I used my Komatsu PC28 to dig up my first stump this weekend and I noticed after a while the boom functions got really slow. I read a couple threads about this type of thing and they mentioned pressure relief valves bypassing when they shouldn't for various reasons. I have a diagram of the circuit and I was hoping someone would explain which one I should concentrate on checking. There are two main reliefs it seems like, then a few others. It's not a terribly complicated system but it's still over my head.
All functions, a couple or just boom up and down?
Boom up was the slowest being almost unusable near the end, but I think everything was slower to some extent. I think the biggest cylinders were the slow ones based on what the print says.
So looking at the schematic, you have 3 fixed displacement pumps. 2 power the boom functions and the 3rd the swing and blade functions. There are 2 relief valves in the main valve assembly both set to 2844psi. Third in the 2 spool control valve set to 2489psi. If all functions slow, i would look at the common denominator, suction.
I found a big 1” low pressure line that was pretty rough looking but not leaking. I replaced that and tested by digging out a pretty good sized hickory stump. By the end my hydraulic oil tank was pretty hot but not too hot to touch. One thing I realized was that if I engage the track motor a little bit, I get some boom lift speed back. I’ll eventually put some gauges on the test points but does that behavior raise any flags?
Getting speed back like that suggests to me that the problem is in the pump or what controls it. Flow and pressure tests need to be done to stop the guessing game. Guessing get expensive when you fix what ain't broke.
It has been a while and of course I have ignored this problem until now because I haven't needed to dig any more stumps. This past weekend I dug up a pretty big poplar stump and this issue showed up again. I dug in to the manual some more and found decent information (on previous gen, fingers crossed they didn't change too much). I'm hoping a pressure relief adjustment will help but I just wanted to ask here if I can get some clarification on whether this makes sense to someone more knowledgeable in hydraulics. Why do they specify that adjustment order? Interesting that RH travel and boom are in the same group after what I see. I do hear some hissing when operating, so is it possible that engaging the travel motor lowers the pressure enough to allow the boom circuit to operate under what the relief valve is set to? Do pressure relief valves wear out? This machine does have a ton of hours on it, so anything is possible. Just trying to understand better before I dig in and possibly make things worse. Thanks for the input!
You’re getting out of line. First thing would be pressure gauges as indicated above, it sounds like one of your pumps are not supplying enough pressure, when hot. If it has a ton of hours, what’s the hydraulic fluid look like. A lot of times, you can take an old worn out anything hydraulic and put in thicker hydraulic oil and get some more time out of it, before fixing or replacing pumps and or valves. Think butter on a dish in the sun.
Relief valves wouldn’t be likely to cause this. In that case even with cold oil there wouldn’t be force in the cylinder even before ever deadheading the function and in that case looking at the first schematic with the boom also the bucket, right travel and boom offset cylinder would also lack force. These four functions share the same relief valve according to schematic and are all supplied by the first pump from the engine. Like said perform pressure test on these three pumps, when starting to work and also after the problem on the boom arises to compress the results (i would expect at least one quarter of less pressure when the boom gets slow). Like also said before it kinda seems this first pump is the problem… When engaging right travel the boom gets additional supply of flow from second pump that why you also get a spike of more force with the boom. But like you said that the machine is old with many hours it can also come down to the basics as always. What is the state of your oil, have you ever changed it? The pump might only get slightly worn, but low viscosity as oil gets hot is great contributor to having less flow. Boom cylinder is probably the biggest and demands more flow to get full force than others, same with drive motors. When oil gets hot and boom weak try traveling straight, just left and right travel no other function engaged. Would you say that right travel is slacking, going slower? That would be easiest to see when traveling uphill… But like said, make pressure tests first and check state of oil.
Fluid looks good and I changed the filter when I first got it since it looked like it hadn't been changed in a while. I will go through the pressure tests for sure, just trying to get an understanding before I do that. Interesting note on performance that maybe I haven't been too clear on. The only two functions that aren't affected are the blade and swing. They seem to be on their own pump and spool, so I guess that makes sense. The reason I was thinking relief valve is that there's a brief moment where things move faster regardless of the oil temp, then I hear the hissing sound and everything is slower. I'm never in a hurry to do things, so it doesn't bother me. Pressure testing should be interesting regardless. I'll post here when I do it to keep the story going.
