Char Lynn 4000 series (Bobcat 743) Shuttle Piston

[onepoket]

So, I have been having drive issues with my 743, it is very jumpy and starts to buck on change of direction to the point of being undrivable for any practical use. I have to creep as slow as possible to get it in and out of my garage without bucking and potentially hitting the door jam.

I had replaced the motor seals last fall getting the thing ready to plow for the winter but when I did that I didnt do anything with the shuttles because the plugs were corroded and I had no replacement plugs.

My feeling is that the shuttles do something with easing the change of direction and maybe if it was sticking that would account for the sudden "pop" into gear when going from forward to backward. The problem is worse on the left motor and that side seems to squeal.

Anyway, I got the thing apart now, machine drained and I have the shuttle out. I only got half the shuttle assembly out which is the plug, the dash pot, spring poppet and shuttle piston. From the diagrams I found there should be another poppet, spring and pot opposing it. Not sure if I should be able to fish them out from the outside or not.

The other thing I see on my piston is that it is not symmetrical. I am unclear if that is how it is made or if it is because of wear. The piston had a sort of channel before a cone shape, on the one side the channel is very defined on both edges, on the other side it is noticeably less defined on one edge.

So, I guess my question is, is this normal, should I replace the piston, is the piston what is causing my whining on the left motor side and drive issues?

I had another theory too that maybe the left drive issue is from a stuck parking brake disc? Thoughts?

I plan on draining the chain case and servicing the brakes too but before I move on from the drive shuttles I want to make sure I don't need to order parts.

 

[Tazza]

I'm not sure you're looking in the right place, the shuttle valve is for changing direction, not when moving.
I'd first check the linkages from the steering levers back to the pump, ensure they are tight and no wear, that can cause bucking of the machine.

 

[onepoket]

I'm not sure you're looking in the right place, the shuttle valve is for changing direction, not when moving.
I'd first check the linkages from the steering levers back to the pump, ensure they are tight and no wear, that can cause bucking of the machine.

Tazza, yeah that is the bigger problem, changing direction.

I redid all the steering linkages last year, new bushings, cleaned and lubricated all the parts.

So, the real issue comes about when changing directions, the left side is worse than the right. The left side whines and screeches pretty bad. When I removed both shuttles from the motors today I noticed that the left side shuttle had the more defined channel toward the inside, the right side had that part on the outside. I was unable to find any documentation on eaton's site regarding the orientation of the shuttle piston. Both sides on my machine had similar sizes when checked with calipers but the orientation was opposite. Maybe this is correct, I don't know.

So, more detail on the problem. All winter I used the machine to plow. It drove fine, I had little "bucking" problems if any. But, the oil I have is, according to my friend at bobcat, too thin. I had ISO 32 in it. My friend told me I should be running ISO 46. Also, I had a leak at the port block (mine is the one with the port block on the left, not on top as you face the machine). During the winter this caused a nice little puddle under the machine but I just kept topping it off when I needed to run it.

My guess is that the cold made my oil run a little thicker and the leak acted as a relief of some sort.

I have fixed the port block leak, it was the back bottom fitting, but I remove the whole block and cleaned the whole thing, new o-rings, etc.

After I fixed the leak, my drive problems started. So I wonder whether the leak allowed pressure relief and it drove smoother. I could be totally off as well, but I wanted to give you a more detailed sequence of events.

The machine will be without oil for at least another week, I'm waiting on a tilt cylinder seal kit. I did the lift cylinders already, but figure while it is drained do as many things as possible.

I also have removed one of the big 1 1/4" plugs on the pump to check the relief valve there. I have had a hell of a time getting them loose and that is why it is just one so far. Any tips on getting those suckers out?

Thanks for sharing your considerable experience!

 

[onepoket]

Tazza, yeah that is the bigger problem, changing direction.

I redid all the steering linkages last year, new bushings, cleaned and lubricated all the parts.

So, the real issue comes about when changing directions, the left side is worse than the right. The left side whines and screeches pretty bad. When I removed both shuttles from the motors today I noticed that the left side shuttle had the more defined channel toward the inside, the right side had that part on the outside. I was unable to find any documentation on eaton's site regarding the orientation of the shuttle piston. Both sides on my machine had similar sizes when checked with calipers but the orientation was opposite. Maybe this is correct, I don't know.

So, more detail on the problem. All winter I used the machine to plow. It drove fine, I had little "bucking" problems if any. But, the oil I have is, according to my friend at bobcat, too thin. I had ISO 32 in it. My friend told me I should be running ISO 46. Also, I had a leak at the port block (mine is the one with the port block on the left, not on top as you face the machine). During the winter this caused a nice little puddle under the machine but I just kept topping it off when I needed to run it.

My guess is that the cold made my oil run a little thicker and the leak acted as a relief of some sort.

I have fixed the port block leak, it was the back bottom fitting, but I remove the whole block and cleaned the whole thing, new o-rings, etc.

After I fixed the leak, my drive problems started. So I wonder whether the leak allowed pressure relief and it drove smoother. I could be totally off as well, but I wanted to give you a more detailed sequence of events.

The machine will be without oil for at least another week, I'm waiting on a tilt cylinder seal kit. I did the lift cylinders already, but figure while it is drained do as many things as possible.

I also have removed one of the big 1 1/4" plugs on the pump to check the relief valve there. I have had a hell of a time getting them loose and that is why it is just one so far. Any tips on getting those suckers out?

Thanks for sharing your considerable experience!

Also, for anyone interested here is the doc on the motor:

https://www.eaton.com/ecm/groups/public/@pub/@eaton/@hyd/documents/content/pll_1333.pdf