Hi guys. I have been searching online for help with this problem and I noticed that there is a lot of good information on this forum so I decided to try my luck here. I have some experience with backhoes and recently bought my first skidsteer with plans to restore it. I have the Bobcat on jack stands with the engine idling and the problem that I am having is that the right side wheels operate perfectly for about 30-60 seconds after starting the bobcat and actually works better in reverse than the left side wheels but after that it gradually gets slower (within 1 minute) until the drive lever no longer operates the wheels. The wheels continues to creep in reverse since it needs neutral adjustment but that should not be related to the complete lack of response from the drive levers. If I shut off the engine for about a minute and restart it, the wheel would work again for a short while. I don't hear any strange noises and the engine does not sound loaded when the wheels stop responding. If I run the Bobcat on the ground it will work fine forward, backward and up and down a steep grade and will turn properly for a short while until the right lever becomes unresponsive. In the following video I had gotten the wheel to move forward and backwards a few times before I started recording - http://youtu.be/0t4uQXa5ch4 This is a Bobcat 863 with cable steering linkages and I have verified that the linkages are operating the spool valves as they should. Currently the cab is removed and there are no electrical sensors attached and the traction lock system has been removed. I am activating the fuel solenoid directly from the battery and I'm starting the engine by activating the starter also directly from the battery. All filters and oil have be replaced and I have cleaned the case drain filters. When I was cleaning the right side case drain filter I did find a piece of hydraulic hose in the canister so I am not sure if that could be related to the problem. I have also removed and disassembled the right drive motor and everything in there looks fine. The manual says that the relief valve on the motor should have shims that the factory uses to to set the relief pressure but I did not find any shims so I'm not sure if it just didn't need shims or if they were missing. I removed the replenishing valves from the hydrostatic pumps and did not find anything strange. The manual says to replace them if they are damaged but I don't see how those can be damaged...what should I be looking for? I am hoping that this is just a simple problem of something becoming clogged after the oil begins to flow. Would a clogged case drain filter cause drive problems? Your help with this problem would be greatly appreciated.
My 742b did that and it needed a new hydro pump assy.
I did some more testing on the Bobcat and I realized that the right wheels only stop working if I move the lever suddenly. Like if I push it back and forward quickly or push it forward and just release it. If I gradually move the lever it will continue to operate the wheels and I tested it by pushing into a heap of gravel. It pushed into to it and the wheels were skidding when it couldn't push anymore but I noticed that it would gradually lose power the longer I ran the Bobcat and would fail to push into the gravel. I am guessing maybe when the oil heats up the pumps don't work as well anymore. I'm getting a pressure gauge for $42US in my country but I'm considering ordering a $19 gauge that I found on Amazon. Any suggestions on where to order testing equipment and where did you get your new pump(s)?
I'm not familiar with the bobcat 863, but the symptoms you describe in a hydrostatic system sound like a lack of charge pressure to me.
Is there anything that can cause a lack of charge pressure beside a bad charge pump? Weak relief valve maybe?
Lots of things, weak relief, bad relief, chunk of hose stuck in relief, excessive internal leakage in pump or motor, collapsed suction hose... HEY! Might want to check the suction lines, see if you have a rubber hose that's come apart internally or the inner wall has collapsed. Or if it's possibly sucking air someplace. Don't know if your pumps have internal charge pumps, or if Bobkitty used an external gear pump to charge both sides. If you had no shims in the motor flushing relief valve, it may have been low on pressure to start with. Or, you may need to add some to get the pressure up where it should be, but you need to measure to know where to start. Need a manual, or at least a schematic and some gauges, take pressure readings.
When you say suction line, do you mean the supply hose from the reservoir to the pumps? From what I saw in the repair manual, the pump seems to be internal between the two hydrostatic pumps. I noticed that there are two o-rings there and I'm hoping that it could be just a simple problem as bad seals. Thanks a lot for all the suggestions. I guess I will be waiting on the gauge before I make any decisions on how to proceed. Here are the pumps and the schematic in case it might be of some help to someone else. Pumps Schematic
okay, now that we know it only has one shared charge pump, if one side always works properly, then it's probably not a charge pressure issue. Still wouldn't hurt to check the charge pressure. The schematic shows a restriction where the charge pressure enters the control valve for the HS pump, and it's possible there's also a screen in this port also. My new guess is that there's some material plugging up the inlet port to this control valve. If it's not a chore, you might remove it and check for contamination. If there are supply and return hoses attached to this valve, check them to be sure they haven't collapsed internally and are blocking flow. :my2c
I was looking at the restriction and thinking it might be blocked but I can't figure out where that restriction is on the real pump. I wasn't sure if it was internal or external. I had removed the plugs on the left and right of the spool in the picture below and a little oil came out of the right hole and a lot shot out from the left one. I just realized that the screened orifice plug is another hex nut under the right plug? Thanks a lot! I will check this in the morning.
Fuzzy pic, but yes, it looks like you remove the hex plug, and then there's another plug down in the same hole with the orifice and a screen, yeah? Hey, you might get lucky!
I uploaded high resolution pictures but the forum reduces them. I removed the orifice plug today and was surprised that I did not find a screen. All I got was what you see in the picture below. I didn't see any muck or hose pieces either and I left the plug out and turned the engine over to blow out some oil from the port just in case there was stuff stuck in there. Afterward I tested the bobcat and it worked great for a couple of minutes as usual and gradually lost power as it warmed up so I'm thinking bad pump or seals. Both wheels seem to lose power simultaneously though so maybe charge pump problem? I will have to test it again on the jack stand to see if the right wheels are working any better but here is a video of the wheels skidding on a paved surface with the bucket stuck behind a wall when I just started it up and running at just a little more than idle speed. - http://www.youtube.com/watch?v=e74xVAVKumk
I had a similar issue on my 753. I ended up replacing the hydrostatic motor with a rebuilt unit. I think it was about $750 (after sending the core in). A year later, 300 Hrs........no problems.
Ok so I installed the gauge to test the charge pressure but the gauge is a 4000 psi gauge and the increments are every 100psi. At idle the gauge shows zero but at 10-20% throttle it reaches 100psi and after that, even if I increase the throttle to maximum, it would only go up to approximately 120psi. Is this normal? Is the pressure relief valve limiting the pressure to 120 psi or is the pump worn and this is the highest pressure that it can achieve? I also noticed that when I operate the drive levers, the pressure drops back to basically zero while the wheels are skidding on the ground. Is that normal? The manual has information about the pressure sender unit mapping pressure to ohms in a table and the highest pressure value in the table is 150psi so I'm not sure if that is the maximum charge pressure.