Bobcat S300 works in high speed but only goes straight in low speed

[lastchancegarage]

Hey, I only work on machines that are near the end of their useful life. This one is no exception. We've been keeping it together for 10 years and it's as well maintained as possible. Regular fluid changes, belts, hoses, little is left in disrepair. It has 7200hours and is on it's second engine. This summer it started erratic bogging down and stick feedback while performing normal maneuvers. When driving in low gear you can go straight forward or reverse but turning it will click loudly and push back on the manual control sticks. If you push into a pile, in low gear, it won't stall but jerks control handles and squeals/clicks. In high gear it behaves almost normal. It isn't perfect, occasional feedback on sticks, but about 90% expected behavior. The loader functions are normal regardless of gear when unloaded. Very occasionally I can get enough of a load in high gear that it seems to be down on hydraulic pressure and or stall. In low gear the drivability is so poor I can't load both the drive and the loader.

I did remove a drive motor, because the left side "feels" worse, at the owners request but found no signs of damage, seals, metal filings, or excess dirt. I cleaned the main case drain and the one from the left side near the motor. Again, no signs of metal or seals or more than a few pieces of dirt. Those haven't been cleaned since I've worked on it and I'm guessing regular fluid changes/blown hoses, keeps things tidy.

There are no codes showing in the computer. The gauges read normally.

Is the next step to check the charge pressure or maybe relief valves? Or is there something I'm overlooking. I've always done well to find the simple things that cause grief but I'm starting to worry this one isn't simple?! Any guidance would be greatly appreciated!

Merry Christmas!

 

[Txhayseed]

I would put a gauge on it and start checking pressures. Sounds like a flow issue or lack thereof. Sounds like your bypassing internally somewhere. A serial number would help. Putting a gauge on and seeing what your pressures are doing is a good place to start.

 

[lastchancegarage]

Serial # 525813507

Any recommendation where to tie the gauge in? I'm not used to checking pressures. Normally I can find a dirty screen or filter somewhere but I'm questioning whether there is something inline I'm not aware of or can't see without disassembly.

Thanks for the feedback.

 

[lastchancegarage]

I checked the charge pressure and it is around 340 when idle, driving forward or reverse with both sticks and in both gears. When i push the left stick only it drops to 0. Is this the left side pump going or is there something else?

Thanks for any assistance.

 

[lastchancegarage]

Today I threw an extra washer in the charge relief and the pressure jumped to about 1000. Too big a washer?! Even at 1000 when I pull/push the left stick it will still drop to 0 without any movement. If you move the right stick and then the left the machine will move but when using the left stick it's always weaker than the right and can drop to anywhere from 1-200 down to 0 intermittently. At this point I would have to guess the main pump has a failure on the swash plate? Can anybody explain the main pump and charge pump relationship? I don't see a separate mechanism on any of the diagrams. Does this mean it only has the one pump per side? And if this is the case it would mean my left pump is incapacitated and I'm only getting pressure from the right side? Does this make sense? Does anybody know if there is more I can do to the pump without removing from the machine and rebuilding? Any other valves or controls I'm missing?

Thanks for any suggestions!

 

[bobatack]

have u checked if the charge pressure is not leaking away on the brake chamber of the left drive motor

 

[lastchancegarage]

Thanks for the suggestion. I ran the machine a bunch today, dozing and moving dirt. It performs almost normally in high gear. The left is definitely weaker and there is a hesitation or weakness until you push/pull the left stick hard. It's really strange. How would I check the leaking on the brake chamber? The brakes come on and off normally and when the charge pressure drops the brakes come back on and then off when charge pressure builds, letting go of the sticks. Mentally I had ruled out the brakes because I had the motor apart and couldn't find visible damage, the brakes turn on and off, and the solenoid works. Am I wrong?

The ill functioning can occur on both sides but it is more prevalent and noticeable on the left. Is it possible for one pump to fail before the other? If it has failed why does it still operate mostly normally except for low gear and fine operation of the sticks?

Thanks again for any feedback.

 

[lastchancegarage]

Thanks for the suggestion. I ran the machine a bunch today, dozing and moving dirt. It performs almost normally in high gear. The left is definitely weaker and there is a hesitation or weakness until you push/pull the left stick hard. It's really strange. How would I check the leaking on the brake chamber? The brakes come on and off normally and when the charge pressure drops the brakes come back on and then off when charge pressure builds, letting go of the sticks. Mentally I had ruled out the brakes because I had the motor apart and couldn't find visible damage, the brakes turn on and off, and the solenoid works. Am I wrong?

The ill functioning can occur on both sides but it is more prevalent and noticeable on the left. Is it possible for one pump to fail before the other? If it has failed why does it still operate mostly normally except for low gear and fine operation of the sticks?

Thanks again for any feedback.

 

[Simon C]

Not entirely sure on your unit, but on similiar units the brake piston if it leaks will leak into case drain. Maybe one of the senior Bobcat guru's will chime in for some testing to see what is happening. You can have more case drain on one side than the other. Don't know how much is too much. Maybe the shuttle valve that some units have for 2 speed is bad and contributing to the problem.
Simon C

 

[lastchancegarage]

Thanks for the reply Simon. I disassembled the 2 sp/brake solenoids and bench tested. All seem to be fine. It shifts up and down on both sides and responds, not well, but you can tell it shifted. The brakes hold and release with the brake switch or the seat bar.

I wonder about case drain or general fluid return to pump. I've checked and cleaned both case drains. They were quite clean to begin. Is there another return line that could be plugged. It really seems like the machine is running out of oil but maybe that's what a cooked pump feels like?

Appreciate you chiming in!