bobcat 743 no drive function

[Tazza]

i did a bunch of checks this morning. here is what i found.
check one: i removed the middle charge pressure hydraulic hose. i purchased an adapter fitting screwed it into the charge hose and screwed on my 5000 psi gauge and capped of the fitting the charge hose goes too. ran the engine and the pressure gauge went up to 1000 psi before killing the motor. so now ive determined that there is charge pressure going into the port block
check two: i ran the engine again with the pressure gauge screwed into the temp sensor port on the port block. ive found that when the engine is running the 5000 psi gauge reads nothing. according to the manual there should be 140-170 psi at that sensor. my guess is my 5000 psi gauge is not sensitive enough to read that low of a pressure. i will get a smaller one and verifiy this.
check three: to determine if there was port block internal leakage i removed the right side hose(right side hose next to the charge pressure hose) and capped of the hose and port in the port block. ran the engine and no change
check four: i removed the left side hose(left hose from the charge pressure hose) which goes to the hydraulic oil fiter housing. i capped off the hose and port block. ran the engine and no change. loosend up the hose fitting when running and foamy fluid and bubles were coming out of it. noticed that when removing this hose its the only hose that has foamy fluid.
check five: ran the engine and loosened up the hydraulic fluid filter in the engine compartment and no fluid came out.
check six: during these checks with the bobcat lifted in the air found the wheels would turn in the opposite direction of stick command intermittently. most of the time the wheels dont turn but when they do its allways opposite. forward stick command would move the wheels backward. rearward stick command would move the wheels forward. since the 743 has a high pressure line and low pressure line connected to the hydrostatic motors i can only assume since there is no high pressure going into the motors that the low pressure side is turning the wheels the opposite direction. the wheels turn in the opposite direction with no force. you can stop them if you hold the wheels.

You're spot on about the gauge being too high to measure the charge pressure, you need a smaller one.
You mention you have no oil comeing from the hose on the filter in the engine bay? oil should be flowing through here! From the setup of my 731, it was the same basic setup as yours, there was a one way valve joining two hoses, i wonder if it could be a problem here too?
I'm at a loss of what to advise you to check next.....

 

[frogfarmer]

i did a bunch of checks this morning. here is what i found.
check one: i removed the middle charge pressure hydraulic hose. i purchased an adapter fitting screwed it into the charge hose and screwed on my 5000 psi gauge and capped of the fitting the charge hose goes too. ran the engine and the pressure gauge went up to 1000 psi before killing the motor. so now ive determined that there is charge pressure going into the port block
check two: i ran the engine again with the pressure gauge screwed into the temp sensor port on the port block. ive found that when the engine is running the 5000 psi gauge reads nothing. according to the manual there should be 140-170 psi at that sensor. my guess is my 5000 psi gauge is not sensitive enough to read that low of a pressure. i will get a smaller one and verifiy this.
check three: to determine if there was port block internal leakage i removed the right side hose(right side hose next to the charge pressure hose) and capped of the hose and port in the port block. ran the engine and no change
check four: i removed the left side hose(left hose from the charge pressure hose) which goes to the hydraulic oil fiter housing. i capped off the hose and port block. ran the engine and no change. loosend up the hose fitting when running and foamy fluid and bubles were coming out of it. noticed that when removing this hose its the only hose that has foamy fluid.
check five: ran the engine and loosened up the hydraulic fluid filter in the engine compartment and no fluid came out.
check six: during these checks with the bobcat lifted in the air found the wheels would turn in the opposite direction of stick command intermittently. most of the time the wheels dont turn but when they do its allways opposite. forward stick command would move the wheels backward. rearward stick command would move the wheels forward. since the 743 has a high pressure line and low pressure line connected to the hydrostatic motors i can only assume since there is no high pressure going into the motors that the low pressure side is turning the wheels the opposite direction. the wheels turn in the opposite direction with no force. you can stop them if you hold the wheels.

