Any help will be most appreciated. my understanding is that the tower boom and the mid boom work together off the same control switch. my 450a tower boom will lift up and down but the mid boom cylinder does not move in or out. I can watch the hose running to the mid cylinder and it jumps like it is loading up with pressure. I took the counter balance valves out of mid cylinder and looked at them ( looked clean and orings looked ok ) . I took the hydraulic lines loose at CB2 ( main control valve assembly ) checked pressure at cb2 which reads 1700 psi when i activate ground control switch. The tower cylinder still moved up so that didn't do me any good. I took the synchronizing valve catridges out and looked them over and did not see anything wrong with them. I think it is a hydraulic issue not electrical. what should I check next or where to start. thanks.
Just for wiggles, disconnect hose connection to LR port on re-level valve, plug hose and valve fittings with proper cap/plug, then see how mid boom reacts.
i will do it.
disconnected LR port on re-level valve and capped off and the lift did eventually extend mid boom. Tower boom went up first pretty smooth about halfway to its full extension and then when mid boom kicked in, it would move a few inches and then stop but if you keep moving the ground control switch it would eventually get all the way to full extension. I check the auxiliary pump ( barnes two section pump ) at both outputs and section of the pump that moves tower lift was reading 1800 to 2000 under load. The other section of the pump would read 2000 under load and when it deadheaded it would jump up to 2500 to 2700. Engine rpm ( just guessing ) 1800 to 2000 when I tested the pump. do you think my auxiliary pump is weak or do you think I have a bad cartridge in a valve body somewhere? lift function for the main boom lifts good but maybe the tower boom takes alot more pressure to move up and down.
Not sure what to tell you davis1676. I suggested blocking off the LR port because this would remove re-level valve from the circuit of normal operation. If it fixed problem completely then problem would be in re-level valve. But since it didn't, mid boom still doesn't respond proper, then problem has to be either one of the four counterbalance valves are at fault or piston seal in mid boom cylinder has failed. Not really sure how to advise further. Can't really switch counterbalance valves around as two of them are same part number, but the other two are different numbers. Hard for me to nail down what your problem is without being hands on with the machine.
well you have been a great help least wise I know it works and something must be wrong with the relevel valve. do you know what pressure should be at the cylinders for tower and mid boom?
I don't know what pressure should be at the cylinders, although that info doesn't matter much. The second pump operates tower lift, mid lift, boom telescope, manual platform level, and platform rotate. If you're able to operate these other functions with no issues, chances are the pump isn't the problem. Besides, even a slightly worn pump will eventually achieve full system pressure, just takes longer to do it. Also a worn gear pump will get hot to the touch fairly quickly when operating functions under load. Blocking off the LR port on re-level valve pretty much removes re-level valve from any fault in tower and mid cylinder extend. If the tower cylinder extends, but the mid does not, it seems you would have a problem with one of the counterbalance valves or bad piston seal in mid cylinder. That's just my take on viewing the hyd diagram.
ok, well with the re-level valve out of the way it does operate the mid cylinder and before it would not. Just goes up really slow. I will have time this weekend to mess around with it more. I work 12 hour days during the week and that takes it toll with the heat in Texas. Makes me sleep on my side of the bed at night anyways thanks for the help.
Did you resolve this problem. I am experiencing the same issue on a 2001 JLG450aj. We have replaced the holding valve on the mid boom cylinder but that didn't work ($600 part). Would love to know how you fixed your problem.
Just ran into this same problem, how'd you fix yours?
You’ve done a solid job narrowing it down. If pressure’s there but no motion, I’d suspect an internal leak in the mid boom cylinder or a restriction in the return flow path. Might also be worth swapping hoses briefly to confirm flow direction.