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I just bought a JD 318 with a jerky hydrostatic unit. I've changed the fluid and filter and no change in performance. I did some searching on the web and it seems everyone just replaces the hydro pump when they have problems. I've run acrossed a couple websites that claim they rebuild them so I know they are rebuildable. Has anyone attempted to rebuild one themselves? I've rebuild several hydraulic cylinders and quite a few automatic car transmissions (including some tricky overdrive units) and I can't believe they can be that difficult. I also have a fullsized milling machine and lathe at home in my shop so if any major machine work needs to be done I should be able to do it myself. Right now I'm waiting for my tech manual from the Deere dealer to see if it has any info on overhalling the hydro pump.

Let me know how it goes, the hydro in my 16 year old JD STX46 is jerky and has just about lost reverse. A new hyrdo is $1300 If they can be rebuilt on the cheap I'd like to go that route.

Before you dive into the pump, you might want to check the linkage to see if any of the components are worn or broken. Also, later model 318s had a hydro damper kit that smoothed the jerkiness out of the system. (The kit is no longer available.) If yours has the kit, it's possible the damper cylinder is shot. When you get your tech manual, take a look at the portion on the hydro drive linkage. The 318s had four different setups during their production run.

Stash said:

Before you dive into the pump, you might want to check the linkage to see if any of the components are worn or broken. Also, later model 318s had a hydro damper kit that smoothed the jerkiness out of the system. (The kit is no longer available.) If yours has the kit, it's possible the damper cylinder is shot. When you get your tech manual, take a look at the portion on the hydro drive linkage. The 318s had four different setups during their production run.

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If it is jerky it doesn't need to be rebuilt. It would be slippery if wearing out. Late models were very crisp, precise, jerky.
gp

Ok, heres an update on what I found wrong. I decided to take the tractor apart this weekend and found the roll pin that holds the control lever on the hydro pump was half gone. This made the lever have about 1/4" of travel without moving the shaft. A simple $.66 roll pin is all that was needed to fix the problem. Now it work has it should.