious improvement.
I am running out of ideas guys and need it to be right before haymaking.

rbargeron · Feb 17, 2017

How many hours on the machine? Regular hydraulic oil & filter changes? Was the drained hydraulic oil clean? Was the level ok? HST failures are rare but they do happen if hyd fluid gets dirty. As the HST filter gets plugged its relief valve could start to sing. I'd try a new HST filter first.

The HST gets oil from the reservoir by a submerged charge pump - if the suction side of the charge pump had a bad seal, low-pressure oil should be drawn in - not air, provided the oil level is right.

Good luck, Dick B

PW1967 · Feb 18, 2017

A complete hyd. system pressure check may be in order to further diagnose hst problems, hopefully In your area you have a dealer, or good mechanic with tools that can perform that test, I'm not familier with your model, but had similar issues with a b2150, If your prepared with the proper fittings and gauges and a good wsm, you could perform the test your self, other than that be prepared for a few hours of shop rate,

With hyd, oil and filter change and no improvement, have you checked all your fittings, especially on the suction side? they could be a little loose and not show signs of leakage but enough to introduce air in system. After running tractor for a while what does your hyd. oil look like?

pc1383 · Feb 20, 2017

Thanks for getting back to me rbargeron...
I did a full HST fluid and filter change when I split the tractor. The old fluid was a bit dark, but otherwise not too bad. This is probably because it was regularly topped-up to keep pace with the inevitable connect/disconnect hose losses and the odd drip leak here and there. Although I have had it about 5 years, it only does low annual hours (200) and so this is my first fluid and filter change. It now stands at 2200 hours. The new fluid and filter made no difference unfortunately.
When I had the gearbox apart, it had 2 pumps side by side... one for the wheels and one for the hydraulics. Each pump takes the form of 7 small pistons (like shotgun cartridges) in a ring. These act directly onto smooth metal plates at either end of the pistons and have no seals as such. As the rings of small pistons rotate, they act against a "wobble" plate, which causes each small piston to pump once per rotation. Thanks.

pc1383 · Feb 20, 2017

Thanks for getting back to me PW1967, I have a nasty feeling that a dealer check may be my only option left. I will check over the fittings once again, but am pretty sure they are tight. Whatever the problem, the symptoms are exactly the same as before the stip-down, (moaning noise and low power), fluid and filter change, so suspect it is something else.
I am not sure exactly what the oil should look like after some use, but would say it was "cloudy"... like an oil/water emulsion. Thanks.

rentthis · Feb 20, 2017

Could you have a quick disconnect fitting that isn't completely connected? That causes all kinds of havoc.

rbargeron · Feb 20, 2017

pc1383 said:
........When I had the gearbox apart, it had 2 pumps side by side... one for the wheels and one for the hydraulics. Each pump takes the form of 7 small pistons (like shotgun cartridges) in a ring. These act directly onto smooth metal plates at either end of the pistons and have no seals as such. As the rings of small pistons rotate, they act against a "wobble" plate, which causes each small piston to pump once per rotation. Thanks.

The side by side units you were looking at are the heart of the HST. One acts as a pump, the other as a motor. (another pump on the engine drives the other hydraulics).

The HST pump/motor pair stay in step with each other in a closed oil circuit. The clearances everywhere are very tight - with any internal leakage replenished by a charge pump. The brass valve plates at the back end of the pump/motor pair form a metal-to-metal moving seal surface that rides an oil film. Microscopic scratches and wear on these plates can cause leakage and loss of power.

If your fluid was discolored dark or was water-contaminated, it could have damaged the unit causing decreasing performance. Dark coloration is really microscopic metal dust (abrasive) and water droplets can flash boil (cavitation) causing subtle erosion damage.

A damaged valve plate doesn't look all that different from a good one. They need to be perfectly flat and mirror-smooth. Otherwise the oil film is too thick, the parts hydroplane, and pressure drops.

Now that your unit is back together a pressure test would be an efficient next step. The test covers the lower-pressure (charge pump & filter - around 300-400 psi) and also checks the high-pressure pump, motor, valve plates, relief valves (10x higher pressure).

Replacing HST internal parts gets expensive - Kubota generally recommends replacing the whole unit if the problem is more than just a relief valve (affecting one direction). On the good side, the HST has an excellent reputation for long life - buying a used one may be an option.

Good luck, Dick B

pc1383 · Feb 20, 2017

Hi rentthis, thanks for the reply. I have run the tractor without any hoses attached, but no change. The bucket and 3 point lift all work fine, as does anything attached to the rear hoses. It is only the wheels that appear to lack power and so feel I must discount anything to do with the hoses.

North Idaho Wolfman · Feb 20, 2017

pc1383,
Do you have the WSM (Service manual) for the tractor?
Check you PM's (top right in orange) for a link to download the WSM.

