low hydraulic pressure problem CAT 988K loader

[KuBaK1]

Hello, i am dealing with strange problem on CAT 988K loader. Hydraulic pressure is low, but only on high idle. Specification is 330 bar, a can meet that on low idle, but when on high idle pressure drops to like 240 bar. We inspected the pump, seems fine. Checked main relieve valve, also seem fine, when i tighten it its responsive, pressure rises, but only on low idle. Tryed to tighten line relieves valves, also seems to have no effect. If you have any tip to try a would be very happy. Also i attached schematic for the machine.

Thank you guys.

 

[FluidPowerUser]

Could be an electronic issue.

The pump displacement is controlled by a proportional solenoid. The spools for the implements are all electro hydraulic too.

Might seem like a strange question, but are you sure that it’s not meant to back the pump displacement off at high idle to limit the power?

It could easily be that the software is backing the pump flow off at high idle, but there is an internal leak somewhere, so the pressure backs off too.

Only way to check is to measure the flow from the pump as the engine speed goes up.

There is one pressure sensors in the circuit, by the pump, so it could be software is compensating for pressure, so it could also be compensating for engine speed and pressure.

 

[KuBaK1]

Hello, thank you for your answer. I think the pressure is escaping somewhere, but cant figure out where. The pump is supposed to destoke 10 % when stall is detected, which seems to be working. When i look at the diagnostic data i can see current drop from 1.8 A which is maximum displacement to like 1.5 A when i reach the end of the stroke of the cylinders, so that is ok. Unfortunately i dont have lfow meter so i do not have any way to test the flow of the pump. Stupid question, but can this be due to the faulty main relieve possibly?

 

[FluidPowerUser]

Does the same thing happen when you stall the lift cylinders in both directions and also when you rack back and tilt forward?

The 10% reduction in the pump displacement is a big clue about what's going on here.

If you see the same pressure reduction on all implement movements, then it suggests that the main relief is not closing properly and the 10% reduction in flow is not able to saturate the relief and then it leads to a 30% reduction in pressure.

If the pressure loss only happens on one of the implements, then it suggests that one of the cylinder seals may be leaking oil.

Because of the square law in relating pressure to flow, you don't need to reduce the flow off by much to see a 30% reduction in pressure.

It's not a stupid question to ask about the relief valve. I was the engineering manager at CAT for the smaller wheel loaders and there were all sorts of the things that could happen, so you have to keep an open mind. Depending on how the software backs the pump off and also how big the relief valve is and the flow capacity it's designed for, the pressure could be anything below 330 BAR. The displacement reduction assumes that the relief valve is off its seat and a percentage of oil flow is going across to the tank line to hold 330 BAR. If the relief is actually passing more oil than expected, then the 10% reduction in flow will result in a bigger than expected drop in pressure.

According to the specs for the machine, the main relief is set to 328 BAR and the pump flow is 580 LPM. The power required to drive the pump is 373 kW and the rated power is 401kW, to SAE J1349. It's not like it's close to stalling the engine. It's unlikely that they have limited the power via the software and it's unlikely that there is a software fault. At this stage, it's seems most likely that the system is losing some oil and the 10% reduction in flow is showing in a 30% reduction in pressure.

Where are you measuring the current? That current seems to be the driving current for the implement spool. These spools works by having equal pilot pressure on both sides of the spool and oil pressure is drained off from one side to get the spool to shift. Dropping the current from 1.8A to 1.5A will make the spool move back towards the neutral position and that will reduce the pressure in the cylinder.

Where are you measuring the pressure?

It could be and it's more likely that that the spool position is not accurate. As the spool moves towards the neutral position, more oil is dumped back into the tank line and that will certainly reduce the pressure.

Try all of the implement functions and see what happens. It could be that the one of the pilot pressure controls valves is leaking and the spool is not holding the position that it should when the current is reduced to 1.5A.

 

[KuBaK1]

Hello, today i did some research in service manual and i starting to think that maybe the problem is in the pump control piston. When i striped down the pump i dint find any problem with the piston rotating group, but i found that the control piston for swash plate had some scoring on its diameter. I didnt tought much about that, but now when iam thinking about it it can cause problems that iam having. If i understand that correctly there is pressure from the pump on one side of the piston (the smaller side) pushing the swash plate to its maximum dispacement and against it there is pilot pressure controled by the solenoid. So the piston needs to seal againts the pump body to hold the pressure and its not sealing properly because the scoring (Its lapped surface, like spools in control valves) then there can be pressure leakage causing the swash plate to return to the minimum displ. on high idle, because on high idle is the highest pressure on the swashplate. Let me know what you think about it, if its nonsence or if this can actualy be causing this problem. I attached som pictures of the pump so you can see what i am talking about. Tha piston is n. 33

 

[FluidPowerUser]

We are looking for something that is related to the speed of the pump.

The servo piston on the pump is balanced between the pump outlet pressure + spring force versus the pilot pressure that is controlled by the proportional valve. The angular velocity of the pump will apply additional force on the swash plate and try to push the swash angle towards minimum. If the servo piston is scored and passing oil, then the pump may be able to push the swash plate back too far and produce less flow than expected and give less system pressure. There are no guarantees, but the damage to the servo piston can be the cause.