I'm working on a 303 mini excavator that the customer stated "won't dig". So after putting my gauges on it I can see why. I've got over 3,000psi on every function except the stick and I can barely get 500 psi out of it either direction. So I pulled the spool out of the valve thinking maybe it was something simple like a stuck spool, maybe some debris binding the spool maybe. Spool looks fine, no scoring whatsoever. What should I check next? Appears to be a simple system, pilot pressure pushes the spool and oil flows to the cylinder. Note: I re-packed all the cylinders a few months ago. Thought maybe it blew the packing out so I pulled it apart. It was fine.
Low pressure in both directions? That (almost) eliminates a bad ine relief valve. Is the stick cycle time OK?. If it's slow, it could be a problem with the pilot valve not shifting the spool enough....
Cycle time seems slow but I haven't actually timed it. I know if you stick it down in hard dirt it won't even drag the teeth through it, let alone the whole bucket. Is there a way to check pilot pressure at the valve? I guess I could plumb in a test port.
I would say probably the easiest way would be to exchange the pilot lines over from one of the neighbouring spools and see if the fault moves with it.
Put the stick cylinder in one of its end positions, pull the non pressurized line of it and plug the line coming from the machine and leave the cylinder connection open and run it against the end position some more, I bet you get a solid gush of oil out of the cylinder and that indicates bad piston seals.
Ok, will do and report back.
You can also check the cyl and spool for holding by putting the bucket teeth straigh into some hard soil then travel For then Rev, this will load the cyl up to line relief pressure. If you have a bad leak the stick cyl will move freely. One more thing, does it travel strong with each track while turning, how is the swing. They are on the same pump.
Good day Sir Low pressure in both directions indicates two way leaking - check if the cylinder holds up or creeps is so this could be a cylinder issue if not your spool may look good these are rarely visibly damaged its more common that the soft material of the valve chest is damaged. Hope it helps if you need a one to one discussion drop an email to [email protected] Kind regards Uffex
Travels and swings strong at the same time. I stuck the boom down in the ground the other day and nothing moves or leaks off if you back up or go forward with boom stuck down.
Ya, I know looking at the spool is a poor way to diagnose whether its good or not. I'm going to pull a line off the cylinder and check for leak-by. If thats not it I'll be pulling the whole valve out to replace or inspect that one section. Thanks for the help.
P.S. - anyone know why Cat felt the need to put this valve under the cab? I had to pull the whole cab off to get good access to it. You can work on it from the side and a small hole in the floorboard but a repair like this is impossible with the cab on. Most other minis I've worked on have the valve in the back or on the right side. Just seems to me like the hyd tank and fuel tank would be much better under the cab and valve out where you can work on it, instead of vice versa.
If it travels and swings good it's not the pump, If you dug into the ground and the cyl held strong, it's not the cyl or the line relief valves. The valve and spool shows it can hold pressure in hold position. Looks to me like the spool is not moving full stroke, could also be a regen circuit if it has one. Try the patern changer first then you can use the Boom pilot oil to work the problem stick circuit. Later Bob
According to this sales brochure the 303 has a stick regeneration circuit which enables high stick-in speeds. I bet that the regen valve is acting up and causing your problem just like the regen valve on my 345B was. $21.00 worth of seals fixed my problem. (Thanks again Per Eriksson) Sales Brochure for 303: http://www.hewden.co.uk/_resources/filestore/catalogue/2011-09-v1-CAT-303CR.pdf You can read about my troubles with the 345B here: HeavyEquipmentForums
That is a great thread scrapper. And this may seem crazy but I'm supposed to go look at a 375 with a shear acting just exactly like that next week. I'll have to hit up Per for regen valve location on that beast (though I bet its close to where it is on your 345) Back to the 303. Guess I better figure out where that valve is, I know its not on the cylinder.
The 303 has no such valve, nor does the 375.
Thanks Per. For everyone else - thanks for the input, I replaced the spool and spool block and she's working great now.