I do not have a 743 to look at here Or the service manual but I do distinctly remember a valve block other than the one on the hydro pumps that is what controls charge pressure and volume. This block also sends the excess flow back to the reservoir. If there was a failure in this block (a valve) the oil would only circulate through the control valve for the lift and tilt circuits. Due to the many variations over the years this may not apply to your serial break. The filter and cooler both have the reduced pressure charge flow so a lack of oil would be verification of no charge oil flow. Given what you have found I would be very surprised if the problem is not the priority flow divider or a complete restriction in one of the lines. Rodents and insects have a way of jamming up open systems and it doesnt take alot to hold back a couple hundred pounds of pressure. If the wheels turn backward once the system is running as it should either the lines are crossed due to a prior fix where the motors were not timed right or the motors were not timed properly when they were rebuilt. Motor timing is to determine directional rotation based on the application. I have know of several instances where the motors were timed for the opposite direction of the intended use it is no big deal just an inconvenience of swapping the large high pressure lines. Hope some of this helps.

 

[busa4]

I do not have a 743 to look at here Or the service manual but I do distinctly remember a valve block other than the one on the hydro pumps that is what controls charge pressure and volume. This block also sends the excess flow back to the reservoir. If there was a failure in this block (a valve) the oil would only circulate through the control valve for the lift and tilt circuits. Due to the many variations over the years this may not apply to your serial break. The filter and cooler both have the reduced pressure charge flow so a lack of oil would be verification of no charge oil flow. Given what you have found I would be very surprised if the problem is not the priority flow divider or a complete restriction in one of the lines. Rodents and insects have a way of jamming up open systems and it doesnt take alot to hold back a couple hundred pounds of pressure. If the wheels turn backward once the system is running as it should either the lines are crossed due to a prior fix where the motors were not timed right or the motors were not timed properly when they were rebuilt. Motor timing is to determine directional rotation based on the application. I have know of several instances where the motors were timed for the opposite direction of the intended use it is no big deal just an inconvenience of swapping the large high pressure lines. Hope some of this helps.

　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
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[busa4]

　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　
　

whoops. failed attempt at attaching a pic of the diagram. its in adobe so i am clueless on how to do this.

 

[busa4]

whoops. failed attempt at attaching a pic of the diagram. its in adobe so i am clueless on how to do this.

since i had no oil flow out of my filter i started staring at the hydrostatic fluid flow chart again. there are two lines in the port block that connect to the case drain system. the one port takes excess charge pressure and returns it to the cooler circuit. i removed this line and removed the valve on the inside and saw the piston had small holes in it. this allows some pressure to enter the case drain circuit. this line appears to be the line that would fill up the case drain system so i should see oil leaking at the filter if i were to loosen it up. this week i will remove the divider valve that this line goes into and both motors go into and see if there is an issue.
the other line goes in to the port block and allows oil from the cooler to re-enter the port block. i dont see an issue with this line except when i removed it it was foamy.

 

[Rojas]

since i had no oil flow out of my filter i started staring at the hydrostatic fluid flow chart again. there are two lines in the port block that connect to the case drain system. the one port takes excess charge pressure and returns it to the cooler circuit. i removed this line and removed the valve on the inside and saw the piston had small holes in it. this allows some pressure to enter the case drain circuit. this line appears to be the line that would fill up the case drain system so i should see oil leaking at the filter if i were to loosen it up. this week i will remove the divider valve that this line goes into and both motors go into and see if there is an issue.
the other line goes in to the port block and allows oil from the cooler to re-enter the port block. i dont see an issue with this line except when i removed it it was foamy.

Don't have the flow chart so I can't say alot here, but don't these valves operate on differential pressure? If the piston has a hole in it the DP=0 so the valve may not open even if the flow is ~nil.

 

[busa4]

Don't have the flow chart so I can't say alot here, but don't these valves operate on differential pressure? If the piston has a hole in it the DP=0 so the valve may not open even if the flow is ~nil.

i have a flow chart but i dont know how to post it. its in pdf format.

