Underpowered 3901 Tractor with BH77 Backhoe (or design flaw)??

[Blumrs3]

Could anyone tell me how to remove the hood from a Kubota L3901HST?

 

[TheOldHokie]

I suspect the BH has to be last in line to prevent back pressure damage. Kubota seems to like BH valves without 'T' lines.

My point exactly.

Dan

 

[TheOldHokie]

I agree. That loss of flow makes no sense. This is a series system so 'flow in' equals 'flow out'. The only other place it has to go is to tank if there are tank connections, and then only if the valve is leaking.

See my comments about pressure drops. Pump.output will fall off as line pressure increases but not this much.

Dan

 

[MBinPA]

I recently purchased an L3901 (July 2021). The machine has proven its worth every time i have used it. I did not want a construction machine. I needed a machine to perform multiple functions. I have to say I expected the backhoe to have more strength, but I have re thought this. Since before I had this machine I would NEVER have accomplished the chores i needed without it. So, I take smaller "bites" of dirt and rock and the job gets done.

Given the supply shortage, you may suggest to the dealer to remove all the hydraulic lines and anything else that could be affected by plastic or debris in the lines and re-plumb.

The L39 is a great machine- Congrats on your purchase- keep pluggin' you'll get it right!

 

[ade678]

Sounds like there's more 'plastic' in the system somewhere. There shouldn't be any drop in GPM if it's an 'open center' system. Full 6.5 GPM should be available all the way to the furthest device, less friction losses( a wee amount).

This is really good point. My very limited understanding of a hydrostatic transmission is that it a positive displacement pump that is indeed connected to the implements in some form of open loop. Any sense of what a flow meter would test for?

 

[ade678]

See my comments about pressure drops. Pump.output will fall off as line pressure increases but not this much.

Dan

Again, this is a really good point. Any idea why the dealer would want to test performance using a flow meter?

 

[TheOldHokie]

Again, this is a really good point. Any idea why the dealer would want to test performance using a flow meter?

The flow rate is what determines the speed of the actuators (cylinders). You are losing 50% of the flow which is why the backhoe is slow.

This flow is coming from the fixed displacement implement pump on the engine and has nothing to do with the HST which has its own variable displacement pump.

Dan

 

[edritchey]

You can't compare it to the B26 it is a TLB from the factory and a completely different hydraulic system. You can compare it to the L2501 with the same BH77 attachment. I would check to make sure yours is plumbed up the same as the L2501 because it sounds like they didn't connect the components correctly in your setup. It should work every bit as good as the other machine or something is wrong. Side note you cannot use anything connected to any of the rear remotes when your backhoe is connected and like mentioned above your 3pt should be in its all the way down position.

Does everything work correctly with out the backhoe connected? If so you might have a internal leak in the backhoe valve somewhere but this is something the dealer should be able to find and correct for you.

Good luck with your new machine.

 

[TheOldHokie]

Side note you cannot use anything connected to any of the rear remotes when your backhoe is connected

Why is that?

Dan

 

[fried1765]

Why is that?

Dan

I don't believe that either!
My L48 TLB has 3 rear remotes, and my backhoe is always connected.
L48 TLB does have 3 hyd. pumps though, so that may be the difference.

 

[Sammy3700]

I have the same configuration but in a 3301 and it works flawlessly. There’s verbiage in the BH77 manual (p7 of the document/p17 of the pdf) that discusses how to prevent damage to the backhoe valve if these other options are also on the tractor.

What are you referring to as the document I have the owners, parts, assembly and parts manuals my page 17 doesn’t cover adding any items. thanks

 

[ayak]

What are you referring to as the document I have the owners, parts, assembly and parts manuals my page 17 doesn’t cover adding any items. thanks

 

[edritchey]

Why is that?

Dan

This is what is says in the manual from mine.

4. It is necessary to have the backhoe detached when
using rear remotes, the 3-point hitch or any other
valves that are downstream of the backhoe valve.
Failure to do this may cause the exceeding allowable
back pressure to break the seal or case of the backhoe
valve.

 

[TheOldHokie]

This is what is says in the manual from mine.

4. It is necessary to have the backhoe detached when
using rear remotes, the 3-point hitch or any other
valves that are downstream of the backhoe valve.
Failure to do this may cause the exceeding allowable
back pressure to break the seal or case of the backhoe
valve.

That warning is 100% correct. BUT on the Lxx01 the OEM remotes are upstream of the backhoe valve.

Dan

 

[edritchey]

That warning is 100% correct. BUT on the Lxx01 the OEM remotes are upstream of the backhoe valve.

