Bobcat 743B Left Reverse Drive Problem

[TBP]

Hi,
I have a drive issue with a Bobcat 743B with over 5000 hrs.
The issue is there is not much drive on the left stick in reverse.
On soft ground on occasions the left wheels will kick in and drive especially if I return the right stick to neutral.
Also it will skid steer right with more power than left.
Not sure what to check or look at I'm new to Bobcats and there drive system.
Thanks

Also posted on heavyequipmentforums

 

[SSdoxie]

There is a combination high pressure relief directional check valve in the pumps for each direction and each drive side. If one is stuck open, you will get this symptom. On the pump locate the 2 drive hoses feeding to the left motor (note: 1 hose will be on left side of pump, other on the right side). Just above each hose connection is a large hex plug, remove the plugs and take the relief valves out and put them in opposite side. If 1 is stuck, problem will move the no drive forward in that side.

Another possibility; there is 1 seal in the distributor of the drive motor that could fail in such manner to allow pressure to by-pass in one direction but seal fine the opposite. If seal in both motors fail or get weak, there would be loss of drive forward on 1 side and reverse on the other.

 

[TBP]

I swapped the 2 reverse valves on the pump that wasn't the problem will move on to the drive motors.
Can the drive motors be swapped to do a test?
Thanks for the info.

 

[TBP]

Another possibility; there is 1 seal in the distributor of the drive motor that could fail in such manner to allow pressure to by-pass in one direction but seal fine the opposite. If seal in both motors fail or get weak, there would be loss of drive forward on 1 side and reverse on the other.

SSdoxie I swapped the drive motors and now it lacks drive forward on the right side so I think I have found my problem.
Do you know if this is an expensive fix?
Thanks for your help

 

[SSdoxie]

If the drive is weak in only 1 direction, the critical/expensive hard parts should be OK. Doing a little deeper research on this motor, I see it uses a Shuttle Valve to direct low pressure oil to case drain. Look at the back of the end cap, there will be a Allen Hex plug to remove. Inside is a set of springs, a spool, and coned seats that the springs push against and the spool is supposed to slide back and forth with change of oil pressure. If this spool sticks it allows high pressure drive oil to escape to tank. I have has some luck (with other Eaton motors using this system) in taking the spool out and polishing it with fine grit Emery Cloth.
Note: There are 2 Allen Screw caps in the end cap of the motor. 1 goes in the side of the end cap, this one has a 65 psi spring and poppet to maintain back-pressure on the case drain to prevent motor from Free-wheeling. The other cap (the Shuttle Valve cap) is on the very end.

A quick test to see if the shuttle valve is stuck: Remove the small case drain hose and put a steel cap on the fitting so oil cannot flow out of case drain then test drive. If drive power is restored, it is the Shuttle Valve sticking. DO NOT operate for any length of time or it could cause severe damage to drive motor and/or pump.

 

[TBP]

A quick test to see if the shuttle valve is stuck: Remove the small case drain hose and put a steel cap on the fitting so oil cannot flow out of case drain then test drive. If drive power is restored, it is the Shuttle Valve sticking. DO NOT operate for any length of time or it could cause severe damage to drive motor and/or pump.

I will give this test a try and see how I go.

Look at the back of the end cap, there will be a Allen Hex plug to remove. 1 goes in the side of the end cap, this one has a 65 psi spring and poppet to maintain back-pressure on the case drain to prevent motor from Free-wheeling. The other cap (the Shuttle Valve cap) is on the very end.

Do I have to dismantle the motor to get to these Allen Hex plugs in the end cap?

I have the same Bobcat part number 6657024 for the drive motors as trull007 did when you helped him with 743 B drive motor failure?

 

[TBP]

A quick test to see if the shuttle valve is stuck: Remove the small case drain hose and put a steel cap on the fitting so oil cannot flow out of case drain then test drive. If drive power is restored, it is the Shuttle Valve sticking. DO NOT operate for any length of time or it could cause severe damage to drive motor and/or pump.

SSdoxie I tried this test today and didnt notice any difference.
But Im wondering if I did it right.
I removed the small hose from the drive motor and put a plug on the fitting to cap it off.
I didnt cap off the hose to the pump just zip tied a bag over the end of the hose to catch any drips (had about 1 litre in the bag much to my surprise when I finished).
Is this the correct way to test?
Thanks

 

[SSdoxie]

Test sounds correct. Seems like problem is internal in motor.

 

[Lwells]

743b feathers to the right when both sticks are forward the main issue is when I am trying to load bucket I veers to the right like there is more power on the left drive motor
If bring the right stick back the left side will kick in but it jerks to the left
Is this a valve sticking?
I read the issue about left reverse and it seems like the same issue but in forward
I checked the right drive motor to look for a large hex plug and did not see one do I need to look at the main hydraulic pump

 

[SSdoxie]

743b feathers to the right when both sticks are forward the main issue is when I am trying to load bucket I veers to the right like there is more power on the left drive motor
If bring the right stick back the left side will kick in but it jerks to the left
Is this a valve sticking?
I read the issue about left reverse and it seems like the same issue but in forward
I checked the right drive motor to look for a large hex plug and did not see one do I need to look at the main hydraulic pump

It could be something sticky or catching in relief/replenishing check valve but it sounds more like steering linkage is loose. Check all rubber bushings and especially the arm that clamps on to the pump shaft. This arm was cast aluminum when 743 was built and were common repair items on machines with some age.

 

[drword]

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.

 

[Jabonee]

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.

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:

 

[drword]

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.

 

[drail]

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

 

[drail]

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]

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?

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:

 

[drail]

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]

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?

Pump is good!! Sounds like a drive motor. Try reversing the hoses on the drive motor.:thumbsup:

 

[drail]

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!

 

[Rushy]

ssue about left reverse and it s

r a large hex plug and did not see one do I need to look at the main hydraulic pu

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.

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 ?
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