Interesting issue after replacing all resolver O rings on Cat 426

[jetranger]

1988 Cat 426

Boom was very slow to come up... so slow in fact that it wouldn't build enough momentum to lock in transit mode.

Over the last couple days I've replaced every O ring on every resolver, both primary and secondary. While doing so, there were only remnants of what were once O rings on each one. I honestly don't know how anything functioned on the backhoe before. Now, everything works better, and much quicker, even at low RPM's.

The issue I'm having is that since replacing the O rings, the pump has too much idle pressure, even with all controls in HOLD mode. I can actually hear the pump whine now, and when starting the engine, it stops cranking after the 3rd or 4th rotation because the pump builds too much pressure. I can only start it if I actuate a hydraulic (any of them) to relieve the pressure build up.

My thoughts - since this backhoe is new to us, maybe the previous owners tried to increase that pressure on the hydraulic pump valve to compensate for the leaky/non existent O rings

- or -

While replacing the O rings, a chunk of one of the old rings is now floating around in the system and causing an issue

I really don't know what else it could be. Everything went smooth with changing them out with Cat OEM parts. All of the check valve balls are back in their place as well.

I though I'd ask opinions on here before adjusting hydraulic pressure or attempting any other fix.

Thank you in advance

 

[Nige]

The hydraulic system uses a piston pump with a compensator valve. From your description it sounds almost as though the pump is not completely destroking for some reason.
If you want to start by checking pressures attached is the test & adjust document.

 

[jetranger]

The hydraulic system uses a piston pump with a compensator valve. From your description it sounds almost as though the pump is not completely destroking for some reason.
If you want to start by checking pressures attached is the test & adjust document.

Thank you for taking the time to reply. You are correct, I don't think it is destroking all the way.

Under the troubleshooting section, it says the signal line may be vented. I can't find anywhere in the manual about how to bleed or vent the signal line. Do you have any tips? Perhaps I got air in the signal line system having every resolver opened up.

I will need to get some gauges to check pressures. I currently don't have any.

Thanks again.

 

[jetranger]

A bit more information...

Still working on acquiring gauges. In the mean time, I read there can possibly be air trapped in the signal line. I loosened the signal line where it feeds back into the pump and actuated all controls. While it was loose/leaking, the pump did destroke completely. I did not see or notice any air bubbles coming out. Upon tightening the nut, the pump engaged again and I could hear the engine load increase. I believe something is causing the signal line pressure to be too high.

I did go back and check the first 4 resolvers I did. I had only done the secondary's on them. When I started the backhoe to swing the other way to gain access to the other side is when I first noticed the issue. So really, I had only changed 4 resolver secondary's when the issue began.

All controls function very well. No binding, sticking, hesitation, etc. The only issue is the pump will not destroke.

Thanks again

 

[jetranger]

The hydraulic system uses a piston pump with a compensator valve. From your description it sounds almost as though the pump is not completely destroking for some reason.
If you want to start by checking pressures attached is the test & adjust document.

I was able to get a gauge on the system this morning. For some reason, signal pressure is all the way up at 2500psi. Pump pressure discharge tap is also at 2500psi with all controls in hold.

Just to test, I backed out the Flow Compensator spool to lower the signal pressure down to destroke the pump. Although it decreased pressure and destroked the pump, everything was very slow and weak afterwards.

I set the spool back to where it was and I'm still having the same issues.

 

[jetranger]

OK, problem solved.

After laying under the backhoe for about 30 minutes trying to trace lines and figure out where I went wrong, it finally occurred to me that I replaced all the O rings, even in the AUX circuit, but hadn't been able to test those because we have nothing hooked up to the AUX hoses. I got in, moved the AUX pedal back and forth a few times, and it easily started right up with no load on the pump. I think the circuit got some debris in it, or the pedal had moved out of neutral and not moved back freely, putting full pressure to the signal line.

Hope this might help someone else down the road.

 

[berky]

glad you got it resolved. thanks for letting us know what the issue was.