Kubota L48 HST issue

[D100jj4]

I have an L48 kubota.
Im having issues with charge pressure.
Charge pressure spec is 370 to 400.
Im only making 100 psi and after running it for maybe 20 mins pressure falls to nothing over the course of me using it. So I changed the oil and filter.
I shimmed up the relief valve and pressure came to 220 and falls to about 150.
220 is as high as I could get it. And pressure doesnt go below 150. Clutch is good. No slippage.
Im guessing either charge pump or hystat pumps.
Not sure what's going on. Does anyone have any ideas?
Tractor works somewhat decent until pressure falls.
So maybe hystat pumps are okay?

 

[100 td]

Probably not it but........You've cleaned the screen as well? Top left. 030 out of picture

 

[rbargeron]

Hi - Can you comment the symptoms that led to testing charge pressure? How many hours on the machine? How many owners?

EDIT: Just looked at your other posts - it's good to keep adding posts to the same discussion until an issue is resolved - hopefully

A L48 with 2000 hours should not be excessively worn unless it has had a very hard life - odds are your HST's main parts are ok.

The charge pump is an internal gear-type pump that supplies makeup oil to keep the main HST pump/motor circuit full. It enables the main pump/motor to move the machine. Machine rolling movement is independent of other hydraulic functions. (that have 3 more pumps - only share the reservoir).

The charge pump gets its supply from the power steering exhaust side return line. If raising the charge relief setting raises the pressure, the inlet screen (030) is not likely restricting flow. Right next to the charge relief valve there's a second relief valve to protect the filter - try raising that one too. If either relief valve is weak/leaking the charge pressure would drop. (see .pdf) Good luck, Dick B

 

[D100jj4]

I cant run machine for very long. It would just stop moving.
This winter i could use longer in colder weather.
So i started checking pressures. That's when i found charge pressure extremely low. I have already shimmed up filter relief and pressure will jot go above 220.
After running unit for 20 mins pressure would gradually fall to about 150 and slowly start to loose power. Unit would slowly move.
Unit has 2000 hrs. Not sure how many owners.
Do you still not think 030 screen is restricted?

 

[rbargeron]

I cant run machine for very long. It would just stop moving.
This winter i could use longer in colder weather.
So i started checking pressures. That's when i found charge pressure extremely low. I have already shimmed up filter relief and pressure will jot go above 220.
After running unit for 20 mins pressure would gradually fall to about 150 and slowly start to loose power. Unit would slowly move.
Unit has 2000 hrs. Not sure how many owners.
Do you still not think 030 screen is restricted?

Please read my edited post above (hit refresh on your browser in case the old version is being cached by your ISP)
Check the second relief valve next to the charge pressure relief.

 

[D100jj4]

Ive checked the filter relief which next to the charge relief. Its shimmed up and did not change pressure.
I also changed filter again. Im going to cut open filter and see if I find anything!!
Any thoughts?

 

[rbargeron]

.......Unit has 2000 hrs. Not sure how many owners.
Do you still not think 030 screen is restricted?

My thought was that if the pump's oil supply was being restricted, shimming the relief valve wouldn't have helped. But we're running out of things to check from the outside. Wouldn't hurt to take off the pipe and see what's in it.

 

[100 td]

My thought was that if the pump's oil supply was being restricted, shimming the relief valve wouldn't have helped.

By adjusting the relief valve, it raised the line pressure. Logic says, therefore some fluid must have been passing the relief, and now less is passing. So is the relief still passing when it shouldn't be? Is there a bit of crap stuck in the relief valve holding it open? May be worth removing the valve for inspection?

Is the filter screen, number 16 on your flow diagram?

 

[100 td]

100 td - scroll down on the flow diagram for the parts key - 16 is the filter

So where is the FILTER-Screen FILTER 54516-36560 if 16 is the Hydraulic Filter FILTER,OIL TA240-59900, in the flow circuit?

 

[rbargeron]

Ive checked the filter relief which next to the charge relief. Its shimmed up and did not change pressure.
I also changed filter again. Im going to cut open filter and see if I find anything!!
Any thoughts?

Are you absolutely sure the clutch isn't slipping?
Is there any free play in the pedal?

In your earlier OTT thread there were comments on the importance of the main clutch in making the machine move. You asked how to test it and Wolfman said check the PTO for power. But since this machine has the backhoe mounted, it is not easy to test the pto.

One way to test it with the backhoe in place would be to connect an implement drive shaft to the pto and carefully put a long crowbar thru the first yoke, long enough so it prevents the pto from turning.

