This is a great forum and I am glad to be a part of it. I purchased an old 743 knowing it had low/no hydraulic power and would very much like to fix it myself. The engine runs great as well as the bucket lift & tilt. However, when I go forward and hit any resistance, or try to turn left or right, it screams and does not want to accommodate. On flat ground it will go forward and backward easily. My fluid level is correct, my auxiliary hydraulics are turned off, and my parking brake is disengaged. Would really like some insight from all of the experts on this forum on what to check next. Thanks a ton in advance!
Bobcat 743 not enough hydraulic power to push dirt or turn left/right
- Thread starter RusticRR
- Start date
Screaming noise sounds like a slipping belt of some sort, or maybe the hydraulic system is bypassing below specified relief valve settings. I would check all of the hydraulic pressures that you can. Any feed back afterwards would help out.
Thank you for your response. The 743 has a shaft drive hydraulic system, so it's not a belt issue, but I will check the hydraulic bypass valves and the system pressure points. Any advice on how to tackle those next steps?
Well, you can remove a hose from a hydraulic cylinder to install a temporary gauge on that line. Then activate the function to that hydraulic line. Doing so will tell you the hydraulic pressure of that function & the pump that drives it. I am not sue if there is a more convenient way to tap into the hydraulic system for testing pressures. I assume there may be, but I would not know where to tell you to look. Assuming there is not, do the above mentioned test will tell you what you have to each cylinder. I would think that would be a better way of testing the ACTUAL hydraulic pressure, instead of what is only at the valve. I hope that helps you out.
Let's not wander too far into the weeds with this... the hydraulic system for the wheel drive is mostly separate from the tilt/lift/aux system. I would remove the Charge Pressure Switch, install a temporary gauge there, and check the pressure. It should be in the 20-80 psi range. Does the charge pressure warning light come on before the engine is started? How long ago was the hydraulic filter changed?
The next step is to remove and cap the hoses from the drive motors. Slowly push the sticks forward and feel how the fight you. If you can install a 5,000 PSI gauge, it will show you even more.
The pumps and motors in these machines are pretty simple to repair.
My guess is a bad vane or gear pump. Since the drive is so weak, you haven't notice it can't pick anything up. Pump is creating just enough to to make loader functions look fine and drive with no load.
Again, my sincerest appreciation for your responses. On Monday I will get a hydraulic testing kit and begin testing all of the areas you indicated. I will let you know what i find.
Also, one more symptom: the right drive motor moves in the oposite direction of the stick. Ie i push forward and the right wheels turn backward, then when i pull the right stick backward the wheels turn forward. Any thoughts?
that sounds like the hoses from pump to motor on right side are reversed. simply reverse the two main hoses to that motor, or at the pump, if that is easier.
It could be that or if you just bought the machine, the previous owner may have opened the drive motor to try and fix the issue and didn't time it correctly. An incorrectly timed motor will turn backwards.
Good suggestions on the drive motor. I called the past owner, and sure enough he had the drive motor apart trying to figure out the hydro issue. He did not re-set the timing. Can you advise on how to time it correctly? I searched the Service Manual, but couldn't find any instruction.
In searching on-line for timing instructions I came across this PDF: http://www.eaton.com/ecm/groups/public/@pub/@eaton/@hyd/documents/content/pll_1332.pdf Take a look at page 5. Is this the same for a bobcat 743 drive motor?
That timing procedure looks right to me. I found it took me a while to do as you keep sencond guessing yourself, was that the right port and was that the right tooth on the valve plate?
Yet, even with the second guessing, each motor i have done was spot on, every time.
One thing that you need to do is put loctite on the 4 bolts that hold the motor together, if you don't, oil can run down the threads and wet the heads of the bolts.
Goti it Tazza, thanks! I was able to test the pressure on the drive motors today. Here are my results: Fight side foward direction - 1000 lbs Right side backward - 800 lbs Left side forward - 1500 lbs Left side backward 1700 lbs I will test the other areas tomorrow. Any thoughts on the above test results?
This is where I hooked up the pressure gauge to test. I simply unhooked both large hoses, put caps on the drive motor inlets, then put a plug in one hose and the testing gauge on the other. Then started the bobcat and set the throttle to 3/4. Then pushed the hydraulic lever either forward or backward depending on the hose I had the gauge connected to. Did I conduct that test correctly?
How far forward did you get the sticks to go? Did you manage to give it all the power you could?
If the pump was in great shape, you should get 5,000 PSI (the max setting of the relief valves in the pump)
If you did it that way, i feel those figures are too low and it may be time to crack open the pump and tidy it up.
Thanks Tazza. I was able to check the Charge Pressure today on the top of the pump. I removed the switch and attached the pressure gauge. It read a consistent 100 lbs at high RPMs. I also checked the pressure as I attempted to move the 743 forward or backward. When I pushed the sticks as far forward as I could, the pressure would drop to 20lbs. Do these test results seem normal? Another question: the previous owner had a theory that someone didn't put the correct AW68 oil in the machine, but instead, a lighter weight oil. The machine seems to be slightly more responsive when you start it cold, but the strength tappers off quickly. To answer your question from above: during the test of the drive motors, I pushed the sticks as far forward as I felt I could. The farther I pushed, the louder the screaming noise became, and the resistance on the sticks became stronger.
I could try to upload a video of the drive motor pressure test if that would help show what is occurring.
