CAT 262B Hydrostat problem

This topic contains 4 replies and was last updated by ben 7 years 5 months ago
Author
ben
February 22nd, 2013 3:30pm
Post
We have a CAT 262B skid steer. If anyone has a hydraulic schematic for this machine and can send it to me I would be very greatfull.



The problem I'm having involves steering, when we move the joystick to the left the reverse alarm beeps and the machine doesnt move for approx 3 seconds, once it starts turning left the reverse alarm shuts off and everything is normal.



I have taken the joystick apart and inspected it but I didnt notice anything worn out or broken... strings all seem ok as do the spools.



I believe my problem exists in the joystick only because my reverse alarm is coming on and the pressure switch for said alarm is installed on the joystick.



Any direction would be great, and like I said before if I could get the schematic the would help alot!



Thanks for your time

Matt
electronexciter
February 23rd, 2013 5:15pm
Matt, I can't send you a schematic due to copyrights, but I will certainly try to help you. The 262B's don't have electronic joysticks, but rather the joystick is a hydraulic valve that controls pilot oil flow to the hydrostatic pump's two swashplate actuators. There is a back-up alarm pressure switch mounted in the reverse function pilot oil line. The wires that you see on the travel joystick are for the horn and continuous flow work tool buttons and a travel lockout solenoid that is connected to the machine's interlock system.



For diagnosis, I recommend that you place the machine on jack stands to minimize frictional load on the drivetrain. I suggest that you plumb a pressure test swivel tee and pressure guage capable of reading up to 580 psi into the pilot hose that is connected to port labeled E on the travel pilot valve. The E port is the reverse pilot line for the RH side of the machine and it should not be hydraulically energized during a LH turn. The labels are probably marked on the bottom of the pilot valve. It might be easier to plumb the pressure guage in where the pilot line connects to the hydrostat pump, although, those connections can be difficult to access also. Record the pressure in the pilot valve E line with the travel joystick in the neutral position at low idle. It should be less than 65 pai, probably close to zero psi. Now move the joystick left to activate a LH turn. The pressure in the E pilot line should not change significantly. If pressure increases in E pilot line, when LH turn is activated, that means either the pilot valve plungers in the joystick valve are leaking from the LH function into the reverse function, or the resolver signal network, part number 218-6119, is leaking. The resolver network is basically 4 double check valves connected together. It is a block that is part of the travel joystick pilot valve. If you were to buy a travel pilot valve it would include the resolver signal network block.



Either way, your joystick is probably the source of the problem. If you don't have a pressure guage available, you can measure your wheel speeds using a phototach and reflective tape on the wheel. The reflective tape would need to be the same distance from the center of the wheel on both sides. During a LH turn, your RH wheels should be turning forward at the same speed that your LH wheel are turning in reverse. If your joystick is leaking, either in the pilot plungers or in the resolver signal block, the RH wheels will turn forward slower than the LH wheels will be turning reverse during a LH turn.



It might be faster to start the diagnostic process by simply replacing the back up alarm pressure switch. If the problem is still there, the joystick pilot valve is probably faulty. The backup pressure switch is supposed to become activated (electrically closed) at 65-81 psi. It is a normally open switch.



If you would like Cat part numbers for pressure test swivel tees and a guage let me know.
electronexciter
February 23rd, 2013 7:07pm
I didn't really address the 3 second delay issue in my previous post. That could also be caused by a faulty travel joystick issue. There might be a check valve ball in your resolver signal network that is sticking and resulting in the delay. It's also possible that you have two separate issues. A faulty travel joystick could cause your back-up alarm problem, and a leaking brake piston seal could cause your delay issue. Each of your travel motors has a wet disc brake pack. They are spring applied, hydraulically released. If one of your brake piston seals is leaking, that could cause a delay in machine movement. You would diagnose that by monitoring the pressure in your brake release line.



First, check some simple things. Is hydraulic oil level ok? Is more than one function affected? If so, oil viscosity could be an issue. Any leaks? Are your engine speeds ok? Low idle should be 1000 +/- 50 rpm. High idle should be 2860 +/- 50 rpm. Is your charge pressure ok? Charge psi should be 450 +/-30 psi at low idle, 468+/- 30psi at high idle. Charge psi test tap nipple is on RH side of pump in center of pump, as viewed sitting in operator seat.



To test your brake release pressure, plumb a pressure gauge into the small pilot line that is connected to port labeled MG on the pump. This pilot line connects to the single speed parking brake valve group that is mounted next to the LH frame rail under the cab. To help identify this line, it also tees and supplies pilot oil to the implement control valve. From the parking brake solenoid valve group, there will probably be steel lines that tee and supply brake release oil to each of the drive motors. The pressure of the oil supplied to the parking brake solenoid valve group should be charge pressure of 450 psi at low idle. If it is lower, that might indicate a leaking brake piston seal.



It might also be a good idea to verify that the oil supply input to the travel joystick is correct. It should be speed sense pressure which is 138 +/- 30 psi at low idle, 290 +/- 22 psi at high idle. The input oil to the travel pilot valve comes from the speed sense valve which is inside the hydrostatic pump.



If you aren't comfortable measuring hydraulic pressures safely, then I recommend that you not perform any of the diagnosis I suggested or you ask a knowledgable coworker for help.



Have fun, be safe.
electronexciter
February 23rd, 2013 7:12pm
If you have a two-speed machine, the brake release oil line, connected to MG port on hydrostatic pump, will go to two speed valve and then to the parking brake solenoid valve group.
ben
February 25th, 2013 8:42pm
Hey EE,

Thanks for responding to my post. Shortly after I posted this on friday I was able to obtain a Hydraulic schematic, after having looked it over I discovered the resolver network vavle and did some presure tests and determined that this was indeed the source of my problem (seems like the shuttle in port F was worn out, either sticking or leaking... and pressure was building at the reverse alarm switch) fortunately my local CAT dealer had the part on hand (updated P/N 343-0877, This is ONLY the resolver network valve, which is the bottom section of the joystick assembly not the whole joystick) ater installing and testing both my problem have been rectified. Apparently this problem has been present on the machine for years or so I was told today, so the guys were very happy that it is now working properly.



Again, thanks for taking the time to respond to me, I really appriciate it. Having the proper working pressures for the system will be very useful in the futur as my company has a fleet of these things.

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