277C Hydraulics bogging engine

[Chrisso]

First place I would check is the load sensing relief valve. You had 700 psi getting past this relief valve during the standby pressure test when there should only be 420 psi. We now know the 700 psi isn't backfeeding from the pump control valve. I wonder if the o-rings are worn?

 

[Mark250]

Went to lift the bucket and found that the bucket moved very slowly and stopped around 2 feet off the ground. Went back down with no issues. tried going back up and only got around 18 inches high. back down no problems. Then the bucket could barely move an inch. The entire time the pump discharge was sitting around 500 psi, never bogged or saw anything like the 4400psi from

Be aware with the signal line disconnected from the pump. The pump out put should not increase. What you are seeing is pretty normal for a pump on minimum output.
The reason to check with signal line disconnected is to make sure that the pump is not delivering high output without a commanded signal from the control valve. Having said that, the swash plate in the pump could hang up due to flat spots on the swash plate pivots and cause problems

 

[Tall.Timber]

I was out of town for a while so I am just getting back to this and getting to work on it. As stated before I do not see the odd jump in discharge pressure with the load sense disconnected. This seems to me that it is possibly an issue with the load sense valve as stated. I am looking at a parts diagram of the load sense valve and from my under standing the adjustment with the smaller spring is for the Margin pressure adjustment and the larger bottom spring is for the High flow pressure adjustment. I am also seeing some pins and orings on the left side labeled "14". Just want to bounce this off you if there is anywhere i should be looking first? It seems like it could be sending the pressure high if the discharge pressure being sent to this valve was getting around the regulating springs and causing issues. I may just start taking it apart and see what i can find.

 

[Tall.Timber]

Is there any reason that the load sense input line coming from the controls would/could be faulty? Just want to make sure i am looking at the right side of this issue.

 

[Chrisso]

Load Sensing Pressure Test
I went outside of the testing procedure and wanted to see what this would do during the bogging event. when bogging the machine with no input to the controls this port saw ~300psi. As soon as i would bumped the controls enough to get the bogging to stop the gauge dropped back to zero. The bogging only happened when this gauge was reading that ~300 psi.

Pump Discharge Test
I again went outside of the testing procedure and saw this pressure with no input during the bogging event and saw that when it bogged the pressure remained at 4400psi. When the bucket was moved enough to stop the bogging the pressure dropped to zero.

So during the bogging event with no operator input, you have 300 psi signal pressure and a pump at max displacement.

For me that pump is doing what it should be doing. Anyone disagree?

 

[Tall.Timber]

So during the bogging event with no operator input, you have 300 psi signal pressure and a pump at max displacement.

For me that pump is doing what it should be doing. Anyone disagree?

That is correct. It is my understanding of the machine that if there is no input to the controls the pump should not be trying to build this much pressure. So it is possible that the issue is inside of the controls and is sending signal pressure when it should not be?

Edit: Confirming question, 300psi signal pressure causes 4400psi pump discharge?

 

[Tall.Timber]

I have an idea that i think might work to help tell what's going on. I plan to leave the load sense input signal disconnected and use a hand hydraulic pump to give this input a little pressure when i try to move the bucket. This should allow me to confirm if it is the hydraulic pump "running away" or if this input signal from the valve body is sending bad pressure. Any reason i wouldn't want to do a test like this? I would of course not send any pressure over the 300psi I see.

 

[Chrisso]

During the bogging event, what happens if you move another machine function... does the load sense pressure come back up and bogging corrects itself?

That's basically doing the same test as you describe above but using another circuits load sense signal.

Can you get to the load sense relief valve I mentioned in post #21 without pulling the implement control valve out?

 

[Tall.Timber]

I will run the controls a bit more tomorrow with the load sense hooked back up and two pressure gauges at the same time to get a better description of what is happening when another function is used. If i am understanding correctly when i run the machine at any given time I should see the pump discharge pressure around 350psi more than the load sense signal pressure.

I am going to check on the location of that load sense relief valve, Based on the attachments above i am pretty sure i will be able to access it.

Also I dont think i mentioned this but during the bogging event the empty bucket will barely move as if it is attempting to lift too much weight. Not sure if that changes things or not. I still plan to check these pressures.

 

[Chrisso]

If i am understanding correctly when i run the machine at any given time I should see the pump discharge pressure around 350psi more than the load sense signal pressure.

That's basically it. And that 350 psi is what they call margin pressure. It's controlled by the margin spool in the pump control valve and set/adjusted by changing the margin spool spring compression.

Margin pressure in most cases (and in yours) needs to be checked or adjusted while an implement is being moved, hence why the procedure calls for that auxiliary hose that connects and loops around your auxiliary ports. I've had success using the raise command to check margin pressure... the problem with that is you only get 1.5 seconds or so to check your gauges while the boom is between roughly mid way up and full stroke. Not ideal but it does get you in the ballpark.

 

[Tall.Timber]

That's basically it. And that 350 psi is what they call margin pressure.

So I took the machine out yesterday with a fresh mind and a better understanding of the how an axial pump like this works. hooked gauges into the load sense port and the pump discharge port. Fired up the machine and saw that the pump discharge was around 750psi and the load sense was 0psi. Ran the machine around for a bit, worked all controls had had the bogging event regularly. The entire time the machine was operating and not bogging i saw ~750psi of margin pressure. I was able to confirm that at the bogging event the machine would surge the discharge pressure to ~4200psi and when I let go of the controls the load sense would drop to 0psi but the discharge pressure would stay hung up at 4200psi. If i bumped the controls a bit i was able to "knock it loose" and it would go back to normal. I decided to address the margin pressure first as that was adjustable and would start to eliminate causes.

This is where it got interesting.

I found the margin adjustment on the load sensing valve and went to loosen the set screw, it felt like it was hand tight at best, loosened it about 2 turns and adjusted the valve about 1-1/2 turns(247 psi per turn). which would give me the 350psi margin pressure. I went to tighten the set screw and it took nearly 4 full rotations to tighten it back down. Very odd. I did also notice at this time that the entire pump was painted OEM back, but this load sense valve was rusted like it had been replaced at some point.

Hoped in the machine and fired it up. Pump discharge ~350psi, load sense 0psi. Perfect. Ran the machine around and did some digging. Did not bog one time, pressures never got hung up like before. No matter how hard i pushed the machine or tired to get the bogging event to happen it could not do it. The machine ran like a top.

I am going to run the machine some more before I call it fixed. But I think that has resolved this issue that i was having. I can only imagine that at some point that load sense valve was replaced or adjusted, and that set screw was not tightened back down correctly. Over time it worked itself higher and higher. This would not really be noticed as it would not slow down the movement or lack power. It kept creeping until the point that the limits of the pump were reached and something was hanging up. My thought currently is that the swash plate was "hyper extended" and the angle created between the swashplate and actuating rod could not be overcome as easily. This would explain the random occurrence and the slow period over which the issue arised.

Again I am 98% sure that this issue is fixed but I am 100% sure this troubleshooting would have been much more difficult without your help. Thank you for your quick responses and extremely helpful graphics, procedures, and descriptions. I feel that i have not only fixed my machine but have learned much about the hydraulic system that will be of benefit in the future. Thank you again!