Komatsu pc120-6 low pressure.

[JRsEquipment]

Komatsu PC-120-6 61000+ SN. At cold start up relief (Or at least pressure is good) around 5000PSI, After warm up pressures start to decline, eventually to about 3400PSI. Stage 2 relief (Swing brake, heavy lift. travel) does not create any more pressure hot or cold. I have tested all pressures in manual, (control pres, LS, PPC, PC etc etc) Even over adjusting these one way or the other trying to get some type of variance in main pressure to no avail. When testing LS pressure differential pressure it is about 300psi differential as it should be, however during the test(see attachment) Instead of getting Pump Pres= 1150 PSI and servo inlet =640 psi I am getting about 2300Psi pump and 2000 Psi for servo inlet. I cant make any sense out of that or if it even relates to low press. There is not a spec for case drain from the pump as in most other machines, however I checked it and in about 10 seconds got about a gallon of oil from case drain and that is just starting machine, I didnt even get to the point of putting the pump under load. So either this pump is bypassing pretty bad or that much case drain is normal for this pump for reasons I am not failure with like some control valve returning through same port on pump IDK. Any help would be greatly appreciated! Thanks

 

[LACHAU]

Komatsu PC-120-6 61000+ SN. At cold start up relief (Or at least pressure is good) around 5000PSI, After warm up pressures start to decline, eventually to about 3400PSI. Stage 2 relief (Swing brake, heavy lift. travel) does not create any more pressure hot or cold. I have tested all pressures in manual, (control pres, LS, PPC, PC etc etc) Even over adjusting these one way or the other trying to get some type of variance in main pressure to no avail. When testing LS pressure differential pressure it is about 300psi differential as it should be, however during the test(see attachment) Instead of getting Pump Pres= 1150 PSI and servo inlet =640 psi I am getting about 2300Psi pump and 2000 Psi for servo inlet. I cant make any sense out of that or if it even relates to low press. There is not a spec for case drain from the pump as in most other machines, however I checked it and in about 10 seconds got about a gallon of oil from case drain and that is just starting machine, I didnt even get to the point of putting the pump under load. So either this pump is bypassing pretty bad or that much case drain is normal for this pump for reasons I am not failure with like some control valve returning through same port on pump IDK. Any help would be greatly appreciated! Thanks

First, you try adjusting the LS Valve.
Your testing differential pressure is 2000 PSI/2300 PSI = 4/5. That means it's not within standard value!!

 

[Jgm]

I so you tried to adjust your main relief and no difference, where is your pressure set at on your secondary, when that pressure is lower, you can adjust your main relief all you want and nothing will happen. Your LS pressure, I would adjust after you figure out your first problem, LS you will only notice in your controls, for finer working.

 

[LACHAU]

Have you checked and adjusted the safety-suction valve?

Please note that: if the safety-suction valve set lower than the main relief valve, the two-stage relief valve will not affect.

 

[Jgm]

How would it not effect it?

 

[Jgm]

If my second stage is set lower, I would not beable to reach or adjust my main relief, with main relief I would only be able to reach my pressure what the secondary is set at.

 

[Jgm]

The middle picture is you secondary with the counter nuts and the third pic, is your main relief with your screw, written wrong on the bottom from thevsketch

 

[John C.]

The Dash 6 line is closed center load sensing. The reliefs only react when an outside force suddenly causes a spike in pressure. A properly adjusted pump will never reach the main relief setting. The pump is supposed to destroke and hold its maximum pressure will little flow.

Three hundred PSI is what I have usually seen when troubleshooting these pumps. I know what the book says but also know what I have seen and can say that probably isn't your problem.

The PC valve cartridge is to the left of the LS valve cartridge in the pump and it has the acorn nut attached. I've uploaded the page in the book that describes how to adjust that valve. I have had issue with that valve in the past. When it gets worn or the oil is dirty, the parts inside the valve get stuck. You can remove the cartridge and take it apart to clean it but make sure you mark or write down somewhere where the adjustments are. It is easy to adjust, just a pain to mess with that acorn nut.

 

[LACHAU]

How would it not effect it?

Ex: Safety-suction valve set at 30 MPa, even when you press the 2-stage button, the pressure is still there, it cannot be increased, it means 2 stage relief valve no longer affect.

The middle picture is you secondary with the counter nuts and the third pic, is your main relief with your screw, written wrong on the bottom from thevsketch

As I noted that, 2 stage relief valve has PX line = from 2 stage solenoid valve

 

[LACHAU]

Please note that cross sectional A-A is Safety-suction valve
and cross sectional B-B is Main relief valve.

