Loader not lifting except at high RPM, diagnosis and repair

[alexander]

We have an MX5000 Utility Special with a LB702 front end loader, and self-installed triple rear remotes (never quite right - leaking). Bought it new probably 15 years ago - only 719 hours on machine now, so only about 50 hrs avg./yr. Performance of the loader has decreased over time. It used to lift at idle, albeit slow. Now, when attempting to lift at idle, the loader doesn't lift. We have to rev the engine to at least 1,300 rpm to lift the loader empty. When there's a load, it needs to be much higher than that, and with a heavy load, it just can't handle it. Rear 3-point system lifts at idle, so the problem appears related to the loader circuit. I replaced the hydraulic filter recently, but does not change the condition. Last fluid change was in 2017, and fluid was milky looking (air, water?) at that time. Looks a little milky still at the site glass. I'm going to add some die to the hydraulic fluid so I can better tell fluid level at site glass, because it is hard to see.

Does the same hydraulic pump supply fluid to both the loader and 3-point system? If so, the problem would seem not to be the hydraulic pump.

I tried searching for relevant threads - found a suggestion to "burp" the outlet side of the hydraulic pump. Is this a potential cause of the problem?

Any suggestions on how to diagnose this problem would be most appreciated.

Thanks,
Al

 

[TheOldHokie]

We have an MX5000 Utility Special with a LB702 front end loader, and self-installed triple rear remotes (never quite right - leaking). Bought it new probably 15 years ago - only 719 hours on machine now, so only about 50 hrs avg./yr. Performance of the loader has decreased over time. It used to lift at idle, albeit slow. Now, when attempting to lift at idle, the loader doesn't lift. We have to rev the engine to at least 1,300 rpm to lift the loader empty. When there's a load, it needs to be much higher than that, and with a heavy load, it just can't handle it. Rear 3-point system lifts at idle, so the problem appears related to the loader circuit. I replaced the hydraulic filter recently, but does not change the condition. Last fluid change was in 2017, and fluid was milky looking (air, water?) at that time. Looks a little milky still at the site glass. I'm going to add some die to the hydraulic fluid so I can better tell fluid level at site glass, because it is hard to see.

Does the same hydraulic pump supply fluid to both the loader and 3-point system? If so, the problem would seem not to be the hydraulic pump.

I tried searching for relevant threads - found a suggestion to "burp" the outlet side of the hydraulic pump. Is this a potential cause of the problem?

Any suggestions on how to diagnose this problem would be most appreciated.

Thanks,
Al

These are constant displacement pumps so the flow rate from the pump increases and decreases with RPM. If RPM improves lift power and speed I would suspect a leak. The pump will basically make the same pressure regardless of RPM. Increasing RPM increases the flow rate from the pump and helps keep up with the pressure loss out the leak. A pressure test ahead of the loader valve will tell you whether or not the pump is producing full pressure and that would be the first thing to check. Just remove the pressure supply hose from the loader and connect your gauge to the hose. Before doing that you need to verify that the outlet block contains a pressure releif valve so as to not deadhead the pump. That test will tell you what pressure the pump and system pressure relief are supplying. My guess based on your description and no knowledge of that machine is the pump will check out fine.

It would be nice to know how the loader circuit is plumbed. The typical Kubota loader circuit is the pump feeds system relief in outlet block, from there to loader valve, and loader valve feeds 3pt. If that is the case and there are no indications of external leakage my primary suspect would be spool leakage in the loader valve. I think we can rule out the relief valve if any in the loader valve since that would also affect the 3pt. Is the problem confined to the lift cylinders or does it affect the bucket cylinders as well?

Dan

 

[alexander]

Thank you Dan!

I will figure out how to do a pressure test out the outlet side of the hydraulic pump. And, I’ll make a circuit diagram and post.
Al

These are constant displacement pumps so the flow rate from the pump increases and decreases with RPM. If RPM improves lift power and speed I would suspect a leak. The pump will basically make the same pressure regardless of RPM. Increasing RPM increases the flow rate from the pump and helps keep up with the pressure loss out the leak. A pressure test ahead of the loader valve will tell you whether or not the pump is producing full pressure and that would be the first thing to check. Just remove the pressure supply hose from the loader and connect your gauge to the hose. Before doing that you need to verify that the outlet block contains a pressure releif valve so as to not deadhead the pump. That test will tell you what pressure the pump and system pressure relief are supplying. My guess based on your description and no knowledge of that machine is the pump will check out fine.

It would be nice to know how the loader circuit is plumbed. The typical Kubota loader circuit is the pump feeds system relief in outlet block, from there to loader valve, and loader valve feeds 3pt. If that is the case and there are no indications of external leakage my primary suspect would be spool leakage in the loader valve. I think we can rule out the relief valve if any in the loader valve since that would also affect the 3pt. Is the problem confined to the lift cylinders or does it affect the bucket cylinders as well?

