I am posting this for a friend who isnt very internet saavy. A customer brought him a 268b skidsteer with an issue of not throttling up. The first story is that cat looked at it and said it needed an injection pump. Going by that this mechanic removed the pump, had it rebuilt and reinstalled. Now the machine idles beautiful, here is the problem. The only way to get the machine to throttle up is to keep the arms or bucket moving. As soon as you stop moving the arms the engine will drop down rpms and no matter what will not increase rpms from an idle, until you move the arms again. When the machine idles down it sounds like the engine is laboring from the hydraulics loading up with extra pressure. Again, as soon as you move the arms the engine throttles right up. He has searched front to back in a parts manual to get familiar with the machine and has mentioned a load sensing valve on the main hydraulic spool valve. Can anyone chime in as to what you think the issue may be? Thanks in advance!
Try posting the machine Serial Number as a start ........... A Parts manual will not tell him how systems operate. For that he'll need a service manual
Well, if you're moving the arms the fluid has somewhere to go under little resistance compared to if the valve were in neutral. That is assuming someone has "adjusted" things. This has a gear or piston pump. I guess the simplest question, what is the pressure between the pump and valve when the machine is "overloaded" in neutral compared to when the boom or bucket is moving?
obviously a parts manual does not tell him how the systems operate. he doesnt own a service manual and the parts manual was given to him. hes just working with what he has been given. I will have him get pressures between the pump and valve block when in neutral and when the arms are moving, as well as a serial number. This particular unit has a piston pump, that has already been verified.
That much is true, but there are resources on here that could get hold of the hydraulic system operation document and/or the test & adjust procedures for him, given a Serial Number.....
Hi, I am assuming your machine has the LBA prefix so will have XPS high flow as standard ,this system has a variable displacement piston pump. from your description I sounds like the hydraulic system is putting load on the engine ? A couple of things to check if you have access to hydraulic test equipment and know how to use it 1/ Main system pressure 2/Signal pressure from the control valve if you do not know how to check these two items please let us know as we may be able to help As a quick and dirty check to isolate your problem from the control side to the pump side. disconnect the signal line going into the pump compensator valve which is located on the side of the pump and cap off start the machine and operate your throttle control and post back what happens To disconnect this hose the cab will have to be raised, please make sure it is correctly supported and that you remove residual hydraulic pressure from the tank and system
Thank you for the reply. I will be getting the serial number on monday when i go back to the shop. I do infact know that it has the piston pump and also has the high flow option. I can do some hydraulic pressure tests as long as i know what psi gauges i need and what ports to check them on.
Hi ,two gauges would be ok, one rated at 5000 psi and another rated at 600psi. At the test port on the left use your 5000 psi gauge, with controls in neutral you should only have three to four hundred psi (Stand By pressure). The test port for the signal pressure is arrowed and is at the back of the pump compensator control valve, with controls in neutral you should have little to no pressure at this port. First off use your 5000psi on this port also, if you notice no pressure swap over to the lower rated gauge For this check it is not about exact pressures, the numbers will tell us where we have a problem Be careful to not operate the controls ,because the signal pressure can go as high as the signal relief setting which is only a few hundred psi lower than maximum system pressure and will destroy your pressure gauge if you have the lower rated one installed Mark
Just to butt in a second.. you can get gauges off ebay in any pressure range CHEAP.. like 10.00 for a 6000psi guage.. I wouldn't be checking pressures on the space shuttle with them but a quickie check on a machine, they would suffice. JMOpinion.
Thanks for the replies guys, it is very much appreciated. I definitely have 5000 and 600 psi gauges. had to do some tests on a terex loader a while back. Im going to print all these responses and run some tests tomorrow. ill report back. thanks guys!
If you would like a copy of the hydraulic system operation document from the Service Manual just drop me a PM with your email address and the machine S/N.
I certainly would like that. ill have the serial number tomorrow.
Morning guys! Here is the serial number cat0268BTLBA00897. If you want to email any documents, my email is [email protected] Thanks a million!
I'm heading out of the door in a couple of minutes. I'll get to it later this afternoon.
Hey Guys, Sorry for the huge delay here but i finally have at least one eye brow raising pressure difference on this machine. At the test port on the left where there should only be a few hundred psi in neutral, we have 2700 psi of pressure. Should be making some more tests today but i assume this may tell us where the issue may be?
anyone?
hi, sorry I missed the earlier post, did you do as I suggested in post 6 and cap off the signal line into the pump. if you have high pressure at the test port on the valve the pump is stroking up for some reason . capping the signal line will tell us weather it is a pump problem or something else. mark
Hi Mark, I will check with the mechanic today, but im pretty sure that pressure was with the signal line plugged off.
Hi Mark, I just called the mechanic and he verified that with the signal line capped and the engine at idle the main pressure is 2700 psi.
OK sounds like your pump is stuck in an up stroke position. some things to check. the compensator valve is the easiest as it bolts on to the pump with a couple of capscrews 1/margin spool stuck 2/ pump servo piston stuck 3/ swash plate support bearing has steps worn in and locking up free movement of the swash plate 4/control orifices in compensator valve blocked
Thank you Mark!!!! I am going to print this and bring it next door and try to get an answer on this. Much appreciated!
I have an update on this machine. Today the mechanic removed the work pump from the machine and found the actuating piston in the pump to have a sealing ring that got knocked off, and was wrapped around the spring that is below it. When the ring came out of its retaining groove it scored the wall of the bore where the actuating piston seals/rides. A bearing repair sleeve is going to be installed to repair the bore and will be reassembled. thank you very much for your help so far!
Bingo! a machine shop repaired the bore, the piston and ring were reinstalled and the pump now makes 300 psi at idle instead of 2700. Thank you all!
This is wonderful info for me! I have a 248B doing the same thing. Are the ports and lines the same?
its tough to remember that far back.....but as i recall everything was pipe thread on the gauges and lines.
Thanks very much. I think I am going to pull the engine out and do some rebuilding? Any tricks I oughta be aware of on that Z?