can dig perfect for awhile then on the next dig the stick stops at the least resistance. shut it off ,push stick joy stick out function,(to drain pilot press) start back up and it will dig normal for several passes.heres what ive tried.... swapped relief valves, removed spools in main stick and secondary stick valves... pilot pressure is at spec,.put guage at stick in pilot line to be sure line press was draining back to tank...tried it in each mode. the machine is owned by a retired doctor who never abused it, has 2000 actual hours on it...it sure has me stumped.....
Have you pulled filters yet? May have something getting in the spool and keeping it hung open to let the stick free wheel. I've seen cylinders that start making metal do this.
yes I did, did every hyd. filter on the machine. I can almost count how many passes( 5 or 6) until it`ll act up again.by the way the spools looked new under a magnifying glass. also ran a magnet in the valve bodies , no metal came out. keep on mind.... after I drain off the stick out pilot pressure it will have force enough to drag the machine side ways...... until it acts up..... then the stick just stops when I let it touch the ground..
Sounds like you've got a floater ! …….. huh you ask ???? I had a JD 290D years ago …. when I bought it it ran pretty good in the guys back yard …. brought it home and the very first job I had it booked for was to dig a bunch of holes for concrete piers ~ 4' square and as deep ….. was into the job for about an hr then all of a sudden as I was pulling in the stick it started to come towards the machine and didn't stop when you let go of the control ….. hit the key to off …. started it up again and 5mins into it and again it happened …. this time I wasn't so lucky (the machine had an oversized clean-up bucket, and the machine had full cab guarding as well, obviously the manufacturer of the cab guard never tried the C/U bucket on the JD because if you got too close it would crash into the Cab protection) ….. SO !!! CRASH into the side corner of the CAB guard, hit the key again and repeat as necessary …… The issue was resolved after hauling it back to the JD dealer where it came from …… they found in the spool bank the one of the "POPPET" relief valves that the stem had broken off (they look like a small stemmed mushroom) and was floating back and forth in the spool journal jamming the spool shuttle open and not letting it return to the neutral position ….. but as soon as the machine was turned off it relieved the back pressure shooting the stem backwards out of the spool until you began again and after a few cycles it worked the metal stem back into the shuttle …… Don't ask how this thing got broken because even the JD dealer was dumb-founded !!! Don't know if this is what's going on with your 312 but thought i'd give an insight of what or where too look, not saying that the 312 has a similar poppet relief but maybe there is something "floating" around in the system like a broken off piece of return spring ????? ……Good-luck !
o k thank you, I`ll dig into it Monday...sure has me crazy...
when you had the spools out did you see if they would freely move in the control valve? they should fall into place on their own from the top.... try tee-ing pressure gauges into the pilot lines on the top and bottom of the stick spools on the control valve... just to verify you are getting full pressure to the spools...