I NEED HELP - intermittent loss hydraulics

[bob2003]

Subject vehicle: JD 750, 4WD, with front loader

Hydraulic system works fine then after about 20 minutes of continuous use (engine, not continuous use of hydraulics) the system will begin to shutter on operation, then progresses to complete failure.

Shutting engine down for as little as a couple minutes resets this cycle, then it is good for another twenty minutes.

I have drained all fluid and replaced it with new. The screen is clear; this model does not have a filter. I checked the rubber hose that connect the 7/8th inch pump intake lines on the left underside of the tractor; it does not have any holes, is not kinked, nor is in collapsing from pressure.

Using an air compressor (after disconnecting both ends) I blew air through the line from where the rubber hose connects and the pump-all clear. I replaced both “O” rings; the one going into the pump and the other on the cover that is by the screen.

Disconnecting the output line from the pump, the fluid is ejected with full force. The hydraulic stop valve is open, the bypass activates with the bucket is tilted backward to its extreme, and the lines are not overheating – just barely warm to touch.

The fluid level remains constant as does the engine oil, thus no fluid is being lost into the engine.

I do not have quick connects, and the problem continues even after disconnecting the line to the front loader and rerouting it back to the main pump.

I’m stumped and so are the JD mechanics. Does anyone have any ideas???? My direct email is bobmorr2003@yahoo.com

 

[Kyle_in_Tex]

Maybe a hose is collapsing due to suction. What type of oil? Is there a filter clogged? Is your joystick valve sticking? Maybe pressure is building up where it shouldn't somewhere and turning off the tractor is allowing it to bleed down.

 

[fish_wisperer]

i would think a return line is plugged or a defective pump. if the system is building up to much pressure the pump will get loud, if its happening downstream of the pressure guage sensor it may not show up on guage, say like if there is a check valve past the pressure sensor that would prevent back pressure to the sensor. just a guess. may be a bad pump though.

 

[fish_wisperer]

correction,,,,,,,,,,, i dont know what i was thinking. worked 80 hrs last week.. if you have loss of pressure beond a check valve then it wouldnt show up on the guage. everything else was right.

 

[fish_wisperer]

im almost sure something is wrong with either the selector valve assembly that your joy stick pluggs into or a pump. im curious to see how this turns out. i was trying to relate this to some fuel system problems i have run into before but that would require two pumps.

 

[bob2003]

Please call me 503-930-9295. Thanks

 

[fish_wisperer]

ill try to remember when i get home. when your loader is messing up check and see id 3pt still raises ok, if so its probably in the selector valve for the loader or a check valve spicific to the loader. if all hydrolics are messing up maybe you have a main check valve just upstream from your pump, there has to be one to keep pressure from back feeding through the pump. just a thought, remember i know nothing about tractors, i work on aircraft.

tom

 

[bob2003]

Yes, it is all hydrl's; I loose everything. I can't find anything that looks like a check-valve, but that certainly doesn't mean it isn't there. I don't work on tractors either!!!

 

[fish_wisperer]

you need to look at a drawing of it, the check valve may be right where the pump mounts. it does sound like a messed up check valve, assuming the pump is fine. does the pump get loud? did you say it draws down your engine or just the loss of hydrolics?

 

[bob2003]

Pump works fine; does not draw down engine. The very minute I shut it off, it corrects the prob and it is back to working fo another 20-30 minutes.

 

[bob2003]

And no, it does not get loud.

 

[fish_wisperer]

machines do weird things sometimes........ shutting down the engine would be relieving hydrolic pressure and allow a spring loaded check valve to reseat itself. if it was just a warm up thing shutting it down for just a minit wouldnt help because it would still be warm. relieving the hydrolic pressure is probably what is helping.

 

[fish_wisperer]

then fluid is returning unrestricted, if it were over pressuring it would get loud. or pump stops putting out properly for whatever reason.

 

[GaryK]

Now we know why the 30/06 rifle was invented /forums/images/graemlins/grin.gif

 

[BigEddy]

In my mind, there are a few possible causes of no flow to the loader / 3pt, either intermittant or otherwise.

A blockage / or restriction in the pressure side of the system that prevents the fluid from getting there. I think we can rule this out, because it would load down the pump and engine, and should be noticeable.

A failing relief valve that is sticking open and allowing the fluid to flow unrestricted back to the resevoir, thereby providing fluid but no pressure in the circuit, and no ability to do work. Should hear it, but you never know.

A pump problem where it is failing to put out pressure due to an internal failure / blown O-ring / leakage due to wear.

A problem in the suction side of the circuit that prevents the pump getting fluid to pump. Could be not enough fluid, a blocked or collapsed suction line or screen, or air leakage.

You've already eliminated a few of the above. The engine is not loading down, therefore there is no restriction in the system. You checked the screen and changed the fluids, so I rule out not enough fluid / wrong fluid.

I'd start with the relief valve as it is an easy check. If it is sticking open, then you will have all the fluid going directly back to the sump. Not sure why 20 minutes would affect that though - it would be more likely affected by hitting relief pressure using the loader.

Second, I'd be looking at installing a pressure gauge in the line upstream of the relief valve to try to see if there is any pressure / flow at all when the problem starts. No pressure at all - no flow - some pressure you have flow but it's going back to the sump.

Third I'd go back to the suction side to see if there is a reason that the pump can't prime. If you have something floating around in the system that can block an inlet, it would manifest itself this way. Works fine until it gets sucked over the inlet port, then no flow until the pump stops trying to suck and it can drop back off. An air leak would more likely be an immediate problem.

Finally I'd look at tearing down the pump or replacing it. However, if you have access to a spare one you could borrow, I'd swap them a lot sooner to rule that out.

good luck.

 

[fish_wisperer]

i like that idea of a weak overpressure reieif valve. ill be curios to see how this turns out.