JCB 930 Freelift Mast Issue

[PeteG]

Hi,

We have a JCB 930 forklift with the 15’ Freelift mast. Serial number is 2365558. Issue is the center cylinder won’t raise before the outer channels do. The way I understand this mast is supposed to work is the inner cylinder raises the carriage to its maximum height, then the outer 2 cylinders continue on lifting. Currently the outers raise fully, then the inner. Once the inner is raised however, and you lower the mast, it seems to operate correctly until is sits overnight. There is a small hydraulic block where all the hoses divert from at the bottom of the mast. I am assuming this is a priority valve/safety (in the event of a hyd failure in the air) that I am thinking is the problem, but unsure of how to test this to be sure. My other thought was if the packing in the center cylinder is bypassing, however the machine only has 800hrs so I’m hesitant to lean that direction. Any help is appreciated, and Merry Christmas to everyone!

 

[92U 3406]

No rollers or chains seizing up on the freemast?

All the forklifts I've ever worked on use single acting cylinders, so if the piston seals were blown, you'd get no lift on that cylinder and oil would puke out the vent/vent line coming off the rod end of the cylinder.

 

[PeteG]

All rollers are spinning freely, and chains oiled up as well. One other thing I noticed, when the load is supported by the center cylinder, it falls pretty quickly when staying still. ~1” per couple minutes. When the load is supported by the outer cylinders, it hardly drops at all.

 

[Vetech63]

Post a pic or 2 of the mast and the cylinders so we can see the set up. I just had a 930 recently with horrible drift, but this one was from the tilt cylinders.

 

[PeteG]

Here’s a picture of the mast. The center cylinder you can see clearly, the other 2 cylinders are behind the mast frame rails on either side. Also the priority valve I am thinking is the issue is the silver box in the bottom of the picture.

 

[Vetech63]

Can you take and post a pic of the other cylinders please?

 

[PeteG]

Here’s a couple better pictures of the other cylinders. They are much smaller diameter than the center one.

 

[PeteG]

Had a little time today to get a parts breakdown. That small valve at the bottom is actually the “lowering control valve” and must control the oil to go to the center cylinder first, and then the outer two after. Looking at the breakdown, they show a plug where the center cylinder’s input hose is, so it leads me to believe the valve must reach a certain pressure and there is an internal spring that then redirects flow to the ports on the other side. I’m just not sure exactly how to test it to be 100% positive that’s the problem. I’m pretty sure it’s number 10 on the diagram, but if anyone has any input that would definitely be appreciated!

 

[TheWiseTortoise]

Had a little time today to get a parts breakdown. That small valve at the bottom is actually the “lowering control valve” and must control the oil to go to the center cylinder first, and then the outer two after. Looking at the breakdown, they show a plug where the center cylinder’s input hose is, so it leads me to believe the valve must reach a certain pressure and there is an internal spring that then redirects flow to the ports on the other side. I’m just not sure exactly how to test it to be 100% positive that’s the problem. I’m pretty sure it’s number 10 on the diagram, but if anyone has any input that would definitely be appreciated!

Did you ever find out what was causing the issue having similar problems with a unit pulled the control valve that you were talking about and inspected for debris or damage could not find any and reinstalled onto machine and started working correctly

 

[PeteG]

Yes was able to tear into the machine and figured it out, just slipped my mind to update the thread. Inside the center cylinder there’s a check valve at the bottom. You have to disassemble the free lift cylinder to gain access. So a seal kit, plus the check valve kit are required. If I remember correctly, there was also an o ring on the check valve that must be seated correctly, but the kit came with a new one.

I attached a diagram of the cylinder, and it shows the part numbers as well. I believe there is a mast number that you can give to the dealer to order parts from, but the JCB website has it organized somewhat based on height/mast type. This was for a 15ft free lift mast on this particular machine.

The mast now works in sequence with the inner carriage raising all the way up, then the outer carriage.