Hitachi EX swing issue

Hitachi EX120-2 Excavator
1995 4,500hrs.

I'm having issues with the swing motor. At first it would act lazy after 2-3 hrs. of use and now it barely swings after 10 mins from cold to warm. Meaning it moves left or right but at about a foot per 5 seconds with the bucket and arm curled in. Forget having a load in the bucket or boom extended out. I pulled the swing motor- full disassembly and reseal after no issue found.
At first thought I thought it was hydraulic oil getting hot and dumping back to tank causing me to loose pressure, It was not.
I used a infrared gun to check temps at cooler after 10 mins of running under load.
Not sure which tube is inlet and what is outlet ( to the cooler) but they both are around 130degs F / 54 deg C
Hydraulic Cooler is flowing air easily - if you lightly hold a piece of paper 12" from screen in front of the cooler it sucks it in to the face of the cooler and holds it.
I read some previous posts about a 100-3 overheating and read about a bypass valve for the cooler; I can not find a bypass valve for the cooler anywhere. On the machine or in the book. The oil is fresh and filters are new. Engine is running perfect and no issue with power or engine over heating. After running for the 10 mins the engine temp is 170-180 deg. F. / 79.5 deg C

When run (example- digging rock off the surface of clearing a dirt driveway; for 10 mins dressing up a dirt road on my property) Every dig function on my hydraulic pressure gauge that I screwed into pressure port on main pump per service manual shows approx. 2500psi. Let go of a stick and it drops back to approx. 750psi- 775psi - right where it should. I do not know what high pressure should be. But 2500psi- 2750psi on spikes feels about right.
If you move the joystick to swing- it bounces up to 2500 then drops back to approx. 750psi immediately. If it gets feather it back and forth (neutral / left /neutral left) it will move quicker.
When unit is first started till it hits that magic 10-12min mark, swing is perfect. Both power and speed.
I dont want to disturb the two relieve valves on the swing motor because cold they work find.
I am not getting full line pressure to the swing motor once hot. This tells me there is a relief valve or spool leaking hot, BUT since I am dropping back to pilot pressure (possible psi co-incidence or not?) just after the initial hit of the pilot control ( both left and right) is working, could I have a leaking pilot control joystick?

I have a Russian copy of a service manual I bought on line that was converted to English and I am not super good on hydraulic schematics. Hopefully there is more info out there than what I have.

Thanks!
Joe

So after scrolling thru a lot of post I find a post from # Northmanlogging dated Aug 27,2019
Called " Swing motor slow intermittent ex120-2"

And I scroll down the post as I read and I find a gem of a down load from #Uffex dated Aug 29th, 2019 showing how to dead head the pump to test the braking feature.
I want to thank both people for posting the information and the diagram. Especially UFFEX.
So i start chasing back hydraulic lines to verify my hoses are correct and the line labeled in the picture from Uffex shows the #6 HOSE PG : ( To solenoid valve) as I follow it back to the valve body just behind the rear corner of the cab running 90 degrees from the main valve bank ( about 8 electrically controlled solenoids horizontally run, hoses top and bottom of bank and the solenoid disconnected is the only one that does not have a pressure set screw and nut in the top). I see the solenoid is unplugged and the steel retaining wire is missing. I plug it back in and start the machine. My pressure is at about 750. Then i hit the joystick left and right and the pressure sores to about 4500psi. I let go back to neutral and the pressure returns to about 750psi. If I use any dig feature the pressure goes to about 2500psi then about 750psi when returned to neutral. Swing is the only thig disabled.
Now according to Uffex's picture... you are supposed to disconnect the solenoid and block the line to stall the main line pressure to approx. 4,550psi. Does this mean i have a blockage in a line? it is doing the exact opposite to what these directions are saying.

I don't have the exact specs at the moment but Main Pump Neutral Pressure should be 500-700 psi. System relief with a function stalled should be close to 5000 psi. If you are trying to check swing stall pressure, don't disconnect any lines. Just place your bucket against a track and swing.

Your picture Hitachi 1 is too close and blurry. Are you saying that when the PG solenoid is plugged in you have no hydraulics? Is it actually the PG solenoid that the plug is attached to? Right now my system is down so I cannot look up information. I'll check when it is back up and running.

However the first thing I would check is the pilot pressure to your swing circuit pilot cap to make sure the spool is receiving full pilot pressure (compare to a working function). If pilot pressure to the spool cap for swing is OK then you may have a sticking Variable Pressure Compensator Valve in the main control valve.

mg2361 said:

I don't have the exact specs at the moment but Main Pump Neutral Pressure should be 500-700 psi. System relief with a function stalled should be close to 5000 psi. If you are trying to check swing stall pressure, don't disconnect any lines. Just place your bucket against a track and swing.

Your picture Hitachi 1 is too close and blurry. Are you saying that when the PG solenoid is plugged in you have no hydraulics? .

Click to expand...

What I found is the electrical plug for the pg solenoid was not plugged in. When I plugged it in- I lost my swing function and the hydraulics would rise to about 4500psi. If the system had no load or no function was operating it was at 750psi. My service manual shows like 777psi. That pg solenoid is normally open and electrically closes. So if it is hydraulically “off” then I must have a short to 24vdc or a bad computer.

mg2361 said:

However the first thing I would check is the pilot pressure to your swing circuit pilot cap to make sure the spool is receiving full pilot pressure (compare to a working function). If pilot pressure to the spool cap for swing is OK then you may have a sticking Variable Pressure Compensator Valve in the main control valve.

Click to expand...

I wonder if the VPC is sticking - would I not have other functions as well? Boom, stick and tracks work fine. I’m going to research where to check the spool cap pressure for pilot pressure. My service manual for the machine is a copy I downloaded on line after paying for it. It is translated from Russian copy, missing the trouble shooting section at the end and pictures are blurry. So the term you are giving me is not matching what my manual is calling items. Lol! I thank you for your help but with out a good picture of where to plug in my gauge I’m stumped. BUT I thank you for your help!

Does anyone have a good solid copy of a technical service manual on line I can get a copy of for this ex120-2? If you pm me I can send you my email address.
Thanks!
Joe. Syleng1

Syleng1 said:

Does anyone have a good solid copy of a technical service manual on line I can get a copy of for this ex120-2? If you pm me I can send you my email address.
Thanks!
Joe. Syleng1

Click to expand...

Here is Hitachi EX120-2 full Technical manual +Workshop manual exactly.
Link to download documents:
http://www.mediafire.com/file/len6liz0wirx0bc/EX120-2+Technical+&+Workshop+Manual.pdf/file

LACHAU said:

Here is Hitachi EX120-2 full Technical manual +Workshop manual exactly.
Link to download documents:
http://www.mediafire.com/file/len6liz0wirx0bc/EX120-2+Technical+&+Workshop+Manual.pdf/file

View attachment 224961

View attachment 224960

Click to expand...

Thank you!
I greatly appreciate your help and energy is me repairing my machine!

Hitachi 17 with a swing issues, brake only locks up some time which I believe it is actually a valve sticking just looking for placement of said valve? Any help
Would be appreciated