JLG 600A main tower not dropping

[DavidL]

When unit is cold, I can operate all controls fine. I will get in the platform and spend an hour working at height. When done, dropping down, all works with exception of lowest, main tower cylinder won't retract. It will go up but not down....I checked I have enough hydraulic fluid when all is down. Main tower after hours of sitting will go down and act normally till next time I am in the basket...

When at the lower station, I can try to lower the tower. I see pressure in the hydraulic line to the filter that changes when the switch is toggled. So the switch is fine.

Getting tired of jumping down from height...

If it was a limit switch, I would predict it wouldn't change once the unit sits (gets cold).

Sometimes just an engine restart and it will again work. Seems that's a matter of how long it was being used (heat?).

Any safety switches come to mind that might cause this?

I noticed that sometimes the boom retract (top) won't work too...but most of the time it will, even when lower tower has issues.
Since most hydraulics work....I am eliminating pump / filter / fluid from the issue. Driving seems to always work.

I wish the machine supported the diagnostic tool....and that I had one for 10 minutes...

 

[TheFixer]

Mine has a sequence valve for the tower lifts. One of the tower lift/extend has to be raised all the way before the other. Same as going down.

Mine are hydraulic push button type limit valves. I did see in the manual, some versions have electrical switches.

One of mine was leaking, and I rebuilt it pretty cheap. Maybe you have one binding, or a cut o-ring inside that is causing it to stick. They are just a simple spring return type piston.

If you find it, you should be able to raise the tower from the ground, then operate the valve manually to see if it is sticking, but not when you are actuating controls. It should push in and pop out.

If I operate mine in the wrong order, I can hear the hydraulic pressure build. That would be a good indication if your control valve for that function is working.

 

[92U 3406]

^^^This

Side of the tower boom there should be a little button on the side of a valve block. If that little button isn't pushed in all the way (ie the tower boom isn't fully retracted) then the tower boom won't lower. It will raise but not lower.

 

[TheFixer]

And I forgot to mention it is adjustable.

 

[DavidL]

On the side of the sliding boom (top), I see an electrical cable running to what I assume is a switch about half way down the length of the boom. Is this the limit switch? Looking at the schematic now....

 

[92U 3406]

That sounds like the limit switch to control the dual capacity envelope if you have that option. The thing I'm thinking of is a hydraulic valve block with a little poppet valve that contacts a stop bolt. It should be located down on the base of the tower boom, behind the counterweight. With the tower boom fully retracted that poppet should be pressed in by the stop bolt.

 

[DavidL]

well, I might have found the problem....I took the cover off of that limit switch and found some grass in it...I extended the boom out horizontal and found a bird had made a dense nest in the nose of the boom...tight enough that I think the limit switch was not finding the hole to know the boom was retracted. 1/2 hr with coat hanger, mechanics pickup tool and needle nose pliers and I hope I have most out. Not sure if I can push it out with a power washer...

After the thundershowers pass, I'll take a look for the valve you are describing.
Thanx for the general description of the system...that is what got me closer to the problem.

Wondering if this is also the root cause for the machine going into crawl mode and not coming out when all is down...because the boom thinks it's still out.

Fargin birds...this is for the birds.

 

[DavidL]

The bird nest wasn't the issue...all cleaned / powerwashed and same problem. I will wait until the tower will drop and then check out the hydraulic valve at the joint. I see what you both are talking about now.

 

[DavidL]

The hydraulic stop valve is a B_tch to remove...there are three allen heads holding it on...none of them will break free including with a impact. Didn't want to apply heat as it's an aluminum body. I was able to remove the plunger and there was a thread chase cobbled up with the spring. The circlip was also very loose on the shaft and the shaft had some burrs on it from the circlip. Cleaned up all of that. I can't remove the other side (where I think the actual o'ring might be) as I can't remove the aluminum housing from the machine...I'll test it today. Not thinking I did anything that will fix the problem. The plunger was moving fine before messing with it, and the adjustment was stop bolt tight to the stop when tower was down...So I think that was always fine.

 

[DavidL]

Well, just used it for four hours and all worked fine...so who knows. Fingers crossed. thanx for the tips!!!

 

[TheFixer]

You don't need to remove the AL block valve body. Just remove the plunger like you did. I put ne O-rings on mine since it was leaking. It needs to move freely. Hopefully that fixed it.

 

[DavidL]

Hi, Thanx for the feedback.
I was able to take the plunger side out...there wasn't any O'rings that I saw. What I saw was a stainless plunger, the "nut" that has the seal to keep fluid from leaking out the plunger, a stainless circlip that keeps the plunger from getting pushed out of the housing, and a split steel tube inserted into the end of the plunger (the housing side). I didn't take off the "nut" off the opposite side of the AL housing. Possibly that is where the O'ring is?

Anyway...since doing this, it has been working correctly....so maybe the thread I found in the housing, or the damage to the plunger from the circlip kept the valve from doing it's job correctly.

 

[92U 3406]

Debris in the hydraulics will cause all kinds of funky stuff. I worked on a 450AJ a while back that had everyone at the shop stumped as well as our tech support team. Ended up finding a tiny piece of o-ring getting intermittently stuck in a 2 way check valve in the main manifold.