The relief valve cartridges will get weak over time because the cone will get a groove worn in it or the spring holding the cone down will get weak. You will need to put a gauge in the circuit to see what is going on. You should be able to adjust the setting up be turning screw. When the cone goes bad you will be able to adjust the pressure up some but it will drift down when the oil gets warm. You can remove the cartridge and take it apart to inspect the cone. There will be a ring on it where it seats in the orifice. If it out of round or you can feel it with your finger nail, it will need to be replaced. It can be a lot of other things as well as mentioned above. These machine were built inexpensive but they don't stay that way forever.
I was able to do some pressure testing this weekend but it confused me even more. I put the gauge at the 1/8 plug on the pump since that was the easiest place to get one in the circuit. The middle pump which feeds the arm and LH drive motor seemed to relieve pressure at one tick mark above 3k psi in both directions when the arm cylinder reached the end of stroke. The pump closest to the engine drives the RH motor and boom cylinder and that one was hitting the relief at a different pressure while booming up or down. I actually had to let off in the up direction at end of stroke because it was loading down the engine and pressure got to over 4K (someone must have been here before). I tested circuit pressure while activating RH drive and boom up and the increase in speed lifting the boom showed up as about a 500 psi bump (1000 to 1500 or so). I ran out of time to play around much more but I’m starting to think I need to get a flow gauge to see what’s going on with the pump output. I can’t load the travel motors enough to get more than about 500 psi. Any tips for testing the circuits with pressure gauges by more experienced guys are welcome.
Right travel is slightly weaker I’d say. If the travel motor is engaged while booming up it diverts all of the flow to the boom if the spool is not fully opened.
What is the Dash series of the machine? I'll see if I have a manual for your series of machine. On Komatsu excavators the general scheme is to provide a high pressure and output when operating the travel circuit. I never had to work on enough problems on the minis to have seen the answers to your specific machine.
PC28UU-2 but all I have found is the manual from the -1
I need to confirm just what you have for pumps in your machine. I have files for the PC27 series machines but those pumps are piston pumps while your schematic shows gear pumps. Could you take some photos of your machine? Photos of the entire machine, of the pumps and of the valves and their locations. Most on here ask for serial numbers without a book I don't have any way to reference that to what you have. All the UU machines I have experience with were gray market units.
They’re definitely gear pumps, three stacked up off the crankshaft. Seems like not much changed from the -1 but I can take some detailed pics for you next time I’m at the machine. From the purple and teal color scheme and Japanese characters on the number plate I’d guess the machine was imported in the 90s, probably grey market like you said.
It looks just like this one but they didn’t pop any covers to show the pumps/valves etc. https://used-farm-tractor.com/excavator-komatsu-pc28uu-2e-14114-5231h/
With the odd boom I'd say it's Grey market for sure. I've read those type of offset booms were a weak point and not imported to N. America new.
About the biggest problem was the extra weight over the front. Spent a lot of time working an EX100 with that same boom arrangement and also had a JB Tilt hitch. Man that thing never stopped.
Purple people eater. According to the drawing you have an open center parallel passage hydraulic system which is common with positive displacement gear pumps. The pump next to the engine feeds on side of the six spool hydraulic valve while the middle pump feeds the other side of the valve. There is no fancy straight travel valve that would join both pumps together when a function and travel are operated at the same time. Instead they joint the functions inside the main block of the valve itself. You can adjust the low side pump at the main relief on the valve to see if pressure would come up but from your symptoms it sounds like front pump has lost some efficiency. Since you don't have a catastrophic failure, there probably isn't any visible metal particles in the hydraulic filters. It's just time that the old machine is getting tired. Maybe you can adjust the main relief up some to make the issue less bothersome and use it like it is. If not you likely need to start looking at aftermarket parts somewhere. Komatsu dealers are not likely to have that in stock anymore.
Had a little PC of the older vintage where the feed lines going into the pilot controls had screens in the fittings. Cleaned the screens and the little guy perked right up. I cant recall which model, but, it was higher hours.
Interesting, I haven't seen reference to that yet. Can you tell from the schematic that it has them or will I just have to go searching?