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Crawlers, Dozers, Loaders & Backhoes Discussion Forum

Bobcat drive issues

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richardw

06-27-2005 18:26:13

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Hi all!

I've got a Bobcat 743B skidsteer that has about 4000 hours on it. When the machine is under a load such as turning or pushing into a pile it makes a growling noise which sounds like it's coming from the hydrostatic pump. It will still turn and spin the tires when pushing but it sure sounds bad. I have a service manual and have studied it thoroughly. I'm wondering if it could be the high pressure relief/replenishing valves on the pump? Some have mentioned here on the forums to replace the sintered bronze filter on the port block. I've got one on order but I can't see how this would help because it just filters the drain fluid from the motors. Any of you guys ever run into this problem?
Thanks in advance for your input.

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Smalltime

06-28-2005 23:39:15

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Re: Bobcat drive issues in reply to richardw, 06-27-2005 18:26:13

Good to hear you found some of the problem. I was wondering if you guys that built your brushcutters could tell me what kind of hyd motor you used for running the cutter heads? Also, are these built as free swinging blades or like the others that are similar to a stumpgrinder with teeth on a large hub?

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richardw

06-29-2005 11:31:50

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Re: Bobcat drive issues in reply to Smalltime, 06-28-2005 23:39:15

The brush cutter I built was just an old rotary sling blade bush hog I had and I installed a hydraulic motor and modified the drive shaft from the gearbox to attach to the hyd.motor. I used a 7.3 cu. in. displacement motor which spins at 470 rpm which is close to the 540 rpm of a standard PTO. So my unit runs a little slower than with a PTO but it's plenty fast enough to do the job. I really like my brush cutter.....I think it may be just slightly too much load on the hydraulics on my 743B. Although when running it doesn't seem to put much load on the engine....just makes it harder to steer.

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gatractorman

06-28-2005 18:17:24

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Re: Bobcat drive issues in reply to richardw, 06-27-2005 18:26:13

Any hydrostatic system with any age/hours will have some noise, you say one side is worse when it gets warm is a classic symptom of worn hydrostats, run it as long as it will, keep on top of filter changes, when the time comes rebuild the pump and motor on both sides and you might wanna be savin' up for that time (lotsa $$$).

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Taylor Lambert

06-28-2005 14:47:52

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Re: Bobcat drive issues in reply to richardw, 06-27-2005 18:26:13

I built a brush cutter for a friends Bobcat last year and when cutting with it he had some problems like you were having. When he attached the mower the fluid that it took t fill the motor and the lines to the mower lowered the amount of fluid in the tank. Try checking the fluid if you haven already. Sounds like cavitation.

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richardw

06-28-2005 18:21:04

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Re: Bobcat drive issues in reply to Taylor Lambert, 06-28-2005 14:47:52

This is also a homemade brush cutter I built. Checked the fluid.....full to the sight window. I wonder if there is a screen filter where the hose comes out of the bottom of the hydraulic tank?

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TimWafer

06-28-2005 12:43:12

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Re: Bobcat drive issues in reply to richardw, 06-27-2005 18:26:13

Sounds like maybe cavatation or air bubbles in the hydraulics. Is the fluid low and you are sucking air? Or a blockage in the supply side somewhere? Maybe the charge pump that supplies oil to the hydrostatic pump isn�t working or up to par. If it is cavatation it will destroy your hydrostatic pump quickly so I wouldn�t run it till you find the cause and correct it.
Good Luck
Tim

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richardw

06-28-2005 18:27:12

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Re: Bobcat drive issues in reply to TimWafer, 06-28-2005 12:43:12

Plenty of fluid in the tank. The charge pump for the hydrostatics is also the hydraulic pump for the lift arms and they seem to work great with plenty of power.

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richardw

06-28-2005 08:42:26

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Re: Bobcat drive issues in reply to richardw, 06-27-2005 18:26:13

I changed the hydraulic fluid and filter but can"t really tell much difference. One thing I have noticed is that the higher the RPM, the smoother and less noisy things are. Even at full RPM, when I am using my bushhog attachment, it gets very difficult to steer like it"s starving for fluid or not enough high pressure. Also one side seems to be worse than the other.