 

[frogfarmer]

since i had no oil flow out of my filter i started staring at the hydrostatic fluid flow chart again. there are two lines in the port block that connect to the case drain system. the one port takes excess charge pressure and returns it to the cooler circuit. i removed this line and removed the valve on the inside and saw the piston had small holes in it. this allows some pressure to enter the case drain circuit. this line appears to be the line that would fill up the case drain system so i should see oil leaking at the filter if i were to loosen it up. this week i will remove the divider valve that this line goes into and both motors go into and see if there is an issue.
the other line goes in to the port block and allows oil from the cooler to re-enter the port block. i dont see an issue with this line except when i removed it it was foamy.

Posted a long paragraph last night but its not here today. Here's a thought if the lift/tilt works; find where the system loses all pressure and volume after that valve by loosening the lines at each break until the problem is found.

 

[frogfarmer]

Posted a long paragraph last night but its not here today. Here's a thought if the lift/tilt works; find where the system loses all pressure and volume after that valve by loosening the lines at each break until the problem is found.

Send me the chart to my e-mail; its listed in my profile.

 

[busa4]

Send me the chart to my e-mail; its listed in my profile.

thanks frogfarmer. ill send it later tonight as soon as the wife gets off the pc.
update: the lower psi gauge came in today and it is now official. charge pressure at the sensor port while the engine is running is 20 psi. it drops to zero when the lift, tilt, and drive levers are moved. ill start in the morning again capping off other ports to find the source of the lost pressure. i already know when removing the center charge pressure line and putting a gauge on it, i got 1000 psi before the engine died

 

[frogfarmer]

thanks frogfarmer. ill send it later tonight as soon as the wife gets off the pc.
update: the lower psi gauge came in today and it is now official. charge pressure at the sensor port while the engine is running is 20 psi. it drops to zero when the lift, tilt, and drive levers are moved. ill start in the morning again capping off other ports to find the source of the lost pressure. i already know when removing the center charge pressure line and putting a gauge on it, i got 1000 psi before the engine died

The charge lines wont have any where near 1000psi as the cooler and filter would rupture. The center line as you describe is most likely the pressure line from the hydraulic pump itself. With 20psi that drops to zero I would guess the spring on the charge valve has broken. I dont think that valve is in the port block. Good news is at least some oil has been going to the hydro pumps that hopefully will have kept them lubed and prevented damage.

 

[busa4]

The charge lines wont have any where near 1000psi as the cooler and filter would rupture. The center line as you describe is most likely the pressure line from the hydraulic pump itself. With 20psi that drops to zero I would guess the spring on the charge valve has broken. I dont think that valve is in the port block. Good news is at least some oil has been going to the hydro pumps that hopefully will have kept them lubed and prevented damage.

thak you all for helping me sort out this issue. firstly i must appoligize to all that helped for wasting all of your time for me being a *******. when i put the charge line on the pressure gauge and got 1000 psi i thought charge pressure was good. when i recieved the 300 psi gauge and put it in the sensor port and i only got 20 psi i stepped back and thought for a minute. tazza told me to clean or replace the charge filter in the beginning so i removed it and cleaned it twice. the filter was un-cleanable as it was clogged beyond cleaning. so what i did was i removed the filter again and drilled 3 holes into it. cleaned it thoroughly and reinstalled it. started the engine and bam!!!! 170 psi charge pressure and for the first time in months the bobcat moved. i have to call bobcat today and get some parts so a new filter will be in that list..

 

[Tazza]

thak you all for helping me sort out this issue. firstly i must appoligize to all that helped for wasting all of your time for me being a *******. when i put the charge line on the pressure gauge and got 1000 psi i thought charge pressure was good. when i recieved the 300 psi gauge and put it in the sensor port and i only got 20 psi i stepped back and thought for a minute. tazza told me to clean or replace the charge filter in the beginning so i removed it and cleaned it twice. the filter was un-cleanable as it was clogged beyond cleaning. so what i did was i removed the filter again and drilled 3 holes into it. cleaned it thoroughly and reinstalled it. started the engine and bam!!!! 170 psi charge pressure and for the first time in months the bobcat moved. i have to call bobcat today and get some parts so a new filter will be in that list..