Then with the clutch pedal down, start the tractor and let it idle. With the clutch pedal still down, reach over and engage the pto lever. Then slowly let up the clutch pedal and see if it will load the engine and stall it. If it slips, you've likely found the problem.

 

[rbargeron]

So where is the FILTER-Screen FILTER 54516-36560 if 16 is the Hydraulic Filter FILTER,OIL TA240-59900, in the flow circuit?

They both appear on the parts page in post #2 (your post) - a full copy including part numbers is attached below.

I'm circling back to whether the main clutch might be the problem. (see post #10)

 

[D100jj4]

I want you thank everyone for all this help first hand.

I disconnect the backhoe and put a bush hog on it.
Blade didn't slow down with machine speed. So I would say the clutch is okay.
But I like what you mentioned. I might try putting a bar in shaft and see if engine bogs down or dies. I will keep you all posted.

 

[D100jj4]

When I originally changed the fluid I took both charge and filter relief out and inspected.
I found no issues or debris in valves. All looks good. Once I achieved 220 pdi I added more shims and didn't not change. 220 is as high as I could get. But to me if suction screen is blocked then maybe after its run for a bit then pressure would fall due to inadequate oil flow. Starving charge pump to make correct pressure.
What do you think. So the 030 tube is the Hst suction tube for charge pump? Is that right?

 

[North Idaho Wolfman]

Did you work the brush hog or just let it spin?

I say this because if you just let it spin the inertia will let it spin even when there is no Input power (like a slipping clutch).

The bar method can work good for eliminating that event.

Take it out run it around and get it hot, then do the test.

We are pushing this point because an HST going out on an L is very very rare!

 

[D100jj4]

I had the bush hog down as far it would go.
My thick field wound slow it down if I disengaged. But I want to try the bar method. Ive got a 6 foot track bar Im going to use.
I do know that these pumps very rarely go bad but i do not know how this unit was used or abused by previous owners my worry would be if they ran it dry.
I will try stalling engine out this weekend.

 

[100 td]

So where is the FILTER-Screen FILTER 54516-36560 in the flow circuit?

They both appear on the parts page in post #2 (your post)

Yes they appear on the parts diagram as I originally posted, but the question I asked is where is the screen in the flow diagram?

The circuit you posted doesn't appear to show a screen in the circuit. Whilst it may not be the cause of the OP's problem, some other tranny circuits have a screen in series with the charge pump inlet. I can only assume the screen would create problems if it was restricted (and it may not be), but if I was investigating any HST problems the first thing I would establish is that all filters and screens are clean or new. Hopefully the OP finds fault with the clutch instead.

 

[rbargeron]

Yes they appear on the parts diagram as I originally posted, but the question I asked is where is the screen in the flow diagram?
The circuit you posted doesn't appear to show a screen in the circuit......

Kubota publishes that same flow diagram in many of its workshop manuals and the screen is not shown in any of them. They may not consider it functionally significant. The pipe carries the exhaust side oil returning to the transmission from the power steering circuit. A portion of the flow is diverted to the charge pump's suction.

Attached is Kubota's HST Troubleshooting page from the L48 workshop manual. Except for the clutch and relief valves most functional problems lead to their recommendation to replace the hydrostatic transmission assembly.

 

[lugbolt]

Split it. Pull the cover off of the charge pump. Bet you'll find the issue there.

OR...

find your old filters and cut them open. Any brass or metal in them? If so, split it and clean the junk out.

 

[100 td]

Kubota must be getting very lazy to not show them, mine is shown in my old manual. As previously stated, whilst it may not be the problem, it's there for a reason. To not clean it and rely on the trouble shooting flow chart alone to replace the transmission would be crazy, but I guess that's what happens eh?

I don't know what that strainer is doing, but I'll have a guess, from the parts diagram, it appears that it's installed in the base of the mid case or mid case spacer, and supplies oil to the HST. As the charge pump is inside the HST, I can't see how exhaust flow from the steering goes directly to the charge pump. If it did, and you had a failure in the steering circuit, you would also ruin your charge pump.

Therefore I assume the strainer and pipe supply oil to the charge pump.
Once again it's probably not the fault, but surely it's worth checking, and I can't understand why it's not a part of the diagnostic.

From the following diagrams, to me as least, it appears that the steering exhaust actually tees into the suction line and plays no part in the charge circuit.

 

[100 td]

Does your manual by chance have a schematic of the circuit?

D100jj4, do you have the skills to split the tractor and pull the HST apart if it came to that or would you pass it to someone to just replace the HST?