Dan

Gotcha That makes sense then.

 

[Old_Paint]

I'm a little confused about where the flow might be going. How can flow just disappear? Pressure, yeah, I get it. But flow? What goes in must come out.

 

[TheOldHokie]

I'm a little confused about where the flow might be going. How can flow just disappear? Pressure, yeah, I get it. But flow? What goes in must come out.

Excessive back pressure causing loss through a relief would be one possibility. Some inlet pressure tests would be informative.

Dan

 

[kubotafreak]

I'm a little confused about where the flow might be going. How can flow just disappear? Pressure, yeah, I get it. But flow? What goes in must come out.

Assuming no bleed off, you can very well see pressure changes across the circuit. Mass flow rate must be satisfied. Imagine a nozzle, the mass flow rate across the restriction is the same. Same concept with a leaf blower advertising 200mph wind out the tip vs another that is rated in cfm. Buy the one based on cfm, because you can "neck" down the tip to get the velocity. Now if there is a valve bleeding off fluid, then the equation changes. If the tech is seeing a loss of fluid "flow" or mass flow, then a valve before it is bleeding off. Chances are something is just not hooked up right, or the valve under the hydraulic filter has not been clocked right. Someone with a standard L series can probably speak better to its configuration. It is a flow director that directs flow to the pb, vs tank.

 

[Old_Paint]

Well, I was going to qualify my statement, and say that pressure drop would be noticeable through the system with bypassing happening. What also would be happening is excessive heating in the oil. I also suspect a relief valve somewhere, and quite possibly in the BH77. It only has two lines with no separate return to tank, so if anyting is between the BH77 and the tractor, it's going to cause restriction and poor performance. From what I've read, backpressure tends to be a common problem on aftermarket remotes. Is this because very few have Return to Tank ports, or what? I know the BH77 only has two hoses to connect to the tractor, with the 3pt hitch being the last thing between it and the tank, assuming I read the hydraulics diagrams right. That also assumes remotes are installed correctly, too.

 

[minthral]

Hi All,

I recently purchased a brand new 3901 tractor with several hydraulic attachments — including a BH77 — and quickly noticed that the backhoe seemed sluggish and underpowered. The most obvious problem is that it won’t smoothly perform dual functions. For example, if I try to raise the boom first, the implement will then only very slowly curl the bucket. It's far inferior to the 2501 525/77BH or 26B TLB setup which I've rented a number of times.

The tractor is now back at the dealership (I called them about 30 minutes after they dropped it off, which was a bummer). They've run a handful of diagnostics on both tractor and backhoe, and they also seem surprised by the slow and uncoordinated performance. The best guess we’ve got comes from the Kubota factory itself: the machine is running too many implements on its hydraulic circuit.

Specifically, the tractor is equipped with (1) a front-loader, (2) a 3rd function valve, (3) a set of 3 rear aux hydraulics (for top & tilt), and (4) a BH77, connected in that order. Kubota’s guess is that the slow backhoe performance is due to the weak hydraulic flow by the time the fluid gets to the implement. A few numbers from tests done at the shop: At source, the pump is putting out 6.5 gal/min (as per factory spec). The flow rate drops to 5.7 gal/min after it crosses the front loader valve, then to 5.1 g/m after is crosses the 3rd function, and finally to 3.8 g/m by the time it crosses the rear auxiliaries and arrives at the backhoe.

Put simply: I'm asking a 3901 to do too much.

Still, I’m surprised that a 3901 can’t fully manage the load (or that that the company would sell it without a “3 implement max” warning.) Anyone experience something similar? If indeed it’s the hydraulics, then perhaps a diverter valve at the the rear aux directly to the BH77 would solve the problem? Or should I ask Kubota to switch to a model strong enough to power the implements I want? I get that a tractor isn’t rigged to function like a skidsteer or a mini ex, but a TLB configuration with a 3rd function and aux hydraulics seems like a reasonable setup.

Thoughts?

Thanks in advance to everyone on the forum for your time and opinions!

I have l3301 with BH77, third function, and 2 rear remotes. Backhoe easily overpowers the tractor. 2 functions are easy and quick. Can do 3, but it really slows down plus that’s not easy and takes lots of skill/practice. Now that I’m familiar with it, I’m doing WOT and can go pretty quick (becomes muscle memory).

I think they plumbed the hydraulics wrong on yours. Make sure the power better beyond port (left most on hydraulic block under floor board goes straight to backhoe). This is the power out port. Make sure other connection is from remotes. This is the power in port.