Have a Hyundai 55, the thumb will drop from full open to closed in 10-15 seconds. The cylinder was rebuilt a while ago and didnt change the problem. The head of the service department tells me that if the thumb opens and closes as it should with the switch then it needs the cylinder rebuilt. Nothing in the valves that control the thumb would cause it to bleed off. Is there any truth in that?
Unless you would have a hold valve on the cylinder, the control valve is what keeps the cylinder from moving when not in use. If your thumb is all the way open, that means your cylinder is completely collapsed.If the seals in the cylinder are bypassing, the oil will go from one side of the piston to the other. So there would only be movement until the cylinder on the other side of the piston is completely full of oil., usually just a couple of inches.
To determine if it is the thumb cylinder seals, open it fully, block it securely with a bit of timber, disconnect the hydraulics and then kick the timber out. If it stays put, your internal cylinder seals are good and the problem is with the valves in the machine. I have limited knowledge of hydraulics but would think that if the thumb works OK then the seals are good because if the seals are that bad that it goes down in 15 seconds then there would be little pressure and the thumb would not grab anything. In other words you would easily push the thumb back with the bucket.
It sounds like you have a port relief valve out of adjustment or failing. I have seen this same thing many times.
When I got my 320 it did the same thing ....... it was a brand new thumb assy that replaced the one that failed ....... long story short it was the spool bank, it was scored so bad from the cyl failure that they had to replace that 1/2 of the bank .... good-luck
Forgot to mention that if I bring the stick in all the way and drop the boom down so that the thumb is actually pushing the cylinder closed it still creeps open. It moves much slower, a minute or two at operating speed and it opens back ip
I guess before we try and give any more armchair diagnostics ...... LOL ..... we need more info like did this just start happening ... or did you just install the thumb on the machine .... or were there any recent changes done to the machine ??????? .... the more info the better to help you out .
Been happening for ~3 years, machine was bought new with thumb. No changes. We had the dealer come out twice to fix it and nothing changed. They tried adjusting/changing something on the valve last time but it's been so long I can't remember what it was. To me it seems like a valve isn't closing all the way and it's extending the thumb cylinder. I have actually had the thumb cylinder rebuilt since the problem started and it had no change.
Well what you describe sounds "exactly" what I had ........ if I remember correctly the Cat mechanic chained the thumb back in the open position and installed an inline pressure guage (on both sides) .... we fired the machine up and watched the line we suspected was the issue ..... but as an extra test he had me reverse the flow and hold it there and watched the guages to see what was going on ........ Because of what he saw, the side that was the problem it started to develope pressure on its own with out actuating the thumb switch when we opened the thumb up and held the pressure on it also started to put some pressure on the back side ...... with this type of testing he determined that the spool bore was damaged to a point where the oil was bypassing internally. Cat was "Awesome".... even tho it was a used Excavator (but still within the 3 yr warranty) they replaced the whole one side of the spool bank complete (2-halves to this machine) To say that you had rebuilt your thumb cyl with no change happening in the end, would be tough to take .... if the machine was new as you say the dealer should have come back until this issue was resolved ..... sounds like pretty pith-poor service follow up on their part ...WoW ! ...... if they still won't do anything for after all this then I would probably try to see if that side is building pressure like they did on mine (you wouldn't have to do both sides, but mine was a case history problem when they took it on trade, the thumb cyl had failed bad and the hyd filter plugged up, the machine hyd filter warning came on the lousey operator kept running the machine causing the system to by-pass and contaminated the WHOLE excavator) But in your case just to see if the pressure builds up, would lead you to check that spool to see if it is stoking both ways properly ..... another thought would be that your hyd pilot line is leaking/by-passing and putting a slight pressure on the spool causing the chain reaction ....... you've got a choice to do it yourself or get a hyd guru to check it out ....... hope this may help and give some more insight on what you could check out ...... Cheers !
Had the dealer out yesterday and he spent 20 minutes checking pressures and watching it drop. We determined it was the valve is letting oil past when it shouldn't. Like I had told them before. They are going to come out and pull it off, tear it down and see if it's fixable, if not throw on a new one. Btw you are right about the service, it's always been pretty pathetic. We had this place come out to CHANGE the hyd oil in a machine. What did they do? They ran it through their filter unit and recycled it, along with a couple gallons of water in the bottom of the barrel. They had to flush it a couple times and do a few oil samples but eventually got it right.
Having the same problem with my CAT 303cr. Sometimes the boom and or thumb will blead down. Other times it will not blead down. Today after a day of work the boom blead down within a beer. Crap!!!