977L hydraulic issue

I had a 977L present this week with a weak hydraulic system. Customer removed pump for repair prior to any testing being done. Pump was examined by disassembly and no defects noted, reassembled and placed back on. Hydraulic test at pump output goes to 2200 psi at hydraulic stall, which is very close to spec, but machine appears to have limited hydraulic pressure...poor lift and dig performance once warm. I tested as per the service manual all the speeds, the cylinder drifts and the make-up valves, and all tests fine. I have not tested pressure at the cylinders themselves yet.
I'm wondering about the actual control valve. Has anyone had any experience with these to offer any more suggestions???
s/n 11K 6300 Thanks!!! [email protected]

I have found that with a Fixed Displacement system you should be able to hold 90% of stall pressure at low idle. If not you have a leak some place. And I think you do since it fades when warm. Get it warm and check drift with bucket up and a full bucket racked back. If it holds good I think the pump might need a closer look.
Come on back with what you find.
Later Bob

I don't know anything about this particular machine, only giving general comments as I design hydr systems for a day job. May be helpful, may be irrelevant.
Is this a gear/fixed pump, or variable pump? If gear pump, the stall psi by itself is not an indicator-It can be bypassing most of its flow somewhere in worn pump or worn cyl or valves, yet hold pressure apparently just fine.
I need to illustrate that pumps produce flow NOT pressure. Pressure is only a resistance to flow caused by a load. Big load, or stall, or relief valve, causes high pressure. Big hole, easy path to tank, generates very little resistance to letting this flow back to tank.
Let's say 25 gpm gear pump. At neutral all 25 gpm goes through control valve center (or unloading section i the valve, but same concept) with very little resistance to tank. At stall with cv shifted and good system, all 25 gpm must go across the relief valve to tank. Let's say RV is set to 2000 psi (there is an increase in pressure with flow, ignoring that for now). So the pump should test out at 25 gpm flow out of the pump, 2000 psi, and full pressure/force at the cylinders.

Now, if pump is very worn, the flowmeter out of pump may read about 20 gpm or less. Let's be ridiculous and say only 5 gpm out of pump (and yes I've seen worse!), 2000psi on the pressure gauge. Only 5 gpm goes across the relief valve, the other 20 gpm goes in a circle across the worn pump parts. Yet pressure gauge alone still looks good, and cylinders should have about full force. They would move against a load very slowly if at all, there's only 5 gpm left to provide motion.
Similarly, if the control valve or cylinders were worn, the pump may be good, 25 gpm out, but 20 leaking through a big hole somewhere. In that case, there would still be 2000 psi on the gauge, but 25 gpm on the flowmeter out of pump.

If pump is variable (especially load sensing) the whole picture gets way more complicated, none of the above applies directly, but same principles apply: pumps move oil by volume (flow) and loads produce pressure.
Even if the pump is variable displacement, load sensing, the fact you have stall pressure leads me to think the pump would be coming on stroke just fine

Ideally, customer should have had the flow and pressure test done before pulling the pump, but 95+% of the time, the customer 'changed the pump because I wasn't getting pressure'......before any diagnostic or help.

At this point, I would try for a proper flow/pressure test, either on machine or flow bench, to verify the pump. You need flow readings at unloaded condition, then at increasing pressures, say 500-1000-1500-2000-2200.

Then, need a schematic to determine what the control valve conditions are supposed to be. It's likely not simple spool valve like a log splitter or farm loader. More likely has an unloading section at the front, various flow compensating in each section, etc. I have nothing to contribute there.

I don't visit back very often, try PM me if possible if I can help in some way.

kcj

Cat has fixed relief valves that use shimms and springs and are not adjustable except to replace the springs and to add or subtract springs. Also the main relief valve in the hydraulic systems and powershift trannys have a small piolt spring built into the main relief valve that will open the valve and dump oil right out of the pump pressure line to reduce back shock on the system if you are lifting and pushing or are in exceptionally good traction. They are not designed to dump all the oil, just enough to hold at operating pressures.

I had a similar problem with a D-8H. Everything looked good and the problem was identicle. Worked fine till warmed up. We did several tests and changed the pump, which was good and inside specs when measured for tolerances and had the same trouble. Then I changed the spring on the relief valve. It was a little better, but it still did the same thing. When I had the relief valve apart, I noticed that the valve had a grey color, (That part looked like it had been sandblasted, while the rest of the valve was shiny), on it on the plunger end, (The tip where the oil would dump through if the relief valve opened to dump the oil.). It still measured within specs, but didn't look right and it could not be gotten to to change that small spring because of design. We put it together and it was better, but still did basically the ames thing, although it took ten minutes for it to happen as oposed to three to five like before. So as a last resort, I bought a new relief valve(90 bucks at Cat). We put it in and the system works just fine. Aparently what happened was that the little spring had weakened and dumped oil when the heat got to the spring.

This was the worst problem I have ever had with a Cat hydraulic system. We had either replaced everything or had measured and speced valves and opened it up several times. This was the last thing left that we knew of. So don't give up. It may be something odd like this.

You should ahve places to put a pressure gage on to check the pressures at various places in the system. Trace them back to the releif valve or as close as you can from the cylanders on back if possible. A Cat Shop Repair manual will show you where the fittings, (Pipe plugs) are so you can attach a gague on them. You will need a 2500# gague unless you are in a newewr maching of the High Drive age. Probably 1980 or newer.

Hope this helps

Chris

we had an 11K6316 for 21 years. the bucket cylinders seemed to have air in them at times, probably a relief valve as stated in other posts. during that time we remanned both final drives, the W8K winch, and the transmission. Hydraulic never caused much problems, repacked the lift cylinders with the barrells still on the machine. Only engine work was reman head during an exhaust manifold replacement.