Good afternoon, I’m new to the site so trying to figure this out as we go. I recently just purchased a 1989 kobelco 903b. Machine runs great and operates great. Until I started tracking around I realized when tracking forward and I want to boom down the machine wants to turn and track hard left. When tracking backwards and I boom down it wants to track hard right. When tracking forward and I stick out it wants to track hard right and when tracking back and curling stick in it wants to turn hard left. I looked at valve body. I see nothing that looks disturbed. Pilot lines look okay. Machine runs and tracks fine and operates fine but when u want to track and try to function the stick or boom it does what it wants. Please help. Great well taken care of machine I just don’t know where to start. Machine has around 5,000 hours and still very strong and tight. HELP !!!!!!! Have pretty good knowledge on machines so I can tear into anything just don’t know what’s the problem.
Good day A hydraulic schematic would answer your question, some early machines did not provide track priority when lowering the boom. The schematic is normally found in the manual, if you have one and post a copy of the schematic we can try to help some more. Kind regards Uffex
Thank you very much I appreciate your time. Attached are some pictures.
Good day Miikey According to my information there is anothe valve between the pump and main valve chest, not shown in the schematic may be there to prevent your issue. What I can say is taht you have one pump providing fluid to one track and boom, when you drop the boom when travelling the fluid will take the easy path instead of driving the track motor it will go to the boom if the refill valves cannot give enough fluid A complete schematic may reveal more. Kind regards Uffex
Pretty sure it only has one pump And the pilot pump. Where would this valve be ? That diagram is right out of the book.
Here’s the pump
Good day Mikey Your schematic shows two hydraulic pumps each serve a MCV block, the boom shares both blocks on raise but only one on lower. If the flow from the pumps meet specifications I can only assume it is a normal feature of this machine. You can check the flow by timing the cylinder movements. If you have a complete manual for the 903B and post it on a google drive post me the link I will read it to see if there are any other clues.Kind regards Uffex
I should mention that you have refill valves. This allows a back pressure to replenish the boom circuit, if this has a malfunction it may contribute to your situation. Kind regards Uffex
I appreciate all your time and effort. Thank you. Which refill valve do you think is bad ? Could you circle it in the pictures.
I will check timing on hydraulics but it’s fast and strong so I can’t see any issues like that.
Good day Mikey When we talk about refill valves these will work or leak, a leak will be obvious. If you look at the diagram point 4.2 you can see a relief valve along side a check valve. It is the check valve that provided refill when a circuit goes into a negative state. The refill rely on return fluid being under a low back pressure, if this back pressure was not working it would for certain cause your issue. As this would effect the arm - bucket which appears to be working oK I would suggest it is unlikely. What we can do if this is a normal feature would be to install a larger check valve to refill the boom down circuit. Before we go down that avenue be sure you have full speed from the engine (full pump flow). If any neighbors have your machine you could just checkout the performance from those.Kind regards Uffex
Okay ran the machine like crazy today full throttle which I always do it stayed super strong and super fast I dunno about super smooth but we got a bunch done lol. Same thing machine operates fine. Except when I want to track and function which will be annoying because I’m in the process of making a thumb to move trees and stumps so I’ll be operating and tracking a lot. So what can we do ? Break it down for me buddy we got this!!!! Help me fix or improve this old girl.
Good day Mikey As to the best of my information, the additional check valve would go some way to resolve your problem, however in view of your end goal for the machine, I would not suggest you go this way. To power a thumb using in combination with the other circuits may not achieve a good result. One could look at adding another pump & control valve but this would be expensive. The 903B is not best suited to your application, more modern machines have priority valve chests preventing this type of situation. I have mapped a diagram of the hydraulics see in yellow the extra check valve. Kind regards Uffex
Hey buddy I found more info in manual. You were right two pumps and I have more hydraulic diagrams and some other check valves !!!!!! What’s your email I will scan them at my work and send them to you. We’re gonna figure this thing out !!!!!!! Going to try to attach manual may be too big.
Manual is to long I will have to scan pages out and email them. I’ll try to do it on here.
Good day Mikey See PM for email address. Kind regards Uffex
Good day Mikey A later development of the two pump system adopted by many manufacturers is that in the event of using the equipment when travelling one pump would connect to both travel motors, leaving the remaining pump to serve the equipment. Most refer to this as a straight travel valve. Another solution that would go some way to resolving the issue would be to install a floating system. Such installation allow the boom to lower (sucking fluid from the tank) using only back pressure fluid making no demand from the two pumps. See attached. kind regards uffex
You could take this or leave it, but most operators don't really try to work and track at the same time. Tracking eats up a lot of flow, and everything runs too slow to do much. It might be easier to adjust your habits than modify or upgrade the machine.
Agreed, pretty hard to make a 1989 vintage machine operate like a new one.
Im not trying trying to make a vintage machine operate like a new one. I’m trying to fix it. I think my buddy uffex figured it out it’s my relief valves are weak either seats are letting to much oil go by and its heading to the track motors in the valve body. There’s also a poppet in there that’s not doing its job. As for tracking and operating at the same time, I get what your saying bud but many people can’t multifunction and it isn’t a problem for them . but it’s extremely annoying that I can’t use the boom and bucket to help the machine. Imagine coming off a trailer and u can’t use the stick and bucket to help track off smoother or maybe your crossing a ditch or your turning around or your tracking over hilly terrain and u need to boom up or down. it just doesn’t make any sense. we call them guys one stick ricks lol. We’re going to figure this machine out !!!!! I’m hoping a couple new pressure reliefs and im back in business I will disassemble and send some pictures.