Komatsu PC290LC-11 no hydraulic or travel functions

I'm dealing with a Komatsu PC290LC-11 that's lost all hydraulic and travel functions. The machine has 11k hours on it. It was moving forward when everything quit, and the pump started cavitating. I topped up the hydraulic oil—it was nearly empty—and replaced both the tank strainers/filters. The suction line at the tank bottom is clear with no blockages. I've also removed and cleaned the Self pressure reducing valve on the main control valve, and confirmed the hydraulic lock out is working properly. The Damper checks out fine. At this point I'm trying to figure out which pressures I should be measuring to track down what's actually failed. Any suggestions on what to test would be appreciated.

I'd check whether the pumps are actually spinning. Sounds like you might've lost the drive connection at the back of the engine.

I've checked the damper connecting the pumps to the engine and it's in good shape.

When you lift the safety lever, do any warning messages pop up and then clear right away? If nothing shows on the display, go into service mode and check the PPC pressure going to each function while you work the joystick. Does the pressure go up and down as you move it? If it's not changing, take another look at the pilot safety solenoid next to the accumulator on the end. Just because it seems to operate doesn't mean it's actually working right. Check both the pin inside the coil and the pin in the manifold to make sure they're moving freely.

What's your pilot oil pressure reading?

I'm not seeing any faults or warnings when I raise and lower the safety lever—just the display icon showing whether it's on or off. I'm planning to check the PPC pressure in service mode and pull the pilot safety solenoid to take a look at it. Haven't measured pilot pressure yet, so that's my next step. I'll get back out to the machine Monday and put a gauge on to see what's reading. Thanks for the help.

I'm seeing PPC pressures rise and drop when I check each function, but they only go up to around 48psi max. I've confirmed the pilot safety solenoid is working in both directions.

48 psi seems way too low—shouldn't it be 450-500? What's your pump standby pressure reading?

Yeah, just 48psi. Right now I'm limited to the monitor for pressure readings since my Komatsu pressure gauge adapter is broken—should have a replacement tomorrow so I can get an actual gauge hooked up. Thanks for the help. The readings I'm seeing are 01100 front pump pressure 455 psi and 01101 rear pump pressure 455 psi.

The unload or standby pressure reading of 455 sounds about right for pump pressure. You could pull the test port plug from the pump output lines and see if anything comes out of the main delivery when you crank it, but with 455 unload pressure showing, that seems good. Check pilot oil pressure—if you don't have a gauge handy, use the monitor to check pilot functions by flicking the safety lock up; the pressure control circuit for the levers should show roughly 32-38 bar. If you're concerned the pump might be the culprit, pull the magnetic plugs from the bottom of each pump and look for metal debris.

Thanks for the reply. Yeah, the shop manual says 455 psi is within spec. I've been watching the PPC pressures on the monitor for boom and travel functions, and I'm getting 0 psi until I pull the safety lever and operate a function. Even then I only see 40-48 psi, but that does tell me my pilot solenoid valve is working. From what I've troubleshot so far, it looks like the self pressure reducing valve might be the culprit. I took it apart and disassembled it, but I don't see any visible damage. The relief valve on the SSRV isn't serviceable if it fails, as far as I know. I've got a new gauge adapter coming today. I did remove the port plugs on the pump and there's definitely good flow there.

I'd cap the line coming out of the reducing valve and check the pressure with a gauge inside the valve itself. There's a small Allen key plug on it, M10*1.25, that you can use for this. Then cap the output line going to the epc stack or Christmas tree and see if the pressure rises. I've seen attachment pedals bypass before and cause the pilot pressure to drop significantly.

When you had the reducing valve apart, did you check that the spool moved freely? Was the poppet and its seating relief in good condition, and did you look at the spring in the relief itself to make sure it wasn't broken?

Also check that the other big plug with the spool underneath it is working properly. I'd pull the swash plate angle sensor out and inspect it too, because I've had those break off before.

Did you check if the coin filter in the valve was clean? I've seen those get blocked before.

I looked at the coin filter and it was clean, no debris in there. On that self pressure reducing valve you mentioned—mine doesn't have an M10x2.5 plug like you said. It's got a smaller one, maybe M6-M8 instead. Is that the one you meant? I rounded up some caps and plugs to seal off the lines so I can try blocking the valve and checking the pressure. Everything inside the reducing valve itself looked fine, the spring isn't broken, and I checked the mag plugs and angle sensors—all good. Thanks for the help.

I think it should be on the side, probably near the ppc pressure sensor on the block, on the cab side of the spr valve. It's got a small head on it and takes an Allen key, but when you pull it out the actual thread is bigger – M10*1.25. Let me know what readings you get.

I've got an update on what I've been dealing with. I went ahead and grabbed a new Self pressure reducing valve and installed it. Right away I had good pilot pressure and the functions came back. But after a minute or two, the main pressure starts dropping, pilot goes down, and I lose all functions while the pump makes noise again. I can pull the strainers and reinstall them—they're clean—and the machine works for a bit before the same thing happens over again. At this point I'm thinking I'm looking at either a negative pressure issue or some kind of blockage. The suction piping checks out, strainers are clean, and everything on top of the tank seals fine. Could a blocked or clogged return line be causing this? I'm checking those now.

Make sure the oil is clean and at a safe working level.

When you mention strainers, are you referring to the return filters inside the tank? And do you have a clean drum available to pump or drain the hydraulic tank into?

I think they meant the suction strainer. I'm sure they can tell the difference between the return filter and suction strainer.

Yeah, from what I know there's just the one suction strainer/filter, but on the return side you've got both a filter and a strainer. I've got a drum and a waste oil tank, so I might have to drain and clean the tank again. Thanks.