Pressure testing difficulties

[grayw0lf]

Help!

I'm trying to test the hydraulic pressure on my Kubota L355SS, but I can't seem to get the pressure to register on the gauge. So, what am I doing wrong?

Per the manual: the feedback rod is unhooked, draft & control levers are all the way up. The gauge is hooked up to the test port in the 3pt cylinder cover. There is obviously pressure, or the loader & shuttle shift wouldn't work at all.

The linkage on the draft control is not hooked up (as when I bought it) & I do doubt the draft control works anyways.

The reason I'm testing is the loader is weak (have to rev high to lift anything over about 400lbs) & slow.

Thanks!

 

[grayw0lf]

Update... I checked the pressure after the loader valve, on its way to the lift arms.
1800psi @ idle. 1950psi giving her some gas.

Specs say to adjust pressure to 1920-2130psi at high idle.

So, I figure I'm safe to adjust to 2000psi @ idle, unless you know of any reason not to? (since I am testing at the loader & not at the 3pt cyl).

 

[J_J]

Spec for your tractor is probably for the 3pt.

If the pump can build up spec'd pressure, and the relief valve goes off, then you are good to go.

Your loader is spec'd to lift a given load.

However, the loader cyl can be bypassing causing you to lose pressure, and therefore lift capacity.

 

[bumperm]

J.J.

Does is matter where one checks hydraulic pressure? I bought a nice big pressure gauge and plugged it into one of the quick connects on the rear of my B3350. When operating that hydraulic spool (with none of the other QC's on the back hooked up), all I get is 1800 psi at about half throttle. Have to check the manual but I think it's supposed to be over 2100 psi or there'bouts.

Is it better to check pressure at the loader QC, - - I was thinking it wouldn't much matter where the measurement was taken . . . ?

thanks,

bumper

 

[J_J]

It doesn't matter where you plug in, if all the relief valves are set the same.

The FEL relief is probably the master relief

 

[kennyd]

Here is a video I made that may help:

 

[Tx Jim]

It doesn't matter where you plug in, if all the relief valves are set the same.

The FEL relief is probably the master relief

I disagree that FEL relief is the master relief. I think 3 pt relief would be the master relief as tractors don't roll off the assembly line with a FEL installed.

 

[bumperm]

I disagree that FEL relief is the master relief. I think 3 pt relief would be the master relief as tractors don't roll off the assembly line with a FEL installed.

I'm not all that well versed in tractor hydraulics, so please correct if I'm wrong on this. There's only one hydraulic pump on most CUTs to run FEL, 3-point and other optional accessories. Some tractors have a second pump for power steering, while others use the one pump.

In an open system, high pressure goes out from the pump and through all the open center valves in series before returning to the reservoir. With nothing operating, no pressure build up to speak of and the fluid is dumping back into the reservoir "no-load". When you operate a valve in this system to direct fluid to a cylinder, you are essentially interrupting the free flow return line and diverting the flow to do work. Thus the high pressure side is sent into the cylinder, and in a two port cylinder, the low pressure side of that cylinder sends some or all of its fluid back to the reservoir. The pressure build up on the working or high pressure side of that cylinder will be dictated by the amount of force or work it's doing.

If we hold the valve open, the cylinder will either come to a stop because it's work load is too heavy, or the cylinder will reach its limit. In either case, the pressure will continue to build until something goes BANG, the tractor engine stalls due to overload, or the relief valve opens before those aforementioned options.

Now when the relief valve opens, it shouldn't much matter where the relief valve is in the system, the first (or only) relief valve to open will dictate the maximum pressure the system can achieve (this is assuming that the relief valve has a high enough flow capacity to do it's job - protecting the system.

On my B3350 Kubota, I know of only one main relief valve. I do believe that the backhoe, on power beyond, has some bypass (relief) valves that will simply bypass a given backhoe function. This simply bypasses into the power beyond return line, but would not necessarily dictate peak pressure in the tractor system except for the backhoe functions.

