I NEED HELP - intermittent loss hydraulics

bob2003 · May 31, 2005

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 · Jun 1, 2005

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 · Jun 4, 2005

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 · Jun 6, 2005

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 · Jun 6, 2005

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 · Jun 6, 2005

Please call me 503-930-9295. Thanks

fish_wisperer · Jun 7, 2005

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 · Jun 7, 2005

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 · Jun 7, 2005

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?