I have a Bobcat 753C that has a similar problem with the drive motors. I am still trying to resolve the problem myself. The main issue is that the left drive is very weak in the forward direction, and normal in the reverse. Forward torque will suddenly emerge when the right stick is shoved way forward. It seems clear to me that the drive motor is able to function in both directions, but there is some misdirection of fluid pressure that is occurring. My best guess at this time is that the Shuttle Valve (in the motor housing) is causing this problem, and I am presently and slowly working on that to try to find out. It is also interesting to me that the right drive motor seems to exhibit the behavior, except that the reduced drive torque is not as extreme. But, the interesting thing about the right drive problem is that it is exactly opposite the left, such that it is the reverse drive that is weaker. I note that the exact same drive motor is employed on both sides of the 753 (i.e., they are not symmetrical mirror images of each other. This would be consistent with a failure mechanism that tends to inherently favor one rotation direction over the other; and thus, it is the same thing going on for both sides, but in opposit drive direction. I have now heard of a few other cases of this same problem in which similar behavior is reported. I have not yet seen a clear resolution described.
Bobcat 743B Left Reverse Drive Problem
- Thread starter TBP
- Start date
Jabonee
Platinum Member
Your describing weak or failing relief valves . Possibly not seating do to contaminate. Try switching the forward/reverse relief valves and see if you reverse the issue. :thumbsup:
Jabonee, thank you for your response, and my apology for delay in getting back to you. I appreciate your suggestion, and I will certainly take a look at that as soon as I can. I am presently hindered somewhat by the fact that my tractor is at my farm, some 70 miles away from my home near Austin, TX; and, several distractions have recently kept me from being able to work on it very much at a time. I have had an old Bobcat M600 for a long time, and have been able to depend on it to just keep going with very little trouble. Recently acquired this 753C, but have not yet had a lot of experience with it. The described behavior is puzzling to me. I am an engineer by profession, and fortunately the troubleshooting has been entertaining to me when time to do it. A while back, I implemented a pressure / flow meter (with two 5000 psi gauges in series with an orifice between them) in the 'forward' supply hose of the troubled left motor. I was able to cram this concoction into the space below the cabin, with a little USB connected 'endoscope' camera mounted to see the two gauges, and I was able to operate the tractor in a normal manner while viewing (on tablet computer) the pressure and flow rate (pressure differential across orifice) in the left drive motor supply hose (the one pressurized for forward drive motion). This seemed to work pretty well, and i was seeing pressures and flow that made sense to me when the motor seemed to be functioning normally. But, during the abnormal 'weak' forward condition, I observed some behavior that I have not yet explained. In particular, during a period in which the motor was still powered forward (weakly), I observed intervals in which the pressure differential (and thus the flow) reversed in polarity, and this slight reversal would continue until I further increased the forward pressure with the left control stick. This makes no sense to me at all; so, I decided that something was probably fooling me in my flow meter rig (perhaps a fluid cavitation , etc.), and I discontinued this experiment. I did get some sensible and perhaps useful data when the motor was appearing to function OK.
I mentioned my pressure / flow experiment mostly for general interest or perhaps amusement. Perhaps some other guys in this forum have done this in a more effective manner, and I would enjoy hearing about it.
Thank you again, Jabonee. I will try to examine your suggestion and get back to you later. You will need to be patient with me on timing.
I have a bobcat 743b that left reverse and right forward wouldnt work unless u jam the stick forward or back, so we parked it about 8 months ago, now i am taking the project on, i pulled both drive motors off and took them apart, both drive motors had one o-ring that was busted into tiny pieces, so ordered 2 seal kits and put the drive motors back together then put them back on the skid steer. Now if you push both sticks forward it goes, or both sticks back it goes, but left stick forward and right stick backward it will not move, left stick backward and right stick forward it will turn some but not like it should. also note it doesnt have the power to go uphill. I capped off both of the small hoses that come from each drive motor, i capped them together where they converge at the inline hydraulic filter (which is also brand new). The symptoms remained the same with a run test. I dont think swapping relief valves from side to side will do anything because of same symptom on both sides. ur thoughts please
Originally I had the same exact problem as described by Trull007, i feel confident that the drive motors are done right. When the bobcat is jacked up off the ground all directions turn as they should, no hesitations. leading me to believe that from sitting 8 months there has to be a seal's' dried up in the hydraulic pump. oil is flowing around sumwhere it shouldnt when bobcat is under load. does anyone know where i should start on this hydraulic pump and/or any other tests i can do?
Jabonee
Platinum Member
If your drive is weak in one direction and normal in the other that a sign that a relief valve is failing. As I posted earlier switch the forward and reverse valves and see if you reverse the problem. You can cheat a little by removing the drive motor hoses and capping them. Start the engine and slowly move the stick. If you can burry the stick and not snuff the engine you have a bad relief valve in that direction. :thumbsup:
I pulled the 2 big hoses off the right drive motor and capped them,started up and snuffed the engine in both directions with the stick, then i swapped the relief valves and did the same test with the same results. is this conclusive evidence that the hydraulic pump is good and whats my next move?
Jabonee
Platinum Member
Pump is good!! Sounds like a drive motor. Try reversing the hoses on the drive motor.:thumbsup:
after some research it turns out there is a little brass plug that sets in the drive motor on the same layer as the O-rings. anyhow I lost both of them (one in each drive motor) I never even knew they were in there. I ended up getting lucky and found both of them. I put them back in and the skid steer works great now!
Hi i would appreciate and advise on the location and position of the Replenish valves . I believe they are the 4 large hex caps on the motor .I have a no fwd movement on the right (left stick ) which of the four do i swap to check if HPRV is playing up the fwd ones on each side ?Your describing weak or failing relief valves . Possibly not seating do to contaminate. Try switching the forward/reverse relief valves and see if you reverse the issue.
Is there a diagram showing position of rv and poppet valves on the pump assy ,also which drive motors and position they go to .
thanks in advance
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