JD 4300 stops moving in forward and reverse after a few minutes - engine still runs

[pmsmechanic]

I'm getting suspicious about the charge pump but I'm only guessing. It seems like after a time it quits charging the hydrostatic pump but how to figure that all out?

 

[texasranger556]

The smaller gear pump on the engine supplies the steering and charges the hydro. There is a poppet valve on the front of the transmission near the spin on hydro filter that regulates the charge pressure. I would suggest pulling that out, paying attention to if there is any debris or if the spring/valve is not able to move freely. Allen head plug.

On mine it was jammed up, only figured that out after splitting the tractor and lapping the valve plate on the hydro pump. Checking that first would have saved a day of work.

 

[texasranger556]

I was able to find the post I made while troubleshooting my transmission issue 5 years ago. It includes the schematics from the service manual.

https://www.tractorbynet.com/forums...344133-jd-4400-hst-transmission-chatters.html

 

[GreenRust]

The smaller gear pump on the engine supplies the steering and charges the hydro. There is a poppet valve on the front of the transmission near the spin on hydro filter that regulates the charge pressure. I would suggest pulling that out, paying attention to if there is any debris or if the spring/valve is not able to move freely. Allen head plug.

On mine it was jammed up, only figured that out after splitting the tractor and lapping the valve plate on the hydro pump. Checking that first would have saved a day of work.

Thanks for the detailed response. Am I correct in guessing you are talking about one of the items I have circled in yellow?

How would I go about checking? Is it as simple as moving them around with the right size hex wrench or is there more to checking them. As you can probably tell this is all new to me.

 

[jthibodeau89]

The charge is definitely provided by the steering half of the engine mounted hydraulic pump. Before opening your wallet and sending to the repair shop, i recommend that you check the charge pressure and relief poppet as Texas suggested. Charge circuit pressure is relatively low, just need a 200 psi gage and appropriate fitting. Did you check the main suction tube for blockage when you replaced the rubber manifold? Power steering is low flow, so steering will work fine while hydro does not have sufficient flow.

Since you stated that the pickup screen is absent, it is highly possible that the charge pressue relief poppet may have debris in it. Or any other relief valve for that matter. Oil flowing across the relief will heat up very quickly.

 

[jthibodeau89]

Greenrust, the charge port is the small one in the center, between the circled ones.

These are all pressure test ports. Do not operate without either the plug in place, or an appropriate gage/adapter installed. The circled ports have 6000 psi oil pressure behind the plug when operating correctly.

 

[texasranger556]

In the thread I linked above I included the service manual instructions for checking the pressure. The charge pressure check port is the middle one. From memory the charge pressure regulator/valve is the identical plug right above it. It's easy to get out, just don't lose the pieces.

The charge oil goes through the spin-on filter prior to the charge pressure regulator, but without a suction screen the spin-on will clog up much faster and allow debris to bypass into the charge regulator and therefore the transmission.

This problem may very well have saved you from a transmission replacement...but still TBD.

 

[Ken]

Wasn't a John Deere but Kubota was used to cleaning pond and ran in water to the top of front tires.. after sitting a few weeks the clutch would seize and fail to move to release the pressure. for shifting forward or reverse. found after splitting the clutch housing the throwout bearing had rusted to the transmission housing. hydraulic pressure wasn't affected.
ken

 

[GreenRust]

I finally gave up about a week ago and decided to hand it off to the dealer. Turns out I wasn't even close. The return valve on the steering control unit failed limiting the amount of high pressure fluid moving back into the system. As luck would have it "The SCU has no serviceable components. If the SCU does not meet specifications, the unit must be replaced.". All things considered it could have been much worse.

 

[jthibodeau89]

Thanks for the update. Something else to consider. I assume this is the hydraulic 'valve' under the dash. Sounds like the failed component caused low precharge pressure.