I have a 1981 Bobact 743. I have a few issues with it and could use some advice. The boom on the machine does not stay up. It slowly falls (About 1 inch every 20 seconds or so). There are no leaks on the cylinders and no leaks to be found in the machine (except leak listed below for another isuue). What should I look for? I was told it could be the control valve. No leaks are present at the control valve. Should I consider a rebuilt on it (how involved is a rebuild on it). The main lift is slow when the machine first starts (everything else works great). Do these early 743 have a float postion on them? If so mine does not work. What would cause the float postion not to work. When I use the tilt I get a squel from it. There is a small drip coming from the end of the tilt cylinder. Am I just getting air in the sytem from the leak or should I look at something else? The deisel engine runs great and has lots of power. However I am getting some oil out of the vent tube. Should I be worried about this? Thank you
Are your linkages worn out and not pulling/pushing enough travel on the valve body spool ? Pj
The up/down foot peddle seem alittle worn out. The boom lift up and down fine when warm. It is slow when you first start the machine. It seem you have to really push on it to make it work. Could bad bushing cause the boom to drop slowly. The tilt control does not drop. The drive and tilt work fine warm and cold.
If the cycle times increase when the system gets warm and the oil loses some viscosity I would be checking for a suction strainer and cleaning it out and whatever sludge is in the tank. If the boom is drifting that fast you have a problem with the control valve but the valve may only have a holding section for the base in which case a piston seal on either side would do the same
You may have a bad seal in the lift cylinder letting oil bypass and causing the downward drift. The tilt squeal means the relief is opening for some reason. If you have a square block on top of the hyd pump there is a bronze filter under the center hose fitting on the left side of the block as you face the machine. It should not affect the lift but worth checking out anyhow.
Thanks for the replys. I have already serviced the whole machine (drained, cleaned and replaced all filters and fluids, including the bronze filter one). I guess it is time to rebuild the control valve. I am thinking of trying this myself. Can it be done in the machine or is it best to remove it. Do I just buy a seal kit or will other parts be needed in it? If a lift seal is bad, how do you test it. Would both cylinder have a bad lift seal? For the tilt squel do I replace the relief or what is the proper repair?
I have no suggestions right of but a question or two. Did this problem just start all of a sudden one day? Also is this squeal in the tilt new or did it start after changing the oil and filters?
Not possible
The machine is new to me. I believe the dirft in the boom has been that way for awhile. The tilt had alittle squel to it before i changed the fluids but seems a little louder after the fluid changes. It seems to go away after running the machine for awhile.
Leaky piston seals in cylinders will definitly cause downward drift or bucket drooping. But also will reduce the lift power so if you can still lift a full bucket as before then you are probably back to the valve. I believe there are load checks at the valve that give trouble also. If you disconnect the bucket from the tilt cyl does it still squeal? I tend go with the easy stuff first like making sure the cyl can move freely and no internal problems such as a slightly bent shaft.
The machine has good power and can lift a full bucket. I have never rebuilt a control valve. Are load checks a serviceable part or do you replace them? Any adive on rebuilding a control valve would be usefull. The tilt cylinder is fairly big. It seems to move freely and will curl a full bucket. I have to remove the tilt cylinder anyway as it has a small leak. I will inspect it and rebuild it. Thank you.
This is true (negatively) with a poppet system but with spools that leak when are new and spools that are this old anything is possible. There aren't really any seals that would hold this pressure. You will have spool to housing clearance or valve seats but no O rings or seals that would hold this pressure
If the cyl is not leaking out of the rod seal there is no way that a cyl will be the culprit...... The rod coming into the bore displaces hyd fluid, this means that the excess fluid being displaced by the rod is coming through the valve or a leaking hose etc. definatly not the piston seal He did say boom and not tilt Pj
start with the linkage. If the hold or neutral position isn't right, the spool could be a millimeter or two over into the lower position. I'm guessing 743's don't have any sort of lift arm bypass valve for dead engine lowering?
