hydrostatic jerks after reversing direction

[sylft]

Looking for some troubleshooting help. Bought a used jd4120 w/750 hrs recently. I've been running a rotary cutter with jd4120 hydrostatic. After warmed up, when I reverse and then go back to forward, the hystat jerks like the pump is going back to neutral. If I keep my foot on forward pedal after it jerks, it will continue driving normally. I have not done any troubleshooting yet, but looking for advice where to start.

 

[oldnslo]

When switching from forward to reverse is everything still smooth? I presume you still have full travel speed and power to the ground since you did not mention either of these items getting worse.

Best guess would be something in the HST control circuit is sluggish or binding. Not familiar with the controls used on the JD models but suspect charge pump pressure is used for the control pilot pressure. If an orifice is slightly blocked, or a small leak some where in the control circuit could cause this problem.

If JD installed a gauge port in the charge system I would start by monitoring the charge pressure to see what it does while doing different functions both with cold tranny and warm tranny.

 

[C4Ranch]

What's the service history? My 1st step would be to know the fluid and filters are good. Then go from there.

 

[boggen]

are you moving extremely fast in reverse, and then going straight to forward motion? or vice vs?

is the pedal / lever loose? double check the linakages from pedal/lever that goes to a switch or some sort of valve. see if anything that needs to be tightened up or is sticking.

John Deere Home Page Redirect
north america english
*top of page* parts (menu shows up) agricultral parts and attachments
*middle of page* search parts
*a link to this point* Agricultural Parts JohnDeere.ca
search parts
enter 4120 into search box
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and a way you go for part diagrams, and part numbers. ((for what ever reason the site seems extremely slow today requiring some extra reclicking in some cases.))

a "shop manual" for the tractor would be a good investment, if you have general skills and tools to work on the tractor. "shop manuals" tend to be a little costly, but generally give full diagrams and step by step instructions of tearing things apart and putting them back together, along with inspection and testing of parts. and general trouble shooting pages.

 

[sylft]

Thanks for giving me some direction. A few more details - occasionally I back up to cut a patch of weeds, then when I drive forward, it seems to go smooth for maybe 3 secs, then the machine jerks almost like I let off the pedal very quickly. If it was due to me on the pedal, I would not expect it to happen so consistently. Forward speed does not seem to matter either, but typically I'm going 1-3 mph. Same speed in reverse.

When I bought the tractor at a sale, there was no service history with it but looked like it was used lightly and maintained very well. I will start with changing fluid and filters. It is due for it in about 20 hours (at 800 smr) so I just planned to get through cutting and then do the service. Pedal linkage might be a good possibility also. I'll have to study it more to see if there's a place to check charge pressure.