Poor boom/bucket function on Kubota SVL95-2S

[HydJoe]

We have a Kubota track loader here that has sluggish boom/bucket functions while driving. It seems to drive normally, or runs boom/bucket funtions just fine when sitting still. But try to raise the boom or tilt the bucket while driving, and response is very slow. If you then stop, function will resume. Our customer recently purchased it used, so no prior history. The seller had not noticed issues when checking it over, but admitted they might have missed something.
I know Kubota skidloaders have pretty complicated load sense/flow sharing, which makes them nice to operate, but possibly harder to troubleshoot. My first thought here is to watch charge pressure while operating it, assuming that low charge pressure would not be able to shift boom/bucket dcvs. But then why would drive speed not be affected as well? Any thoughts if this is a good place to start, or common causes for this ailment? Is it more likely that the valving/controls that "shares" the available flow is malfunctioning?

 

[nikthewho]

you're right. the SVL95-2S prioritizes drive over implement functions, and your symptom is likely flow-sharing valve or priority valve malfunction.

Kubota uses a travel priority system: When driving, a valve diverts most flow to drive motors, leaving minimal flow for implements unless you're at high RPM or low drive demand
Your flow-sharing valve is likely stuck in full-priority mode or the shuttle/compensator isn't releasing flow back to implements during travel

Not likely charge pressure because Low charge would affect everything equally and cause cavitation noise, Sitting-still operation being normal rules out pump wear or charge issues

Check these:

1. travel priority valve: Located between pump and main valve - test for mechanical sticking or contamination; should modulate based on drive demand 2. S shuttle valve: Selects highest pressure signal; if stuck favoring drive circuit, implements starve 3. Pump compensator response: Could be sluggish adjusting during combined operations

test:
1. Check implement circuit pressure while driving vs. stationary - big drop while moving = priority valve not releasing flow 2. Try driving at different speeds - does implement response improve at lower travel speeds? (confirms flow diversion issue)

Start with the travel priority valve - that's the most common culprit for this exact symptom on SVL series.

 

[Vetech63]

Good grief!

 

[HydJoe]

Good grief!

Curious what's the issue here, @Vetech63 ? Does original post sound too newbie-like? Sorry if so! Or do you see something wrong with post #2? I'm not as familiar with hydraulic systems on equipment as I'd like; most of my experience has been with cylinders/machining.

 

[Vetech63]

Its not you lol

 

[HydJoe]

Thanks for the confirm.... Any other input perchance?

 

[Vetech63]

I'm not familiar with your particular skid loader but most I've run across have the hydrostatic hydraulics and implement hydraulic running on separate pumps.

Do the hydraulics on either the travel or implement seem weak or slow? Is there any unusual noises during hydraulic operation? Does the engine seem to load too much?

 

[HydJoe]

Do the hydraulics on either the travel or implement seem weak or slow? Is there any unusual noises during hydraulic operation? Does the engine seem to load too much?

No, travel and boom functions are responsive when used individually. No unusual noises. Engine load is not excessive, maybe rather not enough.
Yes, the work(boom & bucket) pump is a variable displacement pump, separate from the drive pumps. After putting pressure gauges on the charge pressure line and the work pump pressure line, I'm almost certain there's an issue with the control pressure to the work pump, or with the work pump itself. Charge pressure holds steady with engine rpms, between 750 & 1,000 psi. But sometimes while driving, the work pump just would not generate enough pressure to lift the boom quickly; quitting driving always made it build more pressure and be responsive again.

 

[HydJoe]

you're right. the SVL95-2S prioritizes drive over implement functions, and your symptom is likely flow-sharing valve or priority valve malfunction.