Ill check the linkage. I know the bushings on the linkage are a little worn. Even if I push the foot pedal to the lift position with the machine off the boom still drops but not as fast. It might be fun times with the control valve. Thank you
It's been a long time since I've worked on a 743 so I can't recall with complete certainty of how the control valve is set up. However, seems I recall they were an early generation of the same control valve as still used on current machines, minus a few gizmos. The 743 didn't need the emergency lowering valve as they didn't have BICS spool locks. With a dead 743 you simply lower the seat bar, this releases the manual pedal linkage locks, then depress the arm pedal, arms comes down. To my knowledge Bobcat has never used holding valves on the loader arm circuit or the bucket circuit. They typically use a main relief valve and port relief valves for both arm up circuit and bucket curl circuit with anti cavitation feature. As mentioned previously, the spool itself has no seal that holds pressure within the bore of the valve body, if the arms drift down from a fault of the valve spool it would be because of excessive wear of spool and valve bore causing internal leakage. However, there are o-rings on the port relief cartridge, but if they failed, if the relief valve or anti cavitation were faulty, one would think they would cause a problem lifting a load as well, but then it may not be as noticeable as opposed to static holding of loader arms. If the loader arms drift down, raise the bucket about a foot off the ground and support it there by using a floor jack under the bucket. Disconnect one of the hoses that feeds the rod end of the lift cylinder for the loader arms and allow time for residual oil in the hose/pipe to stop draining, do not disconnect one of the hoses feeding the piston end of the cylinder. Slowly release the floor jack to allow loader arms to drift down. If oil comes out of the pipe or hose that feeds the rod end of the of the cylinder then you have failed piston seals. If no oil comes out of the pipe or hose that feeds the rod end, then you have a fault at the control valve. Also, on the 743, check closely the pin that connects the tilt cylinder to the loader arm frame. The pin bosses for that pin connection to the loader arm frame are bad for developing cracks where the pin bosses are welded to frame. I've had to reinforce a number of those machines at those points.
Thank you willie59. That is exactly what I have been looking for.
I don't mean to sound like I'm arguing because I'm really not, but how would a failed piston seal allow the boom to drift ? Pj
Variables PJ, since they don't use holding valves on the cylinders internal leakage at the control valve can allow the cylinders to drift down, we're simply trying to determine if the drift is because of the cylinders or the control valve.
I understand, so basically there is an acceptable valve body leakage amount?
One other way to test for piston seal leakage. Remove line from barrel end of cylinder and cap off the fittings. Now apply pressure to the rod end of the cylinder. If the piston seals are leaking the rod will come out. Just apply pressure slowly at first because in the case the piston has come free from the the ram could be forced right out of the cylinder!
In theory, yes. All manufacturers account for a degree of internal leakage and establish a preferred "maximum allowed". A new spool valve will have minimal or no noticeable leakage resulting in movement of a function cylinder, old (more worn) valves will begin to have noticeable movement, thereby entering the "maximum allowed" spec. Other machines that can't allow leakage (cranes, aerial lifts, telehandler forklifts, etc) install holding valves on the cylinders to prevent cylinder drift. On those applications, with holding valves, a bad piston seal will not allow cylinder to continue drifting, displacement within the cylinder comes into play and the cylinder has to be designed to account for pressure intensification.
Don't want to hijack a thread here but do you think a valve could be my problem on my d6n ? Pj
I worked on the machine over the weekend. I have a few issues it looks like. I tested the lift cylidners as willie59 suggested. I found one bad lift cylinder the other is fine. I looked at the foot pedals. They have alot of wear and need to be rebuilt. I believe I will rebuild the bad lift cylinder and tilt cylinder and the foot pedals as they need it. After those repairs I believe I will still need to rebuild the control valve. Thank you for infomation.
If the valve on the 743 is the same as on the 732 the hardest part will be getting it out of the machine. I resealed the one on our 732 a number of years ago, and it wasn't a big job at all. One thing I did learn the hard way on my 873 was when you take apart the float detent on the lift spool, (probably would be the same on the detent in the auxiliary spool on a 743), put a rag around the detent cup when you take it off, otherwise the little balls in there will go flying around your shop in all different directions. I never did find mine... And of course make sure you keep everything hospitalroom clean!
Rebuild one lift cylinder and the tilt cylinder? If it were me, you've got the machine down to do the repairs, you're getting the parts/seals, I'd complete the job and rebuild both lift cylinders. Just one less thing to give you trouble for quite some time.
Those cylinders use a pin type spanner wrench to remove the glands, they can be a pita to get apart without (even with) proper tools. I took my tilt cylinder to the dealer, they rebuilt it while I waited. Labor cost wasn't much, worth every penny when I saw how hard it came apart. Ed