email sent. thanks tazza
bobcat 743 no drive function
- Thread starter busa4
- Start date
Ok, i'll have a sift through my emails when i get home, see if i can find their email address.
thanks again tazza for the help. i ended up having the pump and both hydrostatic motors rebuilt locally but the bobcat is doing the same thing. i cant seem to find a better hydraulic flow chart. i have lift and tilt function but no drive. the hydraulic fluid in the tank is bubbly and when i shut of the engine you can hear bubbles in the hydraulic tank. from the diagram i have the front vane pump supplies pressurized fluid to the hydraulic valve block for the lift/tilt cylinders and then it exits the hydraulic block as a charge pressure into the top pipe of the rear prtion of the pump assembly. is this correct?
That sounds about right, i know it will take a little bit to get the air out, but it shouldn't take too long. Can you try blocking it up and getting it to spin the wheels?
Its possible you have an air lock as you removed the pump and motors, there will be lots of air. I was sure it didn't take too much to get mine running when i did mine....
there is definitely a lot of air. the hydraulic fluid is mainly bubbles. how do i get the air out of the system? i lifted the wheels and spun each side back and forward many times with no luck. the manual doesnt specify a way to bleed the system. i appreciate all your help tazza. i feel so defeated....
Timethere is definitely a lot of air. the hydraulic fluid is mainly bubbles. how do i get the air out of the system? i lifted the wheels and spun each side back and forward many times with no luck. the manual doesnt specify a way to bleed the system. i appreciate all your help tazza. i feel so defeated....
Thats the only cure. Drive it for a little bit, then stop. Let the air settle for a few hours or over night then do the same, drive it, lift and tilt the bucket, it will make horrid sounds then shut down. Its how i got mine bled from a totally empety system, it took a few weeks, but now its 100%.
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You could possibly try cracking one of the large hoses to the drive motors on each side, operate the drive and it will hopefully squirt out some air and foamy oil. The drive system is a loop, not much air/oil gets out of it, thats why it can take so long to purge.
There is a bleed grub screw on top of the drivepumps in the middle of the two rotating groups. This will bleed out the air a lot quicker.
thanks. i went to check and the bleed screw was covered with sludge. never noticed it. ill try it out and let everyone know what happens.
thanks again. this forum is great. a lot of knowledgeable people on here.
unscrewed the bleed screw and let as much air out as i could. tazza i took your recommendation and cracked open the drive motor lines and plenty of air came out. at one time the left drive started moving then stopped. i still have no drive and i still have plenty of air. im going to just sit there and run the lift and tilt functions for a couple of hours. should i have drive function even if air is in the system?
frogfarmer
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That machine should self prime in a matter of minutes if not seconds. You have a charge presssure or flow problem and could destroy a pump running it with little or no lubrication. Your charge flow as well as pressure needs to be verified. The lift and tilt functions are completely seperate from the drive system although they share the same oil. Even if the hystat pumps are wasted the system should prime and eliminate the air with the charge oil.
i had the hydraulic/hydrostatic pump and both drive motor rebuilt so there is a lot of air trapped into the system. i loosened the center hose(charge pressure) going into the port block(where the filter is located) and hydraulic fluid rushed out quickly but it was foamy/bubbly. how do i get rid of the air in the charge pressure going into the port block? will constant lifting and tiliting of the arm and bucket get rid of it?
If the oil is foamy its all in your reserve tank too, it needs time to settle But as mentioned, the system should prime pretty fast..... Are you sure the hoses went back on the right places? silly question i know, but never hurts to check?
yes. i opened the ports in the tank and it is foamy as well. all the hoses are installed correctly. i number stamped them when i removed them.
frogfarmer
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I realize this is very frustrating but the way the system is set up with a priority flow divider and bypass valve you need the factory service manual with the flow charts for the hydraulic system. Once you have the flow charts you can study the fluid flow and valving to determine where the problem may be. There is a manual online here but the flow charts are hard to follow in the format they are viewed. I work on hydrostat systems of all sizes and when they are right they will self prime very quickly. As I stated before charge pressure and flow need to be checked. There is a special factory setup for this unit but one can be adapted from standard parts. Foam in the oil on initial start after a service is fairly normal. Foam in the oil after extensive run/stop cycles and several days after the service would indicate a problem.
would adding thicker hydraulic fluid help prevent some of the foam? for the past few days after work i have been raising and lowering the lift arm several times and at one point i had movement of the right side wheels for a moment then they stopped. ill install a pressure gauge at the sensor and see what i find.
thanks
frogfarmer
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I would stick with the recommended fluid and the recommended weight. The factory fill has anti foaming additives as well as an additive package for the design of the system. I have run 10w30 motor oil as the recommended alternative on numerous ocassions without any problems. From what you have posted I would guess one of the poppet valves in one of the blocks in leaking or broken allowing the oil to cavitate and draw air into the system. I have worked on 743s but not recently and I dont have the manual for that exact model. I do remember they have several poppet valves to provide flow to the hydros as well as the oil cooler and filter assembly. If one of the valves was continually bypassing it would be very hard if not impossible to purge the air.
i had the pump rebuilt and pressure tested and it pressure tested fine. i asked the hydraulic rebuild shop if any of the pressure relief valves were damaged and he said no. the pump assembly had no major damage to any of the pistons or vane pump. the rebuild shop did a complete re-seal and pressure test of the pump and it passed. the hydrostatic motors were both rebuilt as a set as one of them had slow movement.
i put a 5000 psi pressure gauge on the port block were the sensor is located and i am getting no pressure. ive been staring at this hydraulic schematic and the only two things i can see that would be a problem would be the charge pressure screen clogged which i just cleaned it or the valve to the right of the charge pressure line goin ginto the port block. it appears charge pressure gets diverted back to the cooler line via this line. if there was a problem with this valve it would look like charge pressure would be lost as it would be sent to the cooler.
frogfarmer
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I didnt mean to imply the pumps or the motors had a problem but that one of the valves in the (port block) or the (bypass block) was possibly stuck or broken; these two items are not part of the pumps or motors. If I remember there are 3 or 4 poppet valves that divert flow at a certain pressure from the hydraulic pump. And again it has been a while since I was inside a 743 but I do remember there are 2 (port blocks) besides the main hydraulic valve that divert flow for the hydro system and maintain the reduced pressure of the charge oil. If you are looking at the flow chart and found one of the valves you are on the right track. Along with the flow chart that has all the components numbered there will be a written paragraph outlining how the numbered components all work together to adjust the flow and pressure. Work your way through the system making checks at each component until you find the problem or problems. Since you have verified there is no charge pressure be very careful running the machine because the newly built hydro pumps can be damaged very easily with a lack of lubrication. Hang in there you are learning valuable information that may help you in the future with diagnosing a problem. The machines I saw years ago were a bit overwhelming when I first got started and I have a mechanical engineering degree on the wall.
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.