So, discounting the backhoe, to my way of thinking, if you dead head a valve anywhere on the tractor, the main relief valve should open, and anywhere on the tractor, on the high pressure side of that dead headed valve or cylinder, one should be able to read the attained peak pressure and also see where the relief valve is opening. No??

bumper

 

[J_J]

I disagree that FEL relief is the master relief. I think 3 pt relief would be the master relief as tractors don't roll off the assembly line with a FEL installed.

I am sorry that you disagree with the facts.

Although a tractor with no attachments except the 3pt, has only one relieve valve at the 3pt, it does become the only and master relief.

However when you add a FEL, remotes valve, or BH, they are all upstream of the 3pt relief valve.

Logically, you would set all the reliefs the same to make troubleshooting easier.

With the FEL attached, the loader relief valve is the first relief in line and becomes the master.

The relief pressure is set at the valve for those cyls under that valve control.

So it depends where the pressure is developed as to which relief will activate. Either that valve or an upstream valve.

If I plug in a gage in the FEL circuit, and max out the FEL cyl, then the FEL relief will activate, and no other relief valve.

If I max out a cyl on a remote valve, then that relief or the FEL relief will activate.

So technically, you should set all reliefs downstream of the FEL relief slightly above the FEL relief. The FEL valve becomes the master due to it being set at a lower pressure and will activate first.

So now when any valve on the tractor that max out a cyl, the FEL relief will activate.

You could make difficult for yourself by setting all 4 relief different for specific pressures. However the valve with the lowest upstream pressure will activate.

I believe all this is true for Kubota tractors.

 

[J_J]

I'm not all that well versed in tractor hydraulics, so please correct if I'm wrong on this. There's only one hydraulic pump on most CUTs to run FEL, 3-point and other optional accessories. Some tractors have a second pump for power steering, while others use the one pump.

In an open system, high pressure goes out from the pump and through all the open center valves in series before returning to the reservoir. With nothing operating, no pressure build up to speak of and the fluid is dumping back into the reservoir "no-load". When you operate a valve in this system to direct fluid to a cylinder, you are essentially interrupting the free flow return line and diverting the flow to do work. Thus the high pressure side is sent into the cylinder, and in a two port cylinder, the low pressure side of that cylinder sends some or all of its fluid back to the reservoir. The pressure build up on the working or high pressure side of that cylinder will be dictated by the amount of force or work it's doing.

If we hold the valve open, the cylinder will either come to a stop because it's work load is too heavy, or the cylinder will reach its limit. In either case, the pressure will continue to build until something goes BANG, the tractor engine stalls due to overload, or the relief valve opens before those aforementioned options.

Now when the relief valve opens, it shouldn't much matter where the relief valve is in the system, the first (or only) relief valve to open will dictate the maximum pressure the system can achieve (this is assuming that the relief valve has a high enough flow capacity to do it's job - protecting the system.

On my B3350 Kubota, I know of only one main relief valve. I do believe that the backhoe, on power beyond, has some bypass (relief) valves that will simply bypass a given backhoe function. This simply bypasses into the power beyond return line, but would not necessarily dictate peak pressure in the tractor system except for the backhoe functions.

So, discounting the backhoe, to my way of thinking, if you dead head a valve anywhere on the tractor, the main relief valve should open, and anywhere on the tractor, on the high pressure side of that dead headed valve or cylinder, one should be able to read the attained peak pressure and also see where the relief valve is opening. No??

bumper

On your tractor, if you have a FEL, the flow probably goes through the FEL, which has a relief.

If your tractor has a remote valve, then it has a relief valve.

If your tractor has a BH attached, then the BH has a relief valve.

The relief valve on that valve/circuit or a relief valve upstream will activate.

Your tractor is an open center hyd system and the flow is always flowing. If something stops the flow, there has to be a way for the pressure to escape that circuit, or something will burst, break, crack,, etc.

You may not know which relief valve is relieving.