 

[JRsEquipment]

THANK YOU! all that replied. As far as LS diff pressure I under stand the 3/5 part however if the pressures were as stated in manual (Pump Pres= 1150 PSI and servo inlet =640) Im getting 2300/2000 ish. If i try to adjust LS up, both raise about equally (2800/2500 etc) if I lower LS they will go down and become close to equal around 1500PSI and that is with LS screwed way out. My main point is, it is way higher than the manual says as far as PSI and I can not get 3/5 of differential pressure. weather or not this is an issue IDK. Does anyone have any case drain experience with this pump?

 

[JRsEquipment]

OK, I must confess to being confused by these 3 pages in the manual(see attached) "Testing and adjusting LS valve output pressure" (page1) This test I am getting 2600/1500 PSI listed in table one as 1130/638 is what it should be. However it gives no means or instructions to adjust this pressure. Then moves to Page 2 table 2 to (differential pressure) That is where I am getting the 300 psi differential. I was combineing thes two tests as one or a different view of the same test. Test 2 gives no spec for actual pressure only differential, so I guess it is passing test 2 but test 1 has the correct 3/5 difference however the pressures are way to high. Regardless high pressure is still to low.

 

[John C.]

Did you read my post? The PC valve in my experience has always been where the problems were at. LS pressure only has to be different and the pump will work to maintain that difference.

 

[JRsEquipment]

Did you read my post? The PC valve in my experience has always been where the problems were at. LS pressure only has to be different and the pump will work to maintain that difference.

John C Yes I did thank you. I am going to remove and inspect PC valve tomorrow. I was just trying to get a better understanding of those adjustments.

 

[LACHAU]

Did you read my post? The PC valve in my experience has always been where the problems were at. LS pressure only has to be different and the pump will work to maintain that difference.

Your opinion makes a lot of sense, John.
I also think in your opinion, but I want to explain to him about LS pressure first.

OK, I must confess to being confused by these 3 pages in the manual(see attached) "Testing and adjusting LS valve output pressure" (page1) This test I am getting 2600/1500 PSI listed in table one as 1130/638 is what it should be. However it gives no means or instructions to adjust this pressure. Then moves to Page 2 table 2 to (differential pressure) That is where I am getting the 300 psi differential. I was combining thes two tests as one or a different view of the same test. Test 2 gives no spec for actual pressure only differential, so I guess it is passing test 2 but test 1 has the correct 3/5 difference however the pressures are way to high. Regardless high pressure is still to low.

Please note that: "LS VALVE OUTPUT PRESSURE (SERVO PISTON INPUT PRESSURE) and LS DIFFERENTIAL PRESSURE" are TWO different PRESSURE TYPES.

 

[LACHAU]

 

[JRsEquipment]

OK NOW I understand LS valve out is NOT LS. Stupid me. I still do not understand why it is so much higher than the chart. That being said. Removed cleaned and inspected PC valve and safety valve, both clean and good, the safety valve has a internal O-ring or seal that was pretty flat, but would have to get one Monday if that may be an issue-(not sure if its just spring press or spring plus hyd pressure setting the safety) ran in the safety valve adjustment in and same results. Under no load I have quit a bit of oil returning from the cooler, under full load I have a LOT of oil returning from the cooler which seems to me that it is bypassing somewhere in the main control valve. The main relief has a separate hose back to the tank and if I spike the hydraulics I get a small spurt out of which seems normal......

 

[JRsEquipment]

OK NOW I understand LS valve out is NOT LS. Stupid me. I still do not understand why it is so much higher than the chart. That being said. Removed cleaned and inspected PC valve and safety valve, both clean and good, the safety valve has a internal O-ring or seal that was pretty flat, but would have to get one Monday if that may be an issue-(not sure if its just spring press or spring plus hyd pressure setting the safety) ran in the safety valve adjustment in and same results. Under no load I have quit a bit of oil returning from the cooler, under full load I have a LOT of oil returning from the cooler which seems to me that it is bypassing somewhere in the main control valve. The main relief has a separate hose back to the tank and if I spike the hydraulics I get a small spurt out of which seems normal......

Disregard last sentence, is cooler bypass return not main relief return.

 

[JRsEquipment]

So the way I am seeing it is I dont believe it is a pump or pump control issue.
If I dead head any of the cylinders (including track motors) If I have x gallons a min going through main control valve and returning to tank it must be bypassing= main relief, suction safety or unloading valve. I understand it could be bypassing through cylinder or travel motors etc but not likely all of them. All circuits have the save pressure. I dont visually see anything wrong with relief or safety except maybe the o-ring being flat in safety. I am goin to pull and inspect unloading valve Monday. Any thoughts? Any responses are greatly appreciated. Thank you.

 

[John C.]

If you are sure it isn't the pump or the PC valve, your next step is to remove the compensators in the main control valve. How many hours do you have on this machine and how many hours do you have on the hydraulic oil that is currently in the machine?