I have been working on a hydraulic issue with a cat 426b. 11,000 hours. initial problem was after getting warm front loader would stop lifting. I installed a gauge on pressure port 3 which is signal pressure and found pump was strokiing/destroking rapidly. I changed resolvers, no change, changed entire loader lift valve with new, fixed the stroke destroke problem. i still didn't like the way it lifted so i decided to see if the cylinders were good. blocking off left then right i would watch the gauge i had 1800 psi and no cylinder movement with both. two bad cylinders? took one apart and the packing edge on the rod end was failed even wear but worn badly the rod nearly fell out of cylinder no tension at all. ok so i decide to check the system with no cylinders attached and plugs in those lines. after at operating temp, i can get a consistent 1800 psi from that function and bucket curl back function, clam closed gets me 3000 psi? what am i suppose to have and why do they function differently? Stand by pressure is 800 psi, its suppose to be 320-ish, if i lower my stand by to 320-ish i only get about 1100 psi. on the hoe end everything gives me the 3000 psi! HELP!!!!!!!TIO
Sounds like bad port relief valves on the circuits where you're getting low pressure but that's just a guess. Some systems have relief valves in the load sense signal circuit, I am not familiar with the 426B so I can't say if that's a possibility but look for one in the loader valve assembly?
By eliminating the cylinders and showing the backhoe works you have a problem in the loader valve. From the manual it looks as if there are no individual reliefs for the sensing circuit. Just 4 port reliefs. Try swapping the center cartridges to the outside.
Have you done anything with the clam signal resolvers. In this system the last one that works is usually the one causing the signal loss.
Mobile tech I have swapped and new o-rings on secondary resolver but not the primary resolver. I thought from reading "system operations" that it was the one ahead of the problem that was failed? not the first time I was confused by the manual. Mikebramel, No port reliefs in loader lift or bucket crowd. TIO
TIO, as I understand it the last circuit in the line that is working "normally" is the resolver with the problem. I don't know if this is any help but here's a schematic of what the signal resolver circuit looks like. Reference #1 is the primary resolver for each particular circuit, #2 is the secondary resolver. Only the Steering circuit (J) has a single (secondary) resolver which feeds via outlet K to the pump compensator. The circuits are as follows: - A) RH Stabilizer circuit. B) LH Stabilizer circuit. C) Boom circuit. D) Swing circuit. E) Bucket circuit. F) Stick circuit. G) Lift circuit. H) Tilt circuit. J) Steering circuit. K) Feed to pump compensator.
No port reliefs????? That can't be right can it?
Lantrxco, as you see one port relief only on boom down, nothing on boom up. I am a bit confused about how the spool inside the spool functions and if this strokes the pump differently not allowing full pressure unless it sees volume being used? Because I am not using any volume because I have the ports plugged....TIO
Weird. Riddle me this batman, since I can't find a schematic online... does the backhoe valve get it's pressure supply teed off before the loader valve, or is it coming from a port on the loader valve? I'm beginning to suspect your problem may lie in the steering priority and pressure control section of the loader valve, as it controls flow to the loader sections as part of the priority function for steering. If the backhoe supply tees off ahead of the loader valve, that may be worth diving into. http://new-cat-information.blogspot.com/2015/06/systems-operation-416b-426b-428b-436b_72.html
If you want a Schematic drop me a PM with your email address.
TIO, what's the serial number? Also, I just reread the original post, this machine has a clam (4 in 1?) is that just an added section in the loader valve or a separate aux valve? Could just be bad primary resolvers in the lift and bucket sections also?
Lantraxco, Plugged steering and backhoe resolver lines with no change in pressures. TIO
5yj2??, serial number. All new lloader boom valve. New resolvers in bucket valve, 4 in 1 bucket clam valve last in line in loader section...TIO
Another clue... since the clam section is the last section in the loader valve and works properly, and the backhoe functions all work properly, it would seem to narrow the problem down to the loader lift and bucket sections. Seems? And why are they both malfunctioning? Have you tried activating both lift and curl functions at the same time just for giggles? What's puzzling is that the backhoe signal line comes in at the end of the loader valve and flows through all the secondary resolvers which means at least one side of all of them is working properly, and it should mean that the steering resolver is working correctly. The clam valve gives you proper pressures which should mean that the steering priority valve is allowing oil to flow to the loader functions. What doesn't make sense is that neither the lift or curl functions work correctly and yet there's no common failure point I can see that isn't also used by another function that's working right. It will be something simple. Any possibility there's an O-ring blown out between the clam and bucket sections? Still should affect the clam section if there was though... Have you looked at the compensator spools in the loader and clam sections, again shouldn't affect the lift valve but... ?
If they'll interchange, try swapping the clam and bucket sections temporarily, see if that changes things... ?
Different P#'s, that would be like 6 hours of work, don't think I could get customer to buy that one...next move is to reinstall cylinders after customer has them rebuilt. I will revisit then. Everyone have a good safe weekend.....TIO