Well my friendly old farmer neighbor bought a man lift ... JLG 40HA The guy who sold it to him said it did not move but everything else worked. They got there ... the boom went up then nothing ... no more up or down. He still wanted it and bought it. Well first, two hoses to the boom cylinders had leaks ... her fixed them. Now the boom still will not move. I went over to take a quick look. So it looks like it has safety valves to lock the cylinders ( two lines run to a block on the cylinders .. one valve on each side ...then lines go from the block to the cylinder ends). So my guess is the valves are not getting pressure on the pilot end to open them up. Everything else moves (boom goes in and out, basket moves) and if you break the line I get fluid so all that I assume works. Since he replaced lines, could it be an air block? I see a couple of allen head screws on the block ... are these air bleeds? Anything I should be looking at? I have the manual but no hydraulic diagram in it. Any help would be greatly appreciated!
It's been 30 years since I've worked on one of those machines so I can't offer anything specific. However, I can say, yes, that block on the boom hoist cylinder, called a manifold, does have what's known as a "holding valve". That holding valve locks oil in the cylinder to prevent catastrophic failure if you were to blow a hydraulic hose. With that holding valve you can disconnect the hoses from the cylinder and it won't come down. With that said, do not remove anything other than a hose connections at that manifold. With the boom elevated everything is under extreme pressure. The hex plugs you see are typically to make internal drilled ports inside the manifold, then once drilled, they're plugged. Again, don't remove that stuff. I don't know if you still can, but you used to be able to go to the JLG website and download manuals. But be warned, if you do find the service manual for that machine, the electrical diagrams for JLG machines of that vintage are very difficult to work with. Even with experience of working with them back then those diagrams still gave me a headache working with them.
Hi, I had the same problem after changing the holding valve on the main boom. Took weeks of messing around, but I'll tell you what I did just in case it helps. On my 40ha the platform automatically levels as the boom is raised or lowered. This is where I had an air lock. It has been quite a while, but as I remember it, I did as you did to verify that oil was going where it was supposed to. Then I ran the platform level all the way down and then back again - to push the air through. Amongst doing this, I was also sporadically attempting to move the main boom, so I cannot give you an exact sequence as to how it came to work. But, basically I ran the platform level to push the air out, then attempted the main boom and repeat. It took a lot longer to work the air out than one would expect. I would have expected an almost instantaneous response, but it took holding the switch on for 5-10 seconds (estimating here) to get enough pressure built up to compress the air enough to move the fluid. After about five repeats of this procedure it surprised me by starting to work. Hope that is all you need.
I believe there is a hidden recovery toggle switch on the back of the ground control panel.
Thanks! I will drop by his place in a this weekend and see how it has been working. They needed it ASAP to get the trusses up on a barn they were building. I got it working except for the drive. They ended up towing it around with the tractor and using it just for lifting. I will need to see if the air is worked out or ???
I will take a look ... thanks!
You're on the right track - those are pilot-operated counterbalance valves (also called load-holding or overcenter valves) that prevent the boom from free-falling. They need pilot pressure to open and allow controlled movement. Your issue: Pilot signal isn't reaching the valves to release them. Here's what to check: Pilot lines: Trace from the main control valve / pilot circuit to the counterbalance valve block - disconnected, kinked, or reversed during hose replacement Pilot pressure: Should be 300–500 psi to crack the valve open; insufficient pilot = valves stay closed Air lock: Possible but less likely - counterbalance valves are normally spring-loaded closed, so air alone wouldn't lock them Allen screws: Likely adjustment screws for cracking pressure, NOT bleeds. Don't turn them unless you know the spec - improper setting causes dangerous free-fall or complete lockout. Quick test: At the counterbalance block, carefully crack open the pilot port fitting (small line) while commanding boom up - if you get no oil, pilot circuit is disconnected/blocked. Safety critical: Never bypass these valves - they prevent boom collapse. Fix the pilot issue properly.
Thanks so very much for the advice! I have been tracing lines ... really wish I could find a hydraulic schematic. I have the manual but it does not have one (it does have an electrical schematic). At least I think I have a good handle on how it should work. I found a some small pieces of what I am guessing is rubber in the oil. I have changed the filters ... thinking I may need to flush the system. I found a few splices in the houses ... wondering if they didn't clean the houses when they repaired them. Thanks again!
I have a JLG boom lift that does not maintain a level platform as the boom is raised. I will set it level at stowed position , then as the boom is raised the basket slowly starts to point up. When lowering back down to stowed position it realigns to the level position it was set at. I understand there is two counterbalance valves mounted onto the leveling cylinder and I’m thinking to check them out next. Anyone have any experience with these ?