Caterpillar 305CR - help needed for boom hydraulic problem

[TripitakaBC]

I have a 2006 Cat 305CR, prefix DGT. 5300 hours.

First noticed a problem with the boom not pushing deep on a pit excavation but it then spread to slow lift on boom raise before finally quitting raising the boom altogether. I did some basic troubleshooting, looking for leaks etc and pressure-checked the pump which seemed OK. I checked the boom cylinder to ensure that it wasn't leaking internally and it checked out OK. I also switched control patterns to ensure that it wasn't an issue with the joystick controller. Same issue in either control pattern.

At the amount of hours on the machine I checked the hydraulic filter and pulled out the pleated material and found some metal debris in the pleats. Even though the pump pressure checked fine, I chose to replace the pump anyway.

I now have a new pump, fresh filter and oil. Same problem. I've pulled the line relief valves for the boom (second block backwards in the valve bank) and they all seem OK and have no debris in them.

Here is the kicker: if I dead-head the bucket curl, which operates on the same side of the pump (pressure tap P1) and operate the boom raise at the same time, the boom does raise. The same result if I boom swing to the left to dead-head it and raise the boom, the boom comes up. Without another circuit being deadheaded, the boom doesn't raise.

I'm getting out of my depth at this point so I'm looking for someone that can possibly explain what is going on and suggest potential fixes for it.

 

[uffex]

Good day
When you checked the pressure did you dead end the boom?
Kind regards
Uffex

 

[TripitakaBC]

Good day
When you checked the pressure did you dead end the boom?
Kind regards
Uffex

Yes, initially we figured that was the most likely problem so we started there. The service manual procedure for checking main relief valve pressures is to close the bucket and hold it to test the main relief at P1 and the pressures are good.

We only discovered by good fortune yesterday that operating the boom at the same time as the bucket is closed and held that the boom cylinder works OK, both up and down, but doesn't work without another circuit being actively closed - bucket, boom swing left etc. Tested boom creep overnight and it only dropped about 3 to 4" so I'm thinking that the boom cylinder seals are good.

 

[Nige]

Does your service manual have a copy of the Hydraulic System schematic.?

 

[TripitakaBC]

It does. I can post it here if the rules allow?

 

[Nige]

It does. I can post it here if the rules allow?

Absolutely. Someone like @uffex can use it to help you, at least to make suggestions.

 

[TripitakaBC]

CAT 305CR Hydraulic schematic

 

[uffex]

Good day
You have Nige on board, Im sure he will help you, my suggestion attached.
Kind regards
Uffex

 

[Nige]

I'm not the excavator guy.....
Hoping that someone can offer suggestions based on the posted hydraulic schematic and a description of the described symptoms (as I understand them) of the boom cylinder function being restored if the bucket control is held in the curl position.

 

[TripitakaBC]

Looking at the schematic (first time looking at a hydraulic version but I'm used to electrical schematics) some observations come to mind but I'm acutely aware they may be flawed:

1) The pilot system running through the controls seems not to be a cause as the same problem occurs when the control patterns are swapped. Currently, right side front/back = boom up/down and left side is sick in/out. Swapping the controls results in right side working stick in/out perfectly and left side does not operate boom up/down. My hypothesis is that the controls are not the issue.

2) Boom and bucket are on the same controller and the valves sit next to each other in the MCV. My hypothesis here is that when the bucket is closed and held, fluid is flowing through a return line and backfeeding the boom valve, possibly due to a malfunctioning spool in the boom control valve. I can't work out from the schematic how that might occur because I only partially understand the schematic. I also don't fully understand the principle of the drift control valve.

3) That seems to suggest that the problem (and solution) lies in the boom control valve spool or drift control valve. I have taken these apart already to check for debris and the spool shaft did seem to have more resistance than I would expect when withdrawing it from the valve. If it should slide right out easily, it doesn't, it needs to be pulled. Maybe I should try the bucket spool and see if there is a difference. If the spool is sticking, I can see where the pressure in the pilot system would simply drain back through the resistance and that would explain why the boom makes a *slight* movement when actuated but doesn't lift; more like a pressure twitch. Boom lowering works OK to ground level but it has no 'negative' pressure without closing the bucket.

I hope that makes some kind of sense to someone. If anyone can help me understand what the controls are in blue section, I would be grateful. I think the bottom two are the boom line relief valves, long vertical one is the spool, second from the top is possibly the drift reduction valve and not sure what the top one is.

 

[uffex]

Good day
Sorry I have made a better explanation, see attached.
Kind regards
Uffex

 

[TripitakaBC]

I am clearly reading the schematic incorrectly; I thought that the check valves only allowed fluid through in the direction of the arrow. If that is correct, how is fluid getting past the check valve on the right hand side of the diagram and through the check valve at the bottom? I suspect the simpler answer is that my understanding is completely wrong.

I haven't pulled the bucket control valve yet as I didn't suspect that the issue would be there. I'll need to dig into the manual to see what I'm looking for first.

 

[uffex]

Good day
You are correct just the same as a diode fluid will only pass in one direction, the exception is the bucket check valve which I suspect from your post.If you have a exploded view of the bucket valve, I can point the check valve in question.
KR
Uffex

 

[TripitakaBC]

So these check valves are the load check valves on the top of the main control valve? Is that correct?

This page helped me understand my confusion on the check valve symbol. They seem to be illustrated on the opposite form to diodes. https://www.engineeringclicks.com/hydraulic-symbols/

 

[Cmark]

The check valve symbol is representing a ball against a seat. It blocks fluid in the direction of the "arrow". Counter intuitive if you're comparing it to a diode symbol.

As per uffex's drawing, logically, the circled check valve may be the problem. It could be located in between the valve segments. The parts breakdown isn't very clear. This doesn't explain why boom swing makes the boom operate though. It may be affecting the boom 2 spool.

I suggest you carefully go through each and every function in turn and note which ones do and don't affect the boom operation.

 

[uffex]

Good day
The check valve sits between the two outlet ports I do not have a Cat poarts illustration for that machine.
See illustration may help out.
Kind regards
Uffex

 

[uffex]

Illustration

 

[Nige]

The check valve sits between the two outlet ports I do not have a Cat poarts illustration for that machine.

See attached. For which valve section in particular are you looking.?
There appear to be two sections listed for the boom and a third that is combination boom & work tool

 

[TripitakaBC]

So I pulled the two check valves on the bucket valve but they appeared clean. I couldn't quite figure out the difference between the two (2 and 3 below) so I made sure they were clean and moving freely and reassembled. I didn't have a lot of hope for it to be fixed and initially, it looked like it wasn't but then...behold, a working boom! It just took a couple of tugs on the joystick and it worked OK.

I do want to thank you guys for helping me out here; this was driving me nuts and soaking up way too much time in thinking about the next steps. I'm very impressed with excatech.org, that is a great resource!

 

[uffex]

So I pulled the two check valves on the bucket valve but they appeared clean. I couldn't quite figure out the difference between the two (2 and 3 below) so I made sure they were clean and moving freely and reassembled. I didn't have a lot of hope for it to be fixed and initially, it looked like it wasn't but then...behold, a working boom! It just took a couple of tugs on the joystick and it worked OK.

I do want to thank you guys for helping me out here; this was driving me nuts and soaking up way too much time in thinking about the next steps. I'm very impressed with excatech.org, that is a great resource!

View attachment 298584

Thank you for the kind words great you have it fixed.
Kind regards
Uffex