Kubota uses a travel priority system: When driving, a valve diverts most flow to drive motors, leaving minimal flow for implements unless you're at high RPM or low drive demand
Your flow-sharing valve is likely stuck in full-priority mode or the shuttle/compensator isn't releasing flow back to implements during travel

Not likely charge pressure because Low charge would affect everything equally and cause cavitation noise, Sitting-still operation being normal rules out pump wear or charge issues

Check these:

1. travel priority valve: Located between pump and main valve - test for mechanical sticking or contamination; should modulate based on drive demand 2. S shuttle valve: Selects highest pressure signal; if stuck favoring drive circuit, implements starve 3. Pump compensator response: Could be sluggish adjusting during combined operations

test:
1. Check implement circuit pressure while driving vs. stationary - big drop while moving = priority valve not releasing flow 2. Try driving at different speeds - does implement response improve at lower travel speeds? (confirms flow diversion issue)

Start with the travel priority valve - that's the most common culprit for this exact symptom on SVL series.

Where is this valve that diverts most of the charge flow to the drive motors when driving? Is it built into the control for the variable work pump, or built into the loader function control valve? We removed and disassembled the variable work pump control valve, but can't find dirt or other damage.
Since the work pump is a LS system, there is only about 300 psi on it's output unless a function is calling for flow. So how/where would you best test implement circuit pressure while driving vs. stationary, like you mention in test 1?, since the pump is normally destroked? Auxillaries?
I haven't been able to tell that travel speeds affect implement response; it's either sporadically poor while driving at any speed, or works fine when not driving at all.

 

[nikthewho]

priority valve is typically integrated into the main control valve body or exists as a separate cartridge between pump output and the valve sections. It's NOT in the pump compensator itself - it's downstream. Look for a flow divider spool or priority cartridge in the main valve that has connections to both travel motors and implement spools

Since you correctly note the LS pump destrokes to ~300 psi standby:

1. Install gauge at auxiliary quick couplers (easiest test point with ports exposed)
2. Command boom up while stationary: Note pressure under load (should hit 3000+ psi)
3. Command boom up while driving: If pressure stays high but flow is weak, you have a flow restriction/diversion issue, not pressure loss

Alternatively: Tee into the work port on the main valve (between pump and valve inlet) - this shows available pressure/flow to all implement circuits.

If implement pressure is good but flow is poor during travel, the flow compensator or priority spool is mechanically limiting GPM to implements, not a pressure signal issue. Do you have hydraulic schematic? I can help you identify where travel priority is controlled.g

 

[HydJoe]

Back to this project again. Got hold of a hydraulic schematic, and learned where the feedback from the valvebank enters the work pump controller. Put a gauge there and one on the work pump output. There was a definite correlation beteween the two, leading me to think the pump is responding properly to command pressure. Also did a flow/pressure test on the auxillaries, and got good results.
So why are we not seeing command pressure is the next question? Spools not shifting far enough, due to poor charge pressure, or something malfunctioning in the valve bank feedback circuitry?
We think poor charge pressure, since it's only an issue when driving, and both boom and bucket spool act the same.
Can someone supply the baseline flow/pressure specs for the charge pump output? Not sure if the pump is underperforming, or if flow is getting away somewhere.

 

[HydJoe]

Further testing showed the boom joystick pressure dropping while driving. Hmm...
Looking over the full schematic I noticed a 100 micron screen at the inlet of a small valve block that holds the drive joystick enable and aux. solenoids screen, and began to wonder if that could be the restriction upstream of both joysticks. If you follow the schematic, the charge pressure next goes to the block holding the boom enable, brake, and 2-speed solenoids.
This morning I pulled that screen as well as an identical one where charge pressure enters the main valve block (runs only the float function in there). I couldn’t get any dirt out of either screen, but the main valve block one looked more open; it also was easier to blow through. So I swapped them, figuring the float function has very little flow demand and is rarely used.
Now the boom joystick supply pressure never dropped below 450 psi, even when changing direction and starting 2 boom functions at once. Saw 500 to 530 otherwise, as specified. (I give hard numbers, but the gauge is not calibrated - just close)
And guess what, it responds like I remember Kubota track loaders should! Way more responsive! We just hope it stays fixed.

 

[nikthewho]

Good things