That filter has driven MANY people mad with erratic drive issues... Good to hear you got it sorted out though.

 

[busa4]

That filter has driven MANY people mad with erratic drive issues... Good to hear you got it sorted out though.

tazza. sent you an email.

 

[skidsteer.ca]

tazza. sent you an email.

Fyi
There is no way to post a pdf file here (to my knowledge) but you can up load them to www.mediafire.com then post a link to the uploaded file so others can download and view.
Remember links here must have a space at the end to make them work.
Glad you making progress on the 743
Ken

 

[Tazza]

Fyi
There is no way to post a pdf file here (to my knowledge) but you can up load them to www.mediafire.com then post a link to the uploaded file so others can download and view.
Remember links here must have a space at the end to make them work.
Glad you making progress on the 743
Ken

Ken - sadly he really isn't.....
I have no idea what to suggest next, it was pressure tested, but i don't know if that means it was spun up to give it a real test or not.

 

[busa4]

Ken - sadly he really isn't.....
I have no idea what to suggest next, it was pressure tested, but i don't know if that means it was spun up to give it a real test or not.

update: the skid moves now but after a couple minutes i lose one drive function on both sides. the rebuild shop also didnt time the motors correctly so all my movements are in reverse (forward stick movement moves backward and vise versa). im frustrated with the rebuild shop and may lose my cool with them. im going to call them up and see if they actuall pressure tested the pump with an electric motor. thanks for all that helped.

 

[frogfarmer]

update: the skid moves now but after a couple minutes i lose one drive function on both sides. the rebuild shop also didnt time the motors correctly so all my movements are in reverse (forward stick movement moves backward and vise versa). im frustrated with the rebuild shop and may lose my cool with them. im going to call them up and see if they actuall pressure tested the pump with an electric motor. thanks for all that helped.

Several hydraulic shops do alot of hoses and cylinder rebuilds but dont have a clue when it comes to hydrostat systems. And even the ones who do know realize it can be alot cheaper in the long run to source the reman pumps and motors from outside sources. Hopefully the shop knew what they were doing and just made some mistakes. I would be very surprized if they did a true volume and pressure test of the hydrostats with a motor driving the system as some elaborate equipment would be needed. When the system loses the drive function do you still have charge pressure. Also if the oil is foaming that could cause the system to fade. All of us here understand your frustration as we have been there before. Keep us informed and we will help.

 

[busa4]

Several hydraulic shops do alot of hoses and cylinder rebuilds but dont have a clue when it comes to hydrostat systems. And even the ones who do know realize it can be alot cheaper in the long run to source the reman pumps and motors from outside sources. Hopefully the shop knew what they were doing and just made some mistakes. I would be very surprized if they did a true volume and pressure test of the hydrostats with a motor driving the system as some elaborate equipment would be needed. When the system loses the drive function do you still have charge pressure. Also if the oil is foaming that could cause the system to fade. All of us here understand your frustration as we have been there before. Keep us informed and we will help.

update: everything works now and in the correct direction. the hydraulic shop messed up the rebuilding of both motors. each pump had a seal that was stuck in a pressure port causing the loss of one drive function on each motor. i removed and disassembled both motors and fixed the seal problems and then correctly timed the motors with help from a manual from tazza.

 

[Tazza]

update: everything works now and in the correct direction. the hydraulic shop messed up the rebuilding of both motors. each pump had a seal that was stuck in a pressure port causing the loss of one drive function on each motor. i removed and disassembled both motors and fixed the seal problems and then correctly timed the motors with help from a manual from tazza.

I can't believe they stuffed up that much! How did you go with calling them and complaining?
You are now the resident expert of 743 motors and pumps!!!!