HST Issue - Jerky Motion - SOLVED

[200mph]

I am hoping members can help with a situation I’m having with an L4740HSTC-3 tractor.

Issue: Ground speed can be very erratic and jerky while HST pedal held in constant position. It can act properly and then start acting up.

Background Info:

It doesn’t do it all the time, but it can act erratically when moving forward and may not respond to HST pedal input. I first noticed it in the summertime when mowing a very uneven trail. It almost felt like the tires were slipping and then catching in the dry grass. When snow blowing I was unable to maintain constant ground speed. Sometimes it becomes very jerky like moving the HST pedal very rapidly. It hasn’t gotten any worse, but there are times when it is OK. I don’t trust it when maneuvering close to something of value or that motion control can be assumed.

What I have done so far, but not necessarily in order.

• Verified fluid level OK (Super UDT2, about 120 hours on it with Kubota filters). Fluid is clean.
• Cut both HST and hydraulic filters apart and they were clean. Replaced with Kubota filters.
• No codes stored in the Intellipanel
• Checked HST Pedal sensor for dead spots while rotating. Checked 20+ times on multiple days just to be certain. Resistance changed as it should and never dropped out or spiked. Tested in both directions and both sides of the sensor. (One side increase with rotation and the other side decrease)
• Removed and reattached electrical connectors to most of the input sensors. Nothing looked corroded or was not fully attached.
• Checked sensor outputs and ranges in using diagnostic options in Intellipanel. Everything was in range. Even went through the resetting process of all sensors, just in case something got hammered in memory.
• Monitored sensor range while tractor was acting up. Inputs were all steady while ground speed was oscillating (jerky). I’m not convinced the values showing in the panel react fast enough to show dead spots. IE: Intellipanel responds too slowly to compared to sensor fluctuations.
• Since the servo sensor output was fluctuating around, I replace the sensor with new. It was a long shot, but it didn’t resolve the issue. Old sensor tested good during bench testing but thought something needed to tried.
• Inspected pressure relief valves – Looked brand new with no wear, scaring marks, broken springs, etc…
• Have NOT checked pressured as I don’t have gauges high enough or certain of the correct fitting(s).
• It acts up using Cruise Control, which leads me to believe the HST pedal sensor is not bad. Cruise and HST use same sensor but have not tried swapping them. (I’m 50/50 if I’m remembering this correctly)
• Removed Forward and Reverse Proportional valves today. I really thought one of these would be the problem based on how the tractor is behaving and expected to find the spool valves damaged or binding. Unfortunately, the spools will move using mechanical means. The valve does not appear to come apart. I tried removing the 24mm nut from the coil side, but that didn’t do anything to allow disassembly.
• F/R Proportional valve coils resistance valves value published in WSM.
• Tractor can act up when transmission is cold or warm.
• PTO, Steering, FEL and 3pt hydraulics working as they should.

I hate to replace parts without identifying something wrong, but at this point I’m not certain what to do next.

• Does anyone know what the input voltage is to the F/R Proportioning valves? I’d like to bench test while they are removed from the tractor. Want to verify they are stroking the same and returning home position with every cycle.
  
• How does the proportioning valve work to maintain constant ground speed? Does CPU provide constant voltage or is it pulsed off/on? I could not find any info in WSM on supply voltage or logic (steady on or pulsing).
• Can anyone confirm the correct threads for the pressure ports to test pressures? The WSM calls out 3/8 BPS, but it’s been known to be wrong. I assume BPS is British pipe threads. They look to be straight threads.

I have read a few threads about others with Grand L 40’s and 60’s loosing forward and reverse and replacing at least one of the proportion valves to solve the issue. But nobody stated if the valve passed or failed bench testing prior to replacing. Another user mentioned corrosion, but never stated where this corrosion was. I’m not certain how corrosion would impact the valve unless it was with the electrical connections. Mine are clean inside where the spool valve resides, but outside by the coil retaining nut (24mm) there was corrosion. Electrical connector looks clean. Given the electrical coil seems locked onto the valve, I’m not certain why the 24mm nut is needed.

Without any guidance from the community, I’m thinking about replacing the HST pedal sensor and one of the proportional valves, but hate to spend approximately $500 just to try something.

