John Deere 770G hydraulic overheating.

We've got a John Deere 770G that's overheating in less than 15 minutes of use. I've checked the filter and coolant, and the heat exchanger looks fine—the fan's running at high speed. I'm wondering what could be causing this, since CAT graders like the 12H and 140G don't even have heat exchangers and run without problems. Does anyone have the hydraulic schematic? Also, what should the low standby pressure be?

Temperature checked with an infrared gun? Standby pressure should be 1775-1985 psi.

We're without the right tools. The hydraulic boils fast and gets extremely hot within 20 minutes of running. It's even melting the sight glass.

Yikes!

Check your messages.

We've got knocking and black smoke when we start it up, but both go away after we bleed number 3. It's a common rail setup.

I've tracked down what's causing the overheating to happen so fast. It's the cooling system—the fan slows down after a few minutes of running. I've narrowed it down to the fan control valve. The valve has these parts: a fan speed control solenoid (pressure reducing), a fan direction solenoid, a pressure sensor, and 4 check valves. I really need the exact hydraulic schematic for the cooling system on my John Deere 770G, serial number DW770GX625056.

Check your messages.

Sounds like MG got back to him and he sorted it out.

I'm dealing with a John Deere Grader and running into some trouble. There's constant pressure in the load sensing line, which is making the pump work overtime and causing it to heat up. I've got constant flow coming from the steering valve and from both the left and right direction control valves. I adjusted the flow-reducing-valve in the combination valve on the hydraulic manifold and got the steering valve sorted, but those two control valves keep putting out pressure. I also looked at the load sense relief valve, but I can't adjust it – nothing moves and I'm not sure if it's stuck or how it's supposed to work. The machine is a DW770GX625066. Has anyone come across this before or have the hydraulic schematic? Any help would be appreciated.

What pressures are you seeing on load sense, pump standby, and the differential? Check your messages.

Right when I start the engine, the standby pressure jumps to 1500psi and just stays there. Even when I work the lever, nothing changes—it holds 1500psi the whole time. I haven't checked the load sense line yet, but there's definitely high pressure in it, which shouldn't be there. Could this be a faulty load sense relief valve, or maybe the control valve in that section isn't fully centered and is slightly operated?

Damn.

Check for partially stroked spools first.

I got the 770G working again. I connected a pressure gauge to the pump and adjusted the pressure reducing valve. The pressure dropped from 1500 psi down to 500 psi, which was the system operating pressure at the time. Then I tuned the load-sense relief valve until it hit 2000 psi for max operating pressure. The machine runs fine now.

I'm still wondering though why Deere put a load-sense relief valve in there. The adjustments I made seemed to work the same way as tuning the load-sense and pressure compensator spool would have. Maybe there's something about the design I'm not getting. Either way, the overheating issue is sorted and everything's running properly.

I'd appreciate any photos or details you can share to help track this down.