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Davis In SC

06-28-2005 14:05:44

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Re: Bobcat drive issues in reply to richardw, 06-28-2005 08:42:26

That Bush Hog attachment uses a lot of the volume... Do you have the Hi Flo Hydraulics??

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richardw

06-28-2005 18:29:41

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Re: Bobcat drive issues in reply to Davis In SC, 06-28-2005 14:05:44

No....standard flow. Pump is rated at 12.1 GPM

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Roy Suomi

06-27-2005 20:29:41

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Re: Bobcat drive issues in reply to richardw, 06-27-2005 18:26:13

All hydrostatics have a whine or in some cases a growl at different speeds and loads. To check for problems cut your hydrostatic filter apart to look for metalic particles. A little is normal, a lot is trouble.

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SmallTime

06-28-2005 00:54:37

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Re: Bobcat drive issues in reply to Roy Suomi, 06-27-2005 20:29:41

I agree with checking the fluid and filters for excessive metal. If its any consolation, my 743 has very noisy hydrolics too, lots of whining and groaning. It must be pretty normal with these machines. You might change out your hydro fluid and filter and see if that helps, didnt seem to on mine but you never know. Its loudest when turning or moving and not so much when just lifting. Post back if you come across something, Id be interested to hear. Good luck.

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coleman

07-27-2005 16:53:14

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Re: Bobcat drive issues in reply to SmallTime, 06-28-2005 00:54:37

INteresting thread. thanks. I am looking at a 743 to buy and this load whining is significant, great that it's posssible this machine doesn't need a new pump(s) however, is there any way beside pressure testing to know how much they are putting out, weight/feild test? Also what are the hazards of running 10/30 in these pumps. This yard says they have always done it. can't it harm the seals of the pumps? What else can I look for before getting into this machine? Guy wants 4500, slightly beat with 6000 hours thanks for your help.

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richardw

06-28-2005 18:40:13

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Re: Bobcat drive issues in reply to SmallTime, 06-28-2005 00:54:37

Well, today I changed the case drain filter at the port block (the sintered bronze one) and I must say there was an improvement. Still not 100% but a definate improvement. Each drive motor has a small line coming off it to drain excess fluid and these two lines come together and go thru this filter back into the port block. I don't understand how this could have helped my problem but it did.

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richardw

06-29-2005 11:37:17

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Re: Bobcat drive issues in reply to richardw, 06-28-2005 18:40:13

You know the more I use it....it does sound like cavitation. Sometimes when steering there will be a pause...then it will jump into action like it got a surge of fluid. Where might I possibly look for infiltration of air? Wouldn't it be leaking fluid if I had a crack or loose fitting somewhere?

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TimWafer

06-29-2005 12:28:47

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Re: Bobcat drive issues in reply to richardw, 06-29-2005 11:37:17

Maybe yes and maybe no. It doesn�t necessarily have to be air infiltration. Just a lack of flow or positive pressure supply to the hydrostatics will cause it. When I bought my first used skid steer I had a problem with cavatation. The charge pumps were built into the nose of each hydrostatic pump and one of the supply hoses was packed full of coarse metal shavings. I never did figure where they came from but didn�t know the previous history of the machine either. I cleaned them out and inspected the hydrostatic pumps and all seemed well. That was almost 20 years ago now and thousands of hours use and its still going strong with no problems. The shavings restricted the flow enough to cause problems. Since your Hydrostatics are being supplied by the Auxiliary pump, maybe your mower is using too much flow and not enough is going to the Hydrostatics. If it were me I would look into it further before it runs into big money! Cavatation sounds like marbles running through the pump.
Tim

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richardw

06-29-2005 19:02:40

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Re: Bobcat drive issues in reply to TimWafer, 06-29-2005 12:28:47

Wouldn't say it sounds like marbles....just hydraulic whine exaggerated...and when it does it I can feel it in the steering levers.
Come to think of it....that "pause then surge" thing I was talking about only happens on the left side....so I guess that would eliminate cavitation unless it was inside the hydrostatic pump somewhere.

