Hello Folks, I've been trying to help a friend of mine out with a low power problem on a 120-6 komatsu. So far we have tested PC servo pressure, main relief pressure and LS differential pressure and as far as I can tell everything test out other than pressure. I came into this project 1 day after owner and another friend worked on it for a while a few days previous. I attempted to adjust main relief at the bottom of spool housing. The manual states its the relief to the rear with the hose coming off the end. Made an adjustment of 1/2 turn which should have net around 500psi but no change. Owner's buddy previously had adjusted relief to the front at bottom of spool housing and pressure went from 4k to 4500psi but had been returned to original position. I moved this back and got pressure back up and all seemed fine, machine had lots of lifting power. Owner took to work and machine gradually slowed and lost pressure 3k. I have lots of test equipment available but I'd like some help from someone with experience on these machines to either condemn pump or find the root of the problem. The flow chart in manual basically points to improper PC pressure adjustment or bad main pump. Also only code is a 213 I don't think it pertains to the problem. pc120-6e serial #59800 Thank you all in advance
I advised owner to let machine cool and test pressure again to see if its a heat related issue and to see if pressure comes back up or something has failed.
PC and LS solenoid resistance checked ok.
Tested cold this morning @ 5200psi all functions(releifs turned up) tested warm @ 3800 all functions, so a loss of 1400 at operating temp. I'm leaning towards a loose pump, any ideas?
Bump to the top, ideas or any specific test procedures for pump welcomed.
Have you opened up any of the hydraulic filters to see if anything has come apart lately. Simple check along with is it the right kind of oil in it. Simon
Machine is a recent purchase, all fluids and filters new and running aw46. Im not sure checking filters with so little hours would show anything that wasn't catastrophic.
A couple of things you can check first might be fuel supply to the engine. There is a fuel strainer inside a fitting on the hand priming pump suction port. It is pretty common for that to plug up. Low fuel flow to the engine will impact hydraulic performance. The computer in the machine monitors engine RPM and will destroke the pump when speed drops. There is no fault code for this. The air filter can also be plugged and that should set a code but on a machine that old you might find the sensor wires have been joined to keep the alarm from coming on. Do you have a serial number for the machine? The machine is a closed center load sensing system so you should only get main relief pressure with the swing brake turned on or in travel or power up mode. In normal operation the main relief rarely ever opens. Factory spec as I recall was about 4,600 PSI in normal operation and about 5,000 PSI with the swing brake turned on. What you are describing is that the pump is backing off flow before the engine is fully loaded. That adjustment is on the pump on some serial number machines and on the main control valve on others. If you have a book there is a section titled testing and adjusting PC valve output pressure (servo piston input pressure). Basically it is pressure oil from the main control valve that pushes a control piston in the pump the strokes and destrokes the pump. The idea is that when the system is running over relief the control pressure on the servo piston should be one half the relief pressure. The servo piston at that point will put the stroke at just enough to maintain pump output flow so the pump pressure is about 300 PSI more than what is on the circuit. The service manual describes how to adjust it. If the valve is working right you will notice a big difference in engine sound and the speed of the machine when you adjust it. There can be other issues that can affect pump output as well. The dreaded compensator valves spoken of on larger machines wasn't much of a problem for these. There are companies in this area that will take your money to "rebuild" the pump but all they do is install bearings and seals and not touch the control governors. You are as likely to have the same problem when they are done as you started with. I don't know who is left at the dealer that could do a tear down and rebuild but the last I heard was that they were just doing exchange pumps.
Hi John C, thank you for taking a look. Machine serial is #59800. I tested PC valve pressure @ the top of the pump and compared it to pump output pressure @ the top of the spool bank. PC pressure was 2200 and main at 4200. The manual states that if there's an issue pressure would either be almost zero or the same as pump out put pressure. I will also check strainers and filters on engine before going any further.
That should mean that you have some adjustment possible on the PC valve on the pump. I used to adjust them with the engine shut down because you have to loosen the hose fitting and turn the body of the valve underneath to change the setting. The book describes which way to turn to add or remove load on the engine. As I recall I used to go about a half a flat on the bolt head per try. I used to set them so that with the engine running properly it dropped no more than 1,500 RPM under full load. If you go much more than a flat without changing the load on the engine you should go back the other way. If nothing changes then you could try replacing that valve cartridge. You didn't mention if the pump made any high pitched noise or got hot under load did you?
So the engine should maintain atleast 1500rpm under full load? Currently only pulling down about 200 rpm, so 1950-2000 from high idle at 2100 +/- 100. No unusual noises but there is a large pressure difference from cold to hot.
I got too many zeros in there. The engine should drop no more than 150 PSI under full load. A 200 RPM drop would indicate the engine is being pulled pretty hard. I'll have to try to jar some brain cells back to life trying to remember a few things.
Had Komatsu out, tech said one the of the spools is hanging open causing a constant pressure and probably some bypass. Took auxiliary valve plate off spool stack and It appears to fix problem so at this point we think the aux valve is the problem. Disassembled Aux valve but so far nothing seems overly obvious, possibly a couple of loose nylon seals.