JD 2720 Hydraulic Issues

[smashedtoe]

The technical manual does provide some info (see attached). Not a lot for diagnostics though which is surprising.

 

[oldnslo]

Smashed,
Since the charge circuit is fixed displacement pump we can use the theory that when the pump is turning the oil must go some where and that it will flow to the point of least resistance.

In reviewing the schematic from the service manual it would indicate their four potential flow paths.
1) Across valve O and into the HST for make up oil for leakage losses.
2) Through the PTO circuit. Note: I suspect there should be an orifice in this circuit to control PTO clutch engagement speed but is not shown in schematic.
3) Through valve M to steering. This is where majority of oil would typically since goes through steering unit, then through heat exchanger and back to tank.
4) Across main relief valve L. L would have to be stuck open at pressure lower than M or you would not have any steering.

The service manual shows how to check for steering pressure M, system pressure L. Also shows how to test PTO pressure K. Since steering is not working properly oil is going through another leak point either into the HST, through PTO or something else that could have been damaged when charge pump failed from suspected high pressure during cold start.

My thought on diagnostics would be to install the gauge for checking steering and see what that pressure is. Then with gauge still in steering test port engage the PTO. Does the pressure change? If how much? I significant change would point towards flow loss in PTO circuit and would inspect the solenoid PTO valve. If minimal change then go to next step.

If you have a gauge with capabilities of reading the system 995 PSI I would next leave PTO engaged and then turn steering to lock like testing the system pressure L. If still low pressure then would start by checking valve L which I believe is shown in service manual.

Hope this is written in a manner that makes sense on what first test, second test, etc. is trying to point towards a failing component.

 

[smashedtoe]

Hello oldnslo.
You ask some great questions and bring up some great points.
I am still waiting for my pressure gauge kit to arrive in the mail and will run the tests you suggest. I will definitely share the info with you. Heck, if I could buy you dinner or a bevvie I would do that too just for being so helpful!
Stay tuned bud...more to come.

 

[smashedtoe]

Smashed,
Since the charge circuit is fixed displacement pump we can use the theory that when the pump is turning the oil must go some where and that it will flow to the point of least resistance.

In reviewing the schematic from the service manual it would indicate their four potential flow paths.
1) Across valve O and into the HST for make up oil for leakage losses.
2) Through the PTO circuit. Note: I suspect there should be an orifice in this circuit to control PTO clutch engagement speed but is not shown in schematic.
3) Through valve M to steering. This is where majority of oil would typically since goes through steering unit, then through heat exchanger and back to tank.
4) Across main relief valve L. L would have to be stuck open at pressure lower than M or you would not have any steering.

The service manual shows how to check for steering pressure M, system pressure L. Also shows how to test PTO pressure K. Since steering is not working properly oil is going through another leak point either into the HST, through PTO or something else that could have been damaged when charge pump failed from suspected high pressure during cold start.

My thought on diagnostics would be to install the gauge for checking steering and see what that pressure is. Then with gauge still in steering test port engage the PTO. Does the pressure change? If how much? I significant change would point towards flow loss in PTO circuit and would inspect the solenoid PTO valve. If minimal change then go to next step.

If you have a gauge with capabilities of reading the system 995 PSI I would next leave PTO engaged and then turn steering to lock like testing the system pressure L. If still low pressure then would start by checking valve L which I believe is shown in service manual.

Hope this is written in a manner that makes sense on what first test, second test, etc. is trying to point towards a failing component.

Hi.

Okay, so I put my pressure gauge on to check for the steering pressure M via the steering test port resulting in less than 100PSI (I need a gauge with a smaller range to get exact PSI). When I engage the PTO, pressure jumps up to 350PSI regardless if I am attempting to turn the steering wheel left/right or not. You suggest inspecting the solenoid PTO valve. What exactly does this solenoid valve do?

Another thing I looked into was the system relief valve which appears to be normal and no indication of any debris in the port. What is odd though is the amount of air burping through the system relief valve port and the steering test port when the tractor is running. Almost as if it is sucking air or lacking oil supply?

When I loosen the hydraulic line from the steering cylinder and turn the steering wheel, a combination of air and oil come through. I am starting to get hung up on this air burping through the system and wondering if this is the problem? It would explain the issues I am experiencing. If so, where or how is this air or lack of oil originating?

 

[oldnslo]

In most cases air would be coming from a leak in the inlet or suction side of the pump. Do you recall how the inlet of this pump is routed? Are their O-rings that could have gotten pinched or damaged during assembly? A down tube to the suction screen possibly not tight, etc.

Shaft seals on the pump can also allow air in if damaged.

On certain instances you can get a Venturi effect where oil flowing at velocity can draw air into the system but not real common.

 

[smashedtoe]

There are a total of five o-rings on the charge pump and then an o-ring on the suction line to the sump. I am going to tear the tractor down again and make sure these are all fine as this makes sense. Aside from time, I really have nothing left to lose!