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
D6R yields 50 different results in the Cat system. Will be needing your machine serial number to understand what you've got.
The serial is 9PN00967
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.
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
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?
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.
Are Are there any Active Codes at all on the machine.? SERV CODE icon on the Monitor panel permanently illuminated.?
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.
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.
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.
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.
Hello all, Revisiting this problem in the shop. After plugging into et, and placing pressure gauges on both the priority valve port and the main clutch pressure port, I recognize that when the priority valve is “ON”, the main clutch pressure drops and matches the priority valve pressure. This condition happens when the rpm comes up 1350rpm. I’m reading in the service manual that it is correct for the priority solenoid to be “ON” once the rpm rises to 1350 or greater. I’m planning on diving into the priority valve today, just looking for some insight on what may be hung up in the valve itself. My thought is that when the priority valve is “ON”, it shouldn’t create a reduction in oil going to the clutch system. Thank you!
have you checked the strainer
All strainers and filters are clean and new. I just replaced the priority valve solenoid and have no change in condition. My thought is that when the priority valve solenoid opens at 1350rpm, oil is being sent to the drain side instead of to the transmission. I took apart the entire priority valve and did not find any hung up spools or clogged orifices. Where could the oil be incorrectly going once that priority valve solenoid opens? Based on the pressure chart, I should have 400+ psi at all times at the transmission main relief valve. When the parking brake is on, which means the priority valve is on, the transmission main relief valve pressure drops to exactly what the priority valve pressure is (140psi). To me, I’m recognizing that the transmission main pressure is just mirroring whatever the priority valve pressure is reading.
What does touching the brake pedal do to the pressures.?
When I push the brake pedal in, the pressures drop slightly for about a second and then come back up to previous pressure
OK, so that seems to rule out the brakes as a cause of the pressure loss. I know the question was asked previously but I don't see an answer. What type & grade of oil are you using in the powertrain.?
We use a Kendal 30w sae oil. Which we also use in our other 9PN prefix D6R. I compared pressures in that dozer and it does not have the pressure drop in main transmission pressure relative to the priority valve solenoid opening like this D6R is having.
TO-4 specification I assume.? If the priority valves are the same in both tractors does the possibility exist of running a test using the PV from the other tractor.?
I appreciate the insight on all this. It would be a stretch to take the PV out of the one D6R and use it in the other. Could there be a check valve stuck open in the system maybe? The way I’m picturing this from a flow perspective is that the solenoid is opening, and the oil is just stuck flowing in one direction (the problem) instead of to the transmission. Maybe a check valve that’s between the PV and the transmission in some way?
Do you have the schematic for the power train oil system and the text explanation of how it is supposed to operate.? Another question I don't think has been asked. How did this issue start; suddenly overnight or gradually got worse over time.?
The PV sort of does not direct oil... it does cut off oil ... wich is not the same thing. I just don´t understand what is a solenoid control doing there... as that is suposed to be a safety feature and u can see my point. Maybe u can try run the system with the said solenoid at the X RPM feature offline and check pressures. Some of those tractors also use pressure acumulator, does your have one and its working correct. And @ 20K hour , is the charge pump good enough? Good luck
See the illustration below.
Based on the description of the problem and the steps you took to diagnose it, I can assume that the transmission pump has failed.