For reference:

HST sensor: TD170-52302 (servo position feedback sensor)
F/R Proportional Valves: TD170-59310
HST Pedal Sensor: TD160-32303 (Same p/n used on cruise control)

Look forward to any suggestions someone might have. Thanks in advance!

 

[85Hokie]

That is one of the best most descriptive posts I have every seen - the hydraulic system covered in ways I have no idea how it works!!!

I read it a couple of times - COULD it be fuel related vs hydraulic related??? Just throwing it out there.

 

[Nicksacco]

Yes, excellent wrote-up!

Would you have a recollection whether the engine acted, or sounded differently when the HST was acting up? Was it always at a constant speed and noise?

For example, if your car's wheel is slipping in mud, the engine speeds up and noise goes up. As the wheel gets traction, the engine and associated noise and vibration is less until the wheel finally catches. Then the engine noise isn't as noticeable.

 

[200mph]

Thank you for reading and considering.

The engine RPM's generally remain steady.

When the HST acts up the engine speed will change slightly, but this is due to the HST loading and unloading the engine. IE: The HST acting up is causing engine to change slightly, not the other way around. The swing in engine RPM's would need to be far greater to impact the ground speed like they are. Having said that I'm not completely ruling it out and keeping an open mind on what it can be.

I'm a bit disappointed the supply voltage and theory of operation of the F/R Proportional Valves is not discussed in the WSM. Not certain if another manual exists that perhaps focuses on the HST operation only. Especially the newer ones with all the feedback and control they have on the Grand L's.

 

[tthorkil]

Thank you for reading and considering.

The engine RPM's generally remain steady.

When the HST acts up the engine speed will change slightly, but this is due to the HST loading and unloading the engine. IE: The HST acting up is causing engine to change slightly, not the other way around. The swing in engine RPM's would need to be far greater to impact the ground speed like they are. Having said that I'm not completely ruling it out and keeping an open mind on what it can be.

I'm a bit disappointed the supply voltage and theory of operation of the F/R Proportional Valves is not discussed in the WSM. Not certain if another manual exists that perhaps focuses on the HST operation only. Especially the newer ones with all the feedback and control they have on the Grand L's.

Probably not your problem but has happened on smaller tractor tires as reported on this site - tires slipping on the rim - make a mark from tire to rim on the wheels and check for movement of marks.

 

[85Hokie]

Probably not your problem but has happened on smaller tractor tires as reported on this site - tires slipping on the rim - make a mark from tire to rim on the wheels and check for movement of marks.

The BX series DID have a hellva problem with that - But I would hate to think it would happen on a larger L series. Then again - weirder things have happened.

 

[200mph]

Thanks, but have the tire/rim position marked and it hasn't moved in years.

 

[ruger1980]

Does the issue occur only in forward direction or in both forward and reverse? Have you tried adjusting the sensitivity of the HST through the display?

 

[jimh406]

Could be fuel or air issue. Have checked filters and do you run with a fuel additive?

 

[old and tired]

Does it effect the power steering or the FEL at all?

Jerky standard L's had cracks in the suction hose which would create jerky motions... Do you have a sight glass for hydraulic fluid level or a dip stick; see if the oil has bubbles (if you have a sight glass).

 

[200mph]

Thanks for the replies. I'll try to address each one below.

Ruger1980: Best I can remember is that also happens in reverse, but since reversed is used so infrequently do think of it much. Definitely happens in forward, that's one reason I thought the forward proportional valve would be trashed.

jimh406: Engine power is good. RPM's are not dropping when acting up as described earlier. Fuel has been drained into clean white buckets to check Integrity. Use both Powe Service white (antigel) and also us anti bacteria additive. No indication it is air/fuel related.

Old and Tired: As mentioned earlier, FEL, PTO, 3PT and Power Steering are not impacted by the ground speed issue. Seems to be independent. Air or low fluid was what I was thinking the first time it happened. No bubbles in fluid when viewed on dip stick.

Thanks again.

 

[rbargeron]

From the symptoms (as I understand them) the tractor is acting like its doing unintended, quick changes of gear range without conditions calling for it. Does this L4740HSTC-3 have menu-selectable HST mode controls?

