337 Excavator Hydraulics Question

VernLPoole · Jul 21, 2020

Nearing retirement and having a 15 acre sandbox plus neighbors with many more acres available for potential playing, I finally bought myself a used excavator: 2005 Bobcat 337. I did some dirt work in my 20's, Cat D3 dozer, John Deere backhoe, etc, and have owned a small Ford tractor for years, but the hydraulics on the Bobcat are orders of magnitude more complex than anything I have worked on, so I'm not even sure if the symptoms I'm getting are a problem or are normal. Hoping someone here can help me clarify that and maybe steer me toward what to do if there is a problem. I'm reasonably handy and prefer to fix things myself if I can rather than spend a lot of money, especially since I'm not really generating any income from the machine.

So the symptom is that sometimes the hydraulics get very weak/sluggish below about 1500 rpm. I believe it may be related to temperature because it seems to happen after about 20 minutes of operation, though I can't be sure that's not just coincidence. The last time it happened I put a gauge on the test port and sure enough,at idle the pressure was something like 2200, and if I slowly increased the rpm, the pressure jumped up to normal (~3500) when it reached about 1500 rpm. I have the service manual and am slowly gaining some understanding of the system, but there's not much information about nuances of how things are supposed to behave, or maybe I'm just not understanding the information that is there. If that's how it's designed to function, I can run the machine at higher rpm, but much of the work I'm doing is easier with a light touch: setting stones in a wall, fine grading along a driveway, etc. Any thoughts?

Thanks in advance for any insights you can offer!

Gale Hawkins · Jul 21, 2020

Have your replaced the fluid and filter(s) in the system since you got it? I would like to have a machine like that one.

Do you know why it was on the marker?

VernLPoole · Jul 21, 2020

Gale H: No, I haven't but the guy I bought it from seemed pretty trustworthy and diligent about maintenance, marked the date and hours on filters when changed, etc. I could check the hours on the hyd filter (machine is next door right now) but I seem to recall it was done shortly before I bought it. And yeah, it's a sweet machine... sold because the guy bought another, bigger excavator and his wife laid down the law that he couldn't have two!

ddbackhoe · Jul 22, 2020

Not today or tomorrow, but Im betting that the hydraulic pump is wearing out and will need to be replaced. From age and use the pump can/will develop a bypass just by fluid friction. Just went through a very similar experience with our farm skidsteer. More and more throttle was needed to pick things up. Got to the point where full throttle would barely lift a ~1000 lb. hay bale (machine rated for 3200 lb. lift) Of course, the OEM pump was $1100 but after some research and measuring and head scratching, It was a gear pump that the OEM didnt release data for (Sunstrand). I had to disassemble to get the displacement, then drive measurements, then external measurements and the replacement ended up being 5/8 shorter (Surplus Center) I did have to fabricate a new bracket and get two hose/port adapters (another ~100). The new pump itself was $150.
The people at Dynamic (Dynamic Fluid Components, Inc. - Dynamic Fluid Components) helped make it all happen. They answered my stupid questions, helped me figure out what would be a suitable replacement and even had the local distributor, Delmarva Fluid Power (link didnt work) call me to finish the order.

Id be remiss not to call out Southern Maryland Hydraulics, Inc. - Hydraulic Repairs | Waldorf, MD too. Ron and Les put up with me over the years, not knowing diddly about hydraulics, and spent some time showing and helping me understand something about it all. You would be well advised to find a local shop that you like. Hoses, fittings, seals, etc.etc.etc are in your future :laughing:

VernLPoole · Jul 22, 2020

ddbackhoe, thanks for the response! I have enough of a pessimistic, Murphy's law streak to be willing to believe it's something catastrophic, but the fact that it works fine for awhile (till it warms up?) and then suddenly changes, and the fact that there is such a clear sudden change at 1500 rpm, makes me think the pump works fine but some control mechanism (Two stage pilot spool? Dual sequence valve?) is out of whack, or maybe more likely, the machine is not designed to be operated below 1500 rpm, and I'm just an idiot for wanting to putz around at idle! I thought the point of having variable pilot pressure was to give the joysticks a lighter touch at low rpms, but as I said above, this level of hydraulic sophistication is new to me, so I could easily be making wildly erroneous assumptions. And I can't tell from the service manual if the pilot pressure is actually variable, or it just jumps from 250 psi at idle, up to 435 at some point on the way to high idle, with nothing in between. And would that completely explain the sudden jump in available working pressure at 1500 rpm, meaning everything is behaving as intended? The service manual is great for fixing things that are obviously failed or broken, but I'm still at a more basic level, trying to understand how the system works and what each component is supposed to do to begin with.

Gale Hawkins · Jul 22, 2020

On our 1983 JD 310B backhoe I have to loosen up the hydraulic sump plug so the water can drain out of the sump every year. I have to be careful to not drop the drain plug and lose at of the hydraulic fluid in the sump. I guess it forms in all of the cylinders and it works it way back to the sump. The oil was white when I first got it and it did not perform well. Our pump is green so it was replaced with one off of a farm tractor perhaps. The backhoe was worn out 25 years ago I am sure but it still starts and runs.

SSdoxie · Jul 22, 2020

Vern, What you are seeing is normal. The 337 has load sense hydraulic system. This system uses a variable displacement piston pump to vary the maximum flow of fluid through the control valve to meet operator requirements at any given moment to minimize heat generated and also avoid prolonged engine lugging. The pump performance adjustments are done at max full throttle (max HP) with warm oil (140 - 180F). If/when engine RPM is set too low the pump could not adjust enough to allow full stroke of valve spool(s) without killing the engine, so the PWM solenoid is used to limit pilot pressure so no control spool can go to full stroke. Normally a PWM spool is variable but in this case the controller reads engine RPM and has just 2 voltage signals to the solenoid. It does not vary throughout the full RPM range.

VernLPoole · Jul 22, 2020

Ahh, thank you SSd, it's beginning to make sense now. That's a relief to know nothing's gone wrong. Now if I can just figure out why the blade doesn't hold position when I put it down to stabilize the machine! (It stays up fine, but it floats up when I put weight on it, even when not in float mode...) It's always something!

laurencen · Jul 22, 2020

couple years ago I was informed by the service teck from Bobcat the machines are designed to run full throttle, obviously I do not but full throttle did fix a few issues the service call was for

SSdoxie · Jul 23, 2020

VernLPoole said:
Ahh, thank you SSd, it's beginning to make sense now. That's a relief to know nothing's gone wrong. Now if I can just figure out why the blade doesn't hold position when I put it down to stabilize the machine! (It stays up fine, but it floats up when I put weight on it, even when not in float mode...) It's always something!

There is a circuit/port relief in the blade valve spool to limit maximum pressure in base end of the blade cylinders. Leakage or low setting at this relief might be the problem.