Caterpillar D6R Loss of Clutch Pressure

[dcditommaso]

Hello everyone! First post on here. I’m currently working on a Caterpillar D6R LGP with ≈22,000 hrs on the machine. The problem is that when the machine is operating at elevated rpm’s, the transmission drops out of gear until the rpm’s are reduced. It does this in all gears as well as neutral. After checking main clutch pressure it was discovered that the 425psi pressure drops off to exactly 120psi when the rpm’s are raised above 1350rpm. We checked the main relief valve and didn’t see any problems. Another observation is that the main clutch pressure, in neutral with the parking break on, is also reduced to the same 120psi. Once the parking brake is released, the clutch pressure rises to the rated 425psi and will stay there until the rpm’s are raised.
Wondering if this is a symptom of a check valve malfunction on the priority valve side. Or a speed sensor that’s dropping out at elevated rpm’s?
Thank you all for any insight on this issue

 

[Chrisso]

D6R yields 50 different results in the Cat system. Will be needing your machine serial number to understand what you've got.

 

[dcditommaso]

The serial is 9PN00967

 

[Chrisso]

That helps. By no means a dozer expert here. Anyone else thinking a scavenge pump issue?

Might help if you tell us what oil you're running.

 

[dcditommaso]

I appreciate the recognition on a scavenge pump issue. That checks out. The inability to hold pressure at elevated rpm makes me think it’s an air leak on the suction side of a pump somewhere. I looked at the schematic and recognized that there’s a check valve between the torque converter oil and the power train oil. That may be in play also.

Thank you for the response overall

 

[Chrisso]

I'm not sure an air leak would reduce pressure to 120 psi consistently. Sounds more like a priority valve not doing its job the more I think about it.

Have you checked out the solenoid in the priority valve? O-rings still there?
Have you checked for fault codes relating to this?
Also is the trans oil temp gauge reading correctly?

 

[dcditommaso]

Thank you for the insight on this. We don’t currently have any codes related to the priority valve but I will plan on pulling the solenoid to check for any damages.

 

[Nige]

Are

We don’t currently have any codes related to the priority valve but I will plan on pulling the solenoid to check for any damages.

Are there any Active Codes at all on the machine.? SERV CODE icon on the Monitor panel permanently illuminated.?

 

[dcditommaso]

There are not any active codes on the machine. I plugged into ET to dig deeper and there aren’t any active codes.

When the condition occurs (clutch pressure drops from 425 to 120) there are not any codes that activate. I wanted to note that this loss of system pressure is also seen in the brake valve as well. Looking at the hydraulic schematic it shows that the brake valve and transmission both share a common supply line.

 

[Nige]

Looking at the hydraulic schematic it shows that the brake valve and transmission both share a common supply line.

That’s the purpose of the priority valve. To ensure that in case of any deficiency of oil supply the brakes are supplied first. Which is why it would be a logical place to start.

Try putting gauges on the brake test ports and see what happens when the transmission pressures are fluctuating.

 

[dcditommaso]

Thank you for the insight! I did put the gauge on the brake test ports. The pressure at each side of the brake valve fluctuates in the same way the clutch pressure fluctuates.

 

[Nige]

I did put the gauge on the brake test ports. The pressure at each side of the brake valve fluctuates in the same way the clutch pressure fluctuates.

In that case the problem would appear to be upstream of both brakes and transmission so a close look at the priority valve is the logical next stop in the diagnostics.