PC78US-6 Hydraulic Schematics/Circuit

[lmcnei3]

Let me give some background on my machine; I bought it with a bad final drive and pump. I replaced the final drive and had the pump rebuilt. I'm getting correct system pressure from the pump right now, but operating the boom as a compound function is very slow. When i operate just the boom it takes 8 seconds to cycle up and 8 seconds to cycle down. I pulled off the boom cylinder and inspected the inside of the barrel and rod for damage, and they look smooth as a mirror.

The travel function is also slow.

I don't want to pull apart the control valve unless I have to.

We are suspecting there is debris blocking either screens or orifices. Can someone share the hydraulic schematics on this model machine? Serial number is 9319.

 

[uffex]

Good day
See attached.
Kind regards
Uffex

 

[lmcnei3]

Thanks Uffex.
Would you expect blockages to be in the LS block?
I've attached a hydraulic circuit from the shop manual, might be more clear for some users could benefit from it.

 

[uffex]

Good day
Your post suggests a low flow output, I have not met blocked LS ports in the circumstances you have. I would suggest get the pump tested at a facilty that has the equipment.
Kind regards
Uffex

 

[lmcnei3]

Update:
Pump shop that rebuilt the pump is stating that flow is 35LPM, we were unable to verify in the field due to lack of proper fittings on our flow meter. Attached is before and after pump rebuild.

We swapped suction valves on boom circuit with arm circuit. No change nor improvement.

Checked oil pressure on boom valve output going to boom cylinder and was 4KG lower than relief pressure. Boom holding valve held pressure and didn't drop.

Boom raise to relief is giving spec pressure; 256

Boom lower to relief is under spec pressure; around 220

Poppet inside holding valve moves freely.

Merge divider spool, LS unloading spools, logic valve spools all move freely with no apparent damage.

Screens to logic valve were clean.

We checked spring height on merge divider valve, was 1mm less than spec.

Adjusted swash plate angle 1/4 turn out, and it made function worse. 1/4 turn in from original position didn't change how it operated in original position.

Shop manual is stating safety suction valve is bad/ compensation valve is bad.


I will order fittings for the flow meter to verify the pump output is within spec, then that will isolate problems being in the valve. With the way the machine is acting, tracking slow in both speeds, we should have checked that before.

 

[lmcnei3]

This seems to be a common problem with this model excavator. Someone has had to figure out whats going on with it. A Komatsu technician has been helping me with this along the way, and he has mentioned that the local dealer would be suggesting replacing the pump, and valve. This seems to be the mentality amongst a lot of people. I would prefer to fix the components. Anyone can buy a new part and slap it in, not everyone can repair something that's broken. My concern is would buying a new pump and valve actually fix the problem? What is causing it to happen and what can be done to prevent it.

 

[lmcnei3]

Following up on this post:

Had another mechanic check flow coming out of the pump. It was at spec. He calibrated the pump and everything is working within the specified parameters. A lot of tuning had to be done on both relief valves also, but the machine is finally working properly. John touched on this on one of his posts a while back, and he was correct. Glad I didn't end up buying a new control valve! The machine is very strong, again. It's a little bit slower than new machines, but that's to be expected on a 16 year old machine.

 

[lmcnei3]

Hey guys, Back again.

Had a tech working on this machine for the last month. They are swearing that everything is correct, and this is in my head. I need another opinion.

While tracking forward, the boom wont raise up.

If the blade is raised and held in relief, the engine stalls out.

While tracking with the right track only, both tracks move forward.

Thoughts?

 

[ROAMER_AUS]

Interesting set of problems. I'll just throw out couple of ideas. I don't have intimate knowlege of this machine, so be aware of that.

While tracking forward, the boom wont raise up. Does this machine have travel mode switch where it disables commands while traveling?

If the blade is raised and held in relief, the engine stalls out. You have separate pump and relief valve for swing and blade, Check that it is set as per diagram. Does it happen when you push blade all the way down as well?

While tracking with the right track only, both tracks move forward. Either it activates travel straight valve somehow or you have a bad leak on swivel joint.

 

[lmcnei3]

Tracking forward: Other functions work, as in the arm moves in and out. You can lower the boom while tracking forward, but you cannot raise the boom while tracking forward.
Looking through the troubleshooting diagnostics for H-24 It is calling out an issue in either the merge divider valve or logic valve

Blade relief is set per the diagram, it also stalls the engine if blade is operated while travelling.

Tracking with right track, I think it could be swivel joint issue. The left track doesn't stop completely it tracks slower than the right. This is after travelling with both pedals for a distance.


Also, while doing a PM, we drained oil out from every cylinder and hose and completely replaced all of the oil in the machine.