Dan

 

[TheOldHokie]

Thank you Dan!

I will figure out how to do a pressure test out the outlet side of the hydraulic pump. And, I’ll make a circuit diagram and post.
Al

Do the pressure test on the inlet side of the loader valve. That isolates the test result from problems inside the valve. I assume you plumbed your remotes in after the loader and not before?

Dan

 

[Captain13]

If you have draft control on the tractor, make sure that it’s not in the full down position. That Can cause your problem. At least it did on my M7040 last week. I inadvertently passed the adjustable stop and the up/down of the loader would only work (slowly, I might add), at high rpms. I raised the lever and the problem disappeared.

 

[TheOldHokie]

If you have draft control on the tractor, make sure that it’s not in the full down position. That Can cause your problem. At least it did on my M7040 last week. I inadvertently passed the adjustable stop and the up/down of the loader would only work (slowly, I might add), at high rpms. I raised the lever and the problem disappeared.

How does that work? On most Kubota tractors the loader valve is connected ahead of the 3pt and operates totally independent of it.

Edit: Never mind - I spoke with out enough thought. After just a little more thought I realized most of the loader valves continue to supply the 3pt while the loader is being raised/lowered. Not sure how draft control would booger the loader function up but its not fully isolated.

Dan

 

[Captain13]

it’s as though the draft control was servicing the 3 point hitch as being first in line, if you will, and was directing fluid to the 3 point hitch with minor flow to the loader. All I know is, the loader would not operate correctly with the lever full down. Full down meaning bypassing the adjustable stop. It took me a few minutes to figure it out. I had been using the disk harrow and inadvertently put the lever all the way down. I know it will affect the raise height of the 3 point as well. something to watch for if you suddenly run into the problem like I did.

 

[TheOldHokie]

it’s as though the draft control was servicing the 3 point hitch as being first in line, if you will, and was directing fluid to the 3 point hitch with minor flow to the loader. All I know is, the loader would not operate correctly with the lever full down. Full down meaning bypassing the adjustable stop. It took me a few minutes to figure it out. I had been using the disk harrow and inadvertently put the lever all the way down. I know it will affect the raise height of the 3 point as well. something to watch for if you suddenly run into the problem like I did.

I am not a hydraulics expert but I can speculate. I know the L3901 loader can be raised and lowered at the same time as the 3pt. That is not "normal" behavior in a power beyond valve Shifting the bucket spool will block the three point which is "normal". So what is happening? In general exhaust oil from the cylinder is sent to the tank port. In this case it may be that the lift spool is dessigned to route exhaust oil to the power beyond port. That would put the 3pt valve in series with the loader lift cylinder and allow for simultaneous operation - this would be a slight variation on a standard series spool. The bucket spool on the loader uses regen which loops exhaust oil back to the other side of the cylinder and effectively blocks the PB port. With me so far?

So what happens to the loader lift cylinder if something blocks the 3pt circuit? No way for exhaust oil to get out and pressure increases to the point the pump relief valve opens and excess flow goes back to the tank. If the obstruction is partial and some small quantity of oil can get through the 3pt the loader cylinder extends really slowly. And since this is a series circuit the 3pt back pressure at the PB port is opposing the extend side of the loader cylinder and lift power of that cylinder goes way down.

Like I said not an expert but I believe you when you say you saw the draft position cause the problem and that's the best explanation I can come up with.

Dan

 

[mirceaended]

I believe the thing is that you have a crack in the hydraulic pump. If there is such an opportunity, try to pull it out and examine it for cuts and cracks. I think this may solve your problem. If you do not find any damage, I advise you to lift the entire mechanism and do a thorough check. I can advise you to seek help from the guys from https://teamliftigsolutions.co.uk/loler-testing-kent-and-london/ for them to help you with the lifting mechanisms. I do not know the specialists who could do the repair since I have never used them for work. I hope my advice will help you and wish you good luck.

 

[Nicksacco]

On my L35, I had a similar issue.
The L35 has 2 pumps - the front is the power steering and the rear pump is for bucket.
Both pumps share a large ~1" inlet pipe from the sump.
The problem was small holes (from rust) that formed in the INLET elbow to the bucket pump.
That caused it to suck air.

 

[GreensvilleJay]

Is there any chance it's just low on oil ? Usually a loader will need more oil than 3PH, might explain it. Milky oil means water is in the oil, always a bad situation. Especially if it's been there 'forever and a day'.
As the first step, I'd drain,purge and refill the oil
2nd would be to FIX the leaky fittings. hmm, maybe that should be #1...

Hydraulics should not leak or weep.

 

[TheOldHokie]

This thread is over a year old and the original poster has never come back after the initial exchange. My guess is the problem was resolved long ago.

Dan

 

[Nicksacco]

Eeeesh. 'guess we need to check dates now...