Lull 644B-42 Low steering pressure

[Andy1845c]

Still working through issues on this machine - Where I am at right now is the steering is difficult in either 4 wheel modes. It will steer a little left or right, but then the wheel binds up. In front steer it steers fine. If I jack the back axle up and get the pressure off the wheels it steers fine in 4 wheel modes. So I have verified its not something mechanically bound up.

I took the orbital valve apart and put seals in it. It stopped leaking but didn't improve the steering.

Steering pressure is only about 1100psi on the test port.

Kind of at a bit of a loss on what to check next. In this old thread TractorMech mentioned the compensator valve can cause hard steering.

Lull hydraulic pump

Had a guy ask me to put a hydraulic pump on his lull high reach forklift. Said he has the pump off already and a new one there. Problem is the first pump he says split the case so he put a new one on and fired it up and that pump split also. Im guessing the first thing i should do is back main...

www.heavyequipmentforums.com

I'd like to check this but am not sure where it is. None of the manuals I have found breakdown the valves on the end of the pump.

My serial is ending 1136 - so this is a later machine. Seems they had a few iterations of the hydraulic system over the production run and this one is less well documented in the manual on the JLG site. Is there another manual out there?



This is what my pump looks like.

This is the section of the schematic for the valve block in the end CF is the pressure port on the orbital valve.

 

[tractormech]

If front steer works fine then check to see if you have a blown piston seal on one of the rear cylinders. If it's working good on front steer only then your problem is most likely back there. Nothing wrong with the steering system itself

 

[Andy1845c]

I thought about that- but if I had a cylinder issue do you think it would steer ok in 4 wheel modes with the back jacked up? It’s almost as if the lower pressure is enough to run one axle, but when you series the cylinders as it does in 4 wheel modes, it’s just not enough.

The seal kits for the steering cylinders are available, and I did consider just repacking them to rule this out.

I need to do some more research but I am now getting suspicious there is an issue with the priority valve.

I’ll report back when I know more.

 

[tractormech]

It would steer if it's jacked up. No weight to fight. Seriously doubt there's a priority valve issue. The front steer would probably be weak too.

 

[Andy1845c]

Well I finally made some headway on this today.

I had some others also say it was likely a blow piston seal. I extended each steering cylinder to full stroke and then popped the line off and tried to steer more to see if I had fluid bypassing. I had none on either side. So this morning I pulled the valve block off the pump and one at a time took all the spools out, looked them over and cleaned them all with brake cleaner and compressed air.
Was really hoping to see something obvious, but I didn't.

Put it all back together and it now steers without binding.
Unsure exactly what I did.
I am suspecting the pressure relief on the CF line was opening too soon. But I am not knowledgeable enough to say for sure. It is adjustable but I did my best to not adjust it.

I now have 2200 PSI on the test port where as before I could only get to about 1100.

Steering seems maybe a little slow - but I haven't driven enough of these to compare. At least its working now.

Just wanted to follow up in case someone else has the same issue.

There are a lot of small parts in that valve and I know of no manual for it - so pay attention to how things come apart and work on a surface where ball bearings aren't going to escape.