When you got into the HST unit did you pull the clutch and pressure plate?
I have known a few HST's that a bad clutch was not supplying enough power to run the HST unit and when it did it was noisy.
Would not be the first time that a bad clutch was the cause of what some think is a bad HST unit.
Have you run something on the PTO and tried to use the tractor?
If it's go a bad clutch disk or pressure plate it will effect both equally on that model.

rbargeron · Feb 20, 2017

Wolfman makes a great point (sorry I blew right by it).
If the clutch is slipping the HST can't send full torque to the wheels. If your clutch now disengages near the top of pedal travel, try backing off the linkage so there's more free play (about an inch at the pedal).

pc1383 · Feb 21, 2017

Hi guys, interesting thoughts re the clutch... On the ST30, the tractor is in 2 distinct halves. The engine (and clutch) at the front and the gearbox with the hydraulics at the rear. A drive shaft from the clutch to the gearbox is the only physical link and so logically an issue with the clutch should affect everything that runs off the gearbox. The PTO seems to drive the implements as it should, the lifting hydraulics seem to lift OK and the clutch seems the same as it has always done, so I am thinking it must be an issue with something specific to the forward/reverse drive.

pc1383 · Feb 21, 2017

PS Sorry Wolfman, I do have a service manual in hard copy. The link you sent is for the STa-30 which is a different model. Thanks.

pc1383 · Feb 21, 2017

Hi Dick, sorry I did post a reply, but it has disappeared somewhere... Lots of good information there, but I must disagree about the two 7 cylinder pumps. One has a direct link to the forward/reverse pedal and is clearly supplying power to the forward/reverse drive. The other pumps constantly and must supply the various hydraulic services. NB. On the ST30, there is no other hydraulic pump on the engine. In terms of symptoms, when the gearbox is moaning and I am trying to go forward/reverse, there is a definite throbbing coming through the forward/reverse pedal in tune with the moaning. This makes me think even more that it has something to do with the forward/reverse pump unit. Thanks.

torch · Feb 21, 2017

Is there any chance the "moaning" you hear is just the relief valve dumping pressure? That would suggest the pump is working OK, but something else is restricting the flow downstream. Or the relief valve is stuck open.

rbargeron · Feb 21, 2017

pc1383 said:
........ about the two 7 cylinder pumps. One has a direct link to the forward/reverse pedal and is clearly supplying power to the forward/reverse drive.....

Until I looked into it I didn't realize the ST30 is laid out more like some B and BX machines - with the Type 3 hydrostatic transmission. The Type 3 uses a 7-cyl pump and a 7-cyl motor which comprise the variable speed/reversible drive to the gearing and wheels. The shaft from the clutch turns the variable pump, and continues on, driving the pto(s), and a regular hydraulic gear pump for the loader and 3-point, etc. You're right its not on the engine - it's above and to the rear of the left axle.

The reason you can feel the pulsing is that the pedal linkage connects directly to the swash plate, unlike some larger Kubota HSTs that use a servo regulator to position it. Anything happening to the swash plate (like cavitation) telegraphs through the pedal.

The history of tractors like the ST30 includes many models, with some made by Iseki and sold under the AGCO brand. Operating principles of hydrostatic drives are all the same - so pressure tests and performance assessments are similar across brands and models.

There's some related reading here about an ailing B2150 HST that might be useful.

Take care, Dick B

pc1383 · Feb 21, 2017

Hi Torch, thanks for that. it's worth a look. How do I check the relief valve?

pc1383 · Feb 21, 2017

Thanks Dick, much appreciated. Kubota still stock most ST30 parts (at a price), but there aren't many scrappers on the market to glean parts from. Even tractors with 4000+ hours are fetching good money, so I could really use a simple fix (like a stuck relief valve). The ST is as you say like some of the B series, albeit scaled-up. I had a good run through all functions yesterday (PTO, bucket, drive) and the only real issue appears to be lack of power when presented with even the slightest uphill slope. Are you able to advise about checking if the relief valve is sticking open as Torch suggested?

rbargeron · Feb 22, 2017

The Type 3 HST has two high-pressure relief valves for forward and reverse, 80 and 110 on the attached BX25 parts page. Are forward and reverse both weak? If not, swapping the valves should swap the symptom. If its the same in F and R the reliefs are likely not the cause.

There's also a relief valve for charge pressure (coming from the rear gear pump) but if it were stuck open the other hydraulics would be weak too.

Sorry I'm not more help - my limited experience is with the Type 2 HST in my L48. Dick B

pc1383 · Feb 22, 2017

Thanks for that. Forward and reverse do both seem equally weak, so that's a shame.

pc1383 · Feb 23, 2017

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