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SmallTime

06-30-2005 00:56:30

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Re: Bobcat drive issues in reply to richardw, 06-29-2005 19:02:40

Sounds like a good little attachment. About your pause and surge, Im thinking if it only happens on your left side and you dont have the same symptoms with the right or any of the other functions including any attachments, it might be that left drive motor. Also I may have missed it, but does this surge and pause happen without your cutter running or only with it running? It seems like if it was your cutter robbing volume then your other functions would be acting up also.

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richardw

06-30-2005 08:32:54

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Re: Bobcat drive issues in reply to SmallTime, 06-30-2005 00:56:30

It does it when under a load such as steering while carrying a load..... with or without the bush hog attachment.

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SmallTime

07-01-2005 02:40:06

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Re: Bobcat drive issues in reply to richardw, 06-30-2005 08:32:54

Well, Im far from a hydraulics expert but anytime Ive had problems like that, weak under load, it has turned out to be pump or motor related. It might pay to have someone check your pressure output from the pump. If you can try to isolate it, you might find out what is going, but it sounds like something on the left side is getting weak. That being said, mine makes alot of noise when running too but I dont have any problems like that, Im saying that only because I wonder if you could have a drive chain thats binding or something non hydraulic related.

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richardw

07-01-2005 08:22:26

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Re: Bobcat drive issues in reply to SmallTime, 07-01-2005 02:40:06

I think I'll check the pressure coming into the hydrostatic pump to make sure I'm getting enough charge pressure. Still...if it was a weak charge pump it seems like I would occassionally have problems on the other side. Does the hydrostatic pump have separate internal pumps for each drive motor?

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SmallTime

07-05-2005 03:32:08

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Re: Bobcat drive issues in reply to richardw, 07-01-2005 08:22:26

Just got a chance to check the book. It looks like there are 2 hydrostat pumps, one for each side and the hydraulic pump supplies both.
So I think your right, you would probably see problems with both sides if it was the pump. There are some high pressure replenishing valves also, they perform a number of functions. Maybe something there?

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richardw

07-05-2005 08:14:07

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Re: Bobcat drive issues in reply to SmallTime, 07-05-2005 03:32:08

Wow, thanks for looking that up for me. I pulled all of the replenishing valves and looked at them.....there was just a very slight mark on the poppets where they touch the seat...but all four looked about the same. I was hoping the left side would reveal something damaged but they all seemed to be worn about the same. Called Bobcat dealer to get a price on just the poppet..... wow....$170 each.

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boggs

07-17-2005 19:16:55

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Re: Bobcat drive issues in reply to richardw, 07-05-2005 08:14:07

You said you pulled all the replenishing valves, Could you tell us where they are located? We have the same problem you were having. Thanks

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richardw

07-18-2005 11:18:44

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Re: Bobcat drive issues in reply to boggs, 07-17-2005 19:16:55

When you raise the cab you will see the main hydrostatic pump right on top. It has a small hydraulic pump bolted to the front of it next to you. If you look at the steering lever linkage while moving the levers you will see what they are connected to....that"s the hydrostat pump. On each side there will be two plugs (on my 743 it takes an 1-1/4" wrench). You remove these plugs and there will be a spring and then a poppet. On each side there will be one replenishing valve and one Relief/replenishing valve (They look similar but one has a relief valve built into it). The ones on the right side are tough to get to because you have to remove all the steering linkage to remove the plugs. Which means everything will have to be re-adjusted when you put it all back together. Not difficult....just time consuming.

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boggs

07-21-2005 09:32:55

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Re: Bobcat drive issues in reply to richardw, 07-18-2005 11:18:44

Thank You. We took it to the dealer to run pressure tests, but I think that we will bring it home to fix it. They want $1000 to do the work. I know the valves are expensive, but their labor is worse.

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SmallTime

07-06-2005 10:26:41

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Re: Bobcat drive issues in reply to richardw, 07-05-2005 08:14:07

Ouch!!!! That hurts. Maybe its just something you'll have to watch for a while and see if it gets worse. Sounds like you covered the bases. I dont know if a hydro motor can be intermittent or not but maybe. If they are the same, maybe you could swap motors and see if the problem moves to your right side? Im not sure.

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