John Deere 862 diff lock

[DozerTech78]

Anyone familiar with the old 862's? I'm working on one now. The differential won't lock, when I hit the pedal it goes straight to the floor and doesn't come back up. Its like the system has zero pressure. I know that there's a piston inside the case that locks it together but not sure where I could test the system pressure at if there's a test port. I only have a parts book available, no service manual. Does anyone know where I could check pressures at? Or any ideas of why I'm getting a dead pedal?

 

[Delmer]

This is based off a different JD diff lock, so use with caution. There might be a test port on top of the differential housing near the small hydraulic lines from the diff lock valve. The pressure should be 500PSI engaged. You can disconnect the medium pressure line from the valve to the diff and attach a gauge directly to read the pressure too, of course that eliminates the diff so it might change the feel of the pedal etc. which would indicate to me that the piston is leaking.

I might be inclined to pull the valve and take it apart. Not usually the first thing you do, but it's probably rusted up pretty good, hasn't been used regularly enough, and maybe just has a broken spring or sticking from rust. It should pop back up without any hydraulic pressure at all. The valve is basically on or off, the spools regulate system pressure down to 500PSI, there's no "feathering" of the diff lock valve like a brake valve, AFAIK.

Any history on the problem?

 

[DozerTech78]

Thanks for such a quick reply, maybe I will pull the valve apart like you are saying. At first i was under the impression that the system had to be under pressure for the pedal to rise meaning the machine had to be running, but I tried and still nothing. Its in the process of being moved to a new job and I have no idea the history. They said it worked on a previous job now it just quit. Looking at the parts book i may be able to come up with my own schematic.

 

[DozerTech78]

Hate to double post but just a quick update: pedal does spring back now but there still feels like there is absolutely no pressure behind it. In thinking now ill move on to the pressure testing. There aren't any obvious leaks so I'm leaning toward the piston seal at this point. Ill keep updating

 

[Delmer]

The piston seal has a "recapture" return system to allow the leakage to return to the hydraulic system instead of filling the axle fluid with hydraulic fluid. That's the second (low pressure) line going to the differential, it might also tee into the return from the valve. Take that off and measure the flow into a bucket if you suspect piston leakage.