743 Bobcat Problem

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campbells

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Feb 22, 2008
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I recently replaced worn out hydraulic hoses to the drive tires on my 743 Bobcat. I now have chattering in the hydraulic system when picking up loads I have always picked up with no problem. Is there a way to bleed air from the system or is there another problem?
 

Tazza

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Just keep running the machine, you need to get the air out of the system. Just keep driving and lifting and dropping the arms. It will get out eventually. Make sure your oil level is rite too.
These machines self bleed air through what they call controlled leakage. Basically a small amount of oil leaks out the hydraulic motors and goes through the oil cooler back to the tank.
Give it time and it will get better. If you start it drive it around for say 5 mins lifting and tilting then shut it down for a few hours then repeat. Depending on how much air got in, this may take a few days to go away. On a machine i re-built it took a few weeks of every afternoon when i got home from work i did this, it eventually got clear now its spot on.
 
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campbells

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Just keep running the machine, you need to get the air out of the system. Just keep driving and lifting and dropping the arms. It will get out eventually. Make sure your oil level is rite too.
These machines self bleed air through what they call controlled leakage. Basically a small amount of oil leaks out the hydraulic motors and goes through the oil cooler back to the tank.
Give it time and it will get better. If you start it drive it around for say 5 mins lifting and tilting then shut it down for a few hours then repeat. Depending on how much air got in, this may take a few days to go away. On a machine i re-built it took a few weeks of every afternoon when i got home from work i did this, it eventually got clear now its spot on.
We have been using the machine for about an hour a day for the last four months and it has not gotten any better.
 

OldMachinist

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We have been using the machine for about an hour a day for the last four months and it has not gotten any better.
I would think that by now the air from changing hoses would be gone. You might be sucking air somewhere at a loose hose connection. Is there any foam in the hydraulic tank after running for awhile? Also if your machine has an anti-cavitation valve it might need cleaning or replacing.
 

Tazza

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I would think that by now the air from changing hoses would be gone. You might be sucking air somewhere at a loose hose connection. Is there any foam in the hydraulic tank after running for awhile? Also if your machine has an anti-cavitation valve it might need cleaning or replacing.
WOW that should have cleared up by now...
I agree, check your hydraulic tank for foam. If it was sucking in air you should see oil... Still it never hurts to check. Does your machine have a square block on the hydraulic pump? it could be a blocked charge line filter if it does.
 

skiddz

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WOW that should have cleared up by now...
I agree, check your hydraulic tank for foam. If it was sucking in air you should see oil... Still it never hurts to check. Does your machine have a square block on the hydraulic pump? it could be a blocked charge line filter if it does.
There is a place to bleed air. it is located at the top of the pumps. at the top of the pumps is a charge presure sensor (it is the only sensor screwed directly into the pumps, toward the rear of the pumps just below the fuel tank, next to the sensor is an allen plug, remove this plug and screw a hose into this port and run the hose back into the hyd tank. it is best to have a valve inline on the hose to slightly restrict oil flow. run the machine at no more than 1/2 throttle and this should bleed the air from the system in about 10/15 minutes. If you are still having problems with air in the system, indicated by foaming oil seen through the sight glass, check the previously mentioned filter(if you have the portblock mounted to the top of the hyd pump) otherwise any of the suction hoses/port block between the tank to the hyd pump can be pulling air into the system, to a lesser degree, air can be entering the system through the lift/tilt cyl head seals and rarely if ever, the pintle seals (these are the seals located on the shafts for your forward/reverse at the pumps) if you find you still have air getting into the system, my money is on the large suction hose or the suction hose fitting o-ring right at the hyd pump.
 
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campbells

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There is a place to bleed air. it is located at the top of the pumps. at the top of the pumps is a charge presure sensor (it is the only sensor screwed directly into the pumps, toward the rear of the pumps just below the fuel tank, next to the sensor is an allen plug, remove this plug and screw a hose into this port and run the hose back into the hyd tank. it is best to have a valve inline on the hose to slightly restrict oil flow. run the machine at no more than 1/2 throttle and this should bleed the air from the system in about 10/15 minutes. If you are still having problems with air in the system, indicated by foaming oil seen through the sight glass, check the previously mentioned filter(if you have the portblock mounted to the top of the hyd pump) otherwise any of the suction hoses/port block between the tank to the hyd pump can be pulling air into the system, to a lesser degree, air can be entering the system through the lift/tilt cyl head seals and rarely if ever, the pintle seals (these are the seals located on the shafts for your forward/reverse at the pumps) if you find you still have air getting into the system, my money is on the large suction hose or the suction hose fitting o-ring right at the hyd pump.
Thanks for the help. Hopefully I will get some time today to take a look at it. I will keep you posted. Thanks
 
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campbells

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Thanks for the help. Hopefully I will get some time today to take a look at it. I will keep you posted. Thanks
Sorry it has taken me so long to get back here to the forum. We run a greenhouse business so this time of year is really busy. We got a chance to mess with the bobcat. We did what was recommended by Skiddz and no air came out. Their is also no foam in the tank. I don't know if the pump is just shot or what. Just wanted to update you guys and see if you had anymore ideas of what could be wrong. Thanks for the help.
 

perry

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Sorry it has taken me so long to get back here to the forum. We run a greenhouse business so this time of year is really busy. We got a chance to mess with the bobcat. We did what was recommended by Skiddz and no air came out. Their is also no foam in the tank. I don't know if the pump is just shot or what. Just wanted to update you guys and see if you had anymore ideas of what could be wrong. Thanks for the help.
Raise the arms all the way up...works for me..
 

Tazza

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Raise the arms all the way up...works for me..
Ok, so its ONLY your hydraulics that are chattering? What revs are you running? it could simply be that you aren't running fast enough for the vanes to fully spin out causing the chattering you hear. The only other thing *may* be your pump is starting to get lazy.
 

tsanders

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Ok, so its ONLY your hydraulics that are chattering? What revs are you running? it could simply be that you aren't running fast enough for the vanes to fully spin out causing the chattering you hear. The only other thing *may* be your pump is starting to get lazy.
my 632 has a bleeder valve in the filter housing between the housing and the frame. that is the highest point on the hyd. system but i do not know about your model.
 

Tazza

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my 632 has a bleeder valve in the filter housing between the housing and the frame. that is the highest point on the hyd. system but i do not know about your model.
A bleeding valve would be handy for sure. The thing is this system is designed to self bleed, it should self purge all air on its own. It really does start to sound like your hydraulic pump may be a little sad. As you said you have no foam in your oil means all the air is out of the system.
 

skiddz

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A bleeding valve would be handy for sure. The thing is this system is designed to self bleed, it should self purge all air on its own. It really does start to sound like your hydraulic pump may be a little sad. As you said you have no foam in your oil means all the air is out of the system.
you really wont see any air coming out, the fluid is flowing to fast. but really, if you have been running the machine even if it just a little here and there throughout a few days, any air should self bleed, and definately no indication of air in the hyd tank? do you have a noticable decrease in the strength of the lift arms despite the chattering? how about the speed of the lift arms? does the machine whine noticeably louder when lifting than it did before? it is starting to sound like a pump, possibly when you blew a drive hose the machine ran dry on hyd oil long enough to have damaged the pump? do you have a dealer close enough to you that can flow test the pump.....or posssibly they can come to you? that wouldgive you a better idea of what is going on.
 

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