Bucket truck boom drops by itself.

[COASTBUSTER]

I have a GMC 40 foot height bucket truck. When I get in the bucket and raise up,to any height, then let go of the lift lever,the bucket slowly drops back to its home rest point. Don't see any leaks. Something in the diverter valve? No clue. Any body have any knowledge about these things? Dan:cool2:

 

[J_J]

If your cyl are good, the spools in your valve may be worn and leaking.

If you raise hte boom and shut off engine, does the boom stay up after shutdown ?

 

[LD1]

Not sure what year you are working on, but most lifting devices have pilot operated check valves for the lift. At least on every piece of aerial lift equipment meant to lift a person that I have seen has one. Perhaps it has failed? Or someone didnt know what it was and removed it?

Got any pictures of the bottom end of the lift cylinder?

Regardless of how bad seals in the cylinder are, or how wore out the control valve is, with a pilot operated check valve, the boom should stay where it is put. And total hydraulic line failure or blow out should NOT cause the boom to move or drop at all.

 

[CJONE]

Not sure what year you are working on, but most lifting devices have pilot operated check valves for the lift. At least on every piece of aerial lift equipment meant to lift a person that I have seen has one. Perhaps it has failed? Or someone didnt know what it was and removed it?

Got any pictures of the bottom end of the lift cylinder?

Regardless of how bad seals in the cylinder are, or how wore out the control valve is, with a pilot operated check valve, the boom should stay where it is put. And total hydraulic line failure or blow out should NOT cause the boom to move or drop at all.

THIS.
Also there is a way to bypass the cyl lock valve and the valve may be set to bypass. If you can lower the unit without the system running, the lock circuit is bypasses or missing like LD1 stated. No way in h--l would I operate it in that condition. I have also seen them bypass internally with bad piston packing or the piston has come loose inside the cyl. CJ

 

[COASTBUSTER]

I don't have any pictures as the truck is upstate NY at my hunting cabin. Going up in two weeks and will take pics.p.s nothing was removed from the unit. It was working fine then started to slowly drop. Somethings rotten in Denmark.

 

[dfkrug]

When I get in the bucket and raise up,to any height, then let go
of the lift lever,the bucket slowly drops back to its home rest point.

Does your manlift use an electric-over-hydraulic system or a pump/valve arrangement that
is constantly driven off the engine?

My Genie manlift is the first kind, and for safety it uses a normally-closed solenoid-operated
spool valve that must be activated for the lift cylinder to go down. It is independent
of the lifting valve.

If yours is the second type, I agree with LD1 that you should have a PO check valve in the
system somewhere.

 

[COASTBUSTER]

It's the latter. I will check for cylinder leakage next time I'm near the truck. Just started delving into this problem so I'm not to familiar with all the workings yet. Thanks for the input. Dan