As Kubota has developed the HST transmission, adding electro-hydraulic features like Stall Guard, Three-Range Hydro Dual-Speed, Auto Throttle Advance, and HST response control, my head is spinning trying to imagine their possible failure modes - and how those could interact to produce the symptoms you describe.

Maybe somebody more familiar with how they work will have some ideas. Dick B.

 

[200mph]

Does this L4740HSTC-3 have selectable HST modes?

Yes, the -3 version has all the newer HST features... Perhaps too many

As Kubota has developed the HST transmission, adding features like Stall Guard, Three-Range Hydro Dual-Speed, Auto Throttle Advance, and HST response control, my head is spinning trying to imagine what their possible failure modes might be or how those could interact to produce the symptoms you describe. Maybe somebody who's mastered this new chapter in tractor operation will have some ideas.

From the symptoms (as I understand them) the tractor is acting like its doing unintended changes of gear range without conditions calling for them?

Definitely not gear change as in L M and H. It acts like it is getting an erroneous signal from HST pedal. If trying to simulate it would be like stepping off and on pedal rapidly. Because of this, I tried testing pedal sensor monitoring resistance change with rotation. Probably cycled it 100 times over several days hoping I'd see some crazy glitch. Also used the display in diagnostic mode to watch input voltage for HST pedal while it was acting up, but all looked steady. But it is always possible the display updates too slow to see a signal glitch. I don't have an oscilloscope or similar to monitor in real time. Additionally, no codes are recorded in the panel.

I wish I could find more detailed literature on these newer control systems. The WSM is limited in what it covers.

Hypothesizing one of the proportional valves is not making good electrical contact at sensor or possibly the computer. I have never pulled the CPU harness as it is difficult to get to. Then again, I can not find any literature describing how proportional valves control speed or if there is another sensor responsible for this.

It's not dropping between hi and lo as the dash doesn't indicate a change (turtle and hare symbols).

 

[Rickyboy]

great explanation . i don't know about your model but i cracked the input shaft housing on standard drive and had the similar issue till it busted fully out.

 

[ruger1980]

it's getting to the point where you really need the WSM to diagnose further. And as far as gauges you can probably do everything with a 500psi gauge. Charge and servo pressure are the major pressures you would need to look at

 

[ruger1980]

Hypothesizing one of the proportional valves is not making good electrical contact at sensor or possibly the computer. I have never pulled the CPU harness as it is difficult to get to. Then again, I can not find any literature describing how proportional valves control speed or if there is another sensor responsible for this.

Basically the HST is a variable displacement hydraulic pump and a fixed displacement motor running in a closed loop. Variable current is sent via PWM signal to the proportional control valves. This directs charge oil to either side of the servo piston increasing displacement of the pump.

More current = more charge oil to the servo = more displacement = higher travel speed.

It could be as simple as a loose connection. The best way to determine that is to measure the signals from the potentiometer on the pedal and the current to the PWM's. The best way to di that is with an oscilloscope or a diagnostic tool built for said purpose

 

[Daferris]

Not sure if it's useful for you but Amazon has a interface that turns a Windows PC or and Android phone/tablet into an oscilloscope. It's not full featured but it's also only $89.

https://www.amazon.com/Oscilloscope-OSC482M-Channels-Bandwidth-Resolution/dp/B07TB8V2X1

 

[200mph]

it's getting to the point where you really need the WSM to diagnose further. And as far as gauges you can probably do everything with a 500psi gauge. Charge and servo pressure are the major pressures you would need to look at

The WSM doesn't provide the necessary detail to diagnose further unless a more recent version such as one for the Grand L60's cover it. I've been through the WSM for the 40's but it lacks detailed information except for the vary basics.

 

[ruger1980]

I don't have a copy of the 40 series tractors WSM so I cant say what content it has or how useful it is.

 

[Nicksacco]

All new to me this version of HST.
I'm going to read while keeping an eye on this thread.

All I got is this link and my assumption is these systems are getting their wires crossed!

TractorData.com Kubota L4740 tractor transmission information

www.tractordata.com