why does my jlg 450a main boom settles

[donald thompson]

1 have a 1999 JLG 450A, #0300041500, dual fuel lift. the upper/main boom settles about 1.5 feet in 5 to 7 minutes. slower if the lift is shutoff and no one is in the basket, and a bit faster if the boom is extended all the way out.

 

[skata]

Does it settle and stop? Or go all the way down?

 

[donald thompson]

it will go all the way down

 

[skata]

It's either worn seals inside the cylinder, or something wrong with the lock valve on the cylinder, that's the valve block mounted on cylinder that prevents the cylinder from moving in case of hose burst.

 

[BigWrench55]

Leaning towards the holding valve on the boom cylinder.

 

[donald thompson]

if my memory is right i think we replace the seals on that cylinder about a year ago and the seals on the holding valve. that does not mean i did it right

 

[donald thompson]

more info maybe? when the lower/mid booms are lower, it bounces. if i press the boom retract switch it stops the bouncing, even if the boom is not extended. release the switch after 2-3 sec and everything works fine until i do it again.

 

[Txhayseed]

I would check the counter balance valves.

 

[92U 3406]

2nd the counterbalance valves. Replaced many of those due to drift or leakage.

 

[donald thompson]

counterbalance valves, holding valve, lock valve, these are all the same valve right? in the block on the cylinder?

 

[Vetech63]

counterbalance valves, holding valve, lock valve, these are all the same valve right? in the block on the cylinder?

Correct. Different manufacturers call them different names but they are the same thing.

 

[donald thompson]

thank you all. i guess i'll order a new valve and see if that fixes it

 

[donald thompson]

replaced the holding valve and it appears to have worked. thanks to everyone