4200 HST Charge Pump Failure

[hthmontana]

My JD 4200 HST won't move in forward or reverse after I engage the pedals once. All of the implements work fine and both the rear and mid PTO function normally. I flushed the system, changed the filter, and cleaned the suction tube screen (It was clean but I took an air hose to it to make sure any minor stuff was out of the screen. The fluid is near the top of the sight window and clean. I checked the charge pump pressure and on initial startup the pressure reads about 110 PSI as soon as I engage a drive pedal the pressure drops to 50 PSI and doesn't recover until I turn the engine off and then restart the engine (I am not sure if that is normal since the service manual only talks about checking the pressure and never mentions engaging a drive pedal, I assume the pressure should recover when the pedals return to the neutral position). I split the tractor last year to replace a broken shift fork, what a pain... SO to be clear the issue is that the drive pedals don't engage the HST to move the tractor fwd or reverse, in some cases it will work the first time a pedal is engaged and then stop and in some cases it won't work at all even after a fresh engine restart. Any help you can offer is much appreciated! I am hoping I don't have to split the tractor again and if I do need to replace or repair the charge pump I would sure like to see a part number or the location of the dang thing cause I don't see it in the service manual.

 

[oldnslo]

The charge pressure should stay reasonable stable from stationary to driving. The charge pump is there to keep the HST circuit full of oil. Why the pressure drops and the tractor does not move would indicate this flow is going somewhere. Trying to determine where the flow is going will be a challenge.

If you do not have a hydraulic schematic for this tractor I would suggest getting one? This will help identify where the charge pump is and potential leak paths. On some models the power steering pump is also the charge pump, on other models there is a charge pump integral to the HST pump. I do not know how the 4200 HST is made.

 

[dfkrug]

The good news is that you do not have to split the tractor if the charge pump is the problem.
It is indeed part of the implement pump/PS pump, on the left front of the tractor. I would
have initially suspected your inlet screen, but you say you have cleaned it. Also note that
the inlet manifold to this pump is rubber, and can get holes in it.

This photo shows the rubber manifold, with the pump at the left end of it.

 

[hthmontana]

Well the strange part is that all of my implements work just fine and I would expect that they would fail to work too if the charge pump was failing. Keep in mind when the tractor will no longer drive in FWD or Reverse the charge pump pressure reads 50 PSI and yet all the implements work fine. Here are the service manual images:

 

[oldnslo]

hth,
Do you get any noise while trying to move like oil being pressurized. If charge stays at 50 PSI the tractor should still be trying to move. Have you tried moving the Hi-lo range selector to see if this makes any difference?

Can you take a pressure reading of the drive circuit like you are checking the forward or reverse relief valves? Looks like deere installed test ports by the motor for doing this. Curious to see what this pressure does since it should increase to move.

3rd picture shows line for charge pressure connected to the steering unit so suspect return line from steering goes through the oil cooler and then feeds the charge circuit.

 

[dfkrug]

Well the strange part is that all of my implements work just fine and I would
expect that they would fail to work too if the charge pump was failing.

The power steering pump shares the same outer housing and main shaft as the implement pump,
but the rotors and seals are not the same. One could fail without the other failing. The PS side of
the pump uses a flow divider to feed the HST charge circuit.

Is the PS weak?

 

[hthmontana]

I don't hear oil trying to be pressurized. I think the charge pump is internal otherwise the test port would not be on the transmission case?

 

[hthmontana]

The power steering pump shares the same outer housing and main shaft as the implement pump,
but the rotors and seals are not the same. One could fail without the other failing. The PS side of
the pump uses a flow divider to feed the HST charge circuit.

Is the PS weak?

Are you sure about that? PS is not weak at all and the Charge Pump test port is right below the HST pump test ports.

 

[oldnslo]

hth,
The flow is split in the transmission housing so yes the same pump feeds the power steering and charge system. Did you ever check for pressure in either forward or reverse loop? I.E. the high pressure drive loop. Unless you have the brakes locked or are trying to pull a load a 2,000 or 3,000 PSI gauge would be sufficient for checking to see if you are getting any drive pressure.

 

[Egon]

Have all the control levers and three point rate of drop valve in the proper position?

 

[dfkrug]

hth,
The flow is split in the transmission housing so yes the same pump feeds the power steering and charge system. Did you ever check for pressure in either forward or reverse loop? I.E. the high pressure drive loop. Unless you have the brakes locked or are trying to pull a load a 2,000 or 3,000 PSI gauge would be sufficient for checking to see if you are getting any drive pressure.

Dividing flow between the PS and charge pump happens inside the PS valve, which is on top
of the trans tunnel. The 3 test ports (charge, forward, and reverse) are all accessible inside
the big hole in the trans tunnel underneath, right next to the HST spin-on filter.

Note that the PS runs at only 1500psi, but the HST runs at 6000psi. Other CUTs I have
opened up have HSTs that relieve at 4000-5000psi. I had to get a 10K psi gauge.