Bobcat 743 Hydrostatic Pump Surging, Drive Erratic

[onepoket]

Okay, so I've posted before about this machine as I've been working through some of the aging components and the bonehead things the previous guy did.

Here is a rundown of what I have done so far minus the engine stuff because that is working fine:

• new seals on drive motors
• new shuttle pistons on drive motors, mine were very worn
• new seals for tilt and lift cylinders
• new hydro filter, changed hydro fluid
• drained and cleaned chain case and filled with new oil
• removed and cleaned port block, changed all o-rings
• removed and cleaned/polished relief valves from hydro pump
• replaced all steering linkage bushings
• flattened the steering blocks that the pintle cams press against
• replaced the pintle cams
• fixed the transmission warning light and related sensors

• I think that is about it as it relates to hydro systems.
  
  My problem is this, when moving the sticks slowly forward or backward, the pump sounds as if it surges up and down and this causes the drive to be erratic and jerky. If I have the machine with a decent straight area I can push the stick and get past the surging zone and the drive smooths out and is fine. As soon as I go to turn and back one of the sticks off into the "surging zone" the drive gets jerky and bucks.
  This happens at any engine RPM.
  
  My question is, is this a problem with the hydrostatic pump? Do I need to get wafer plates, rotating piston groups and seals for the TA1919v10?
  I feel like there isn't much else that it could be but I have pretty much no experience on these machines so I'm sure you guys know better than me. Also, the other hydraulic functions work fine except the aux detent but I keep the aux locked out anyway. It may be sucking air through a gland nut on one lift cylinder but it is very slight if that is possible anyway, still not sure about that one.
  
  Thanks for any info.

 

[Bswwood]

If you have a service manual follow the procedure for checking charge pressure. Not sure of your serial# but you might have a charge filter which is a brass cartridge that may be clogged.

 

[onepoket]

If you have a service manual follow the procedure for checking charge pressure. Not sure of your serial# but you might have a charge filter which is a brass cartridge that may be clogged.

Thanks. I did check the charge pressure but I only had a 2400 PSI gauge so it was hard to tell. Nearest guess is that it was at about 76 PSI. I have a new gauge coming so I can get a more accurate reading. I was testing using the sensor fitting on the hydraulic oil filter in the engine bay.

I have the version that does not have the block on the pump, I have the port block on the left hand side of the frame as you look from the front of the machine.

I will post back when I have more pressure readings. I plan on taking readings at the aux ports as well and maybe the motor hoses. Its hard though because when the machine is blocked the wheels seem to run fine, the surging only happens when the machine is under the load of moving its own weight apparently.

At this point I'm really trying to determine whether the pump is shot, can be fixed with seal kit and wearable parts, or just needs to be replaced all together. The machine is unusable, I don't use it for business but I need it to function to work on my property.

 

[Bswwood]

Thanks. I did check the charge pressure but I only had a 2400 PSI gauge so it was hard to tell. Nearest guess is that it was at about 76 PSI. I have a new gauge coming so I can get a more accurate reading. I was testing using the sensor fitting on the hydraulic oil filter in the engine bay.

I have the version that does not have the block on the pump, I have the port block on the left hand side of the frame as you look from the front of the machine.

I will post back when I have more pressure readings. I plan on taking readings at the aux ports as well and maybe the motor hoses. Its hard though because when the machine is blocked the wheels seem to run fine, the surging only happens when the machine is under the load of moving its own weight apparently.

At this point I'm really trying to determine whether the pump is shot, can be fixed with seal kit and wearable parts, or just needs to be replaced all together. The machine is unusable, I don't use it for business but I need it to function to work on my property.

Hopefully when you get your new gauge you have enough hose to run under the cab and monitor charge while driving. See if charge drops when surging

 

[onepoket]

Hopefully when you get your new gauge you have enough hose to run under the cab and monitor charge while driving. See if charge drops when surging

Ok, I did a pressure test, but not the charge pressure because I am still waiting on a fitting, I can't find one locally.

I tested the pressure at the aux connections with a 6000 psi gauge.

Pushing left on the stick, engages my top connector and that had a max PSI of 1600, but it was weird, if I pushed the stick left it initially went to 1500 then if I went to center and back to left it would surge to 1600, not sure if that is significant or not.

Pushing the stick right engaged the lower aux, that read 2200 psi.

I started another thread asking about my aux spool, I think it may be missing a washer and snap ring. I feel like the stick is supposed to self center but mine doesn't so maybe the spring is broken and or the snap ring is causing it to not have tension at all.

here is a link to my other thread Am I missing a Washer and Snap Ring, 743 Control Valve

 

[onepoket]

Ok, I did a pressure test, but not the charge pressure because I am still waiting on a fitting, I can't find one locally.

I tested the pressure at the aux connections with a 6000 psi gauge.

Pushing left on the stick, engages my top connector and that had a max PSI of 1600, but it was weird, if I pushed the stick left it initially went to 1500 then if I went to center and back to left it would surge to 1600, not sure if that is significant or not.

Pushing the stick right engaged the lower aux, that read 2200 psi.

I started another thread asking about my aux spool, I think it may be missing a washer and snap ring. I feel like the stick is supposed to self center but mine doesn't so maybe the spring is broken and or the snap ring is causing it to not have tension at all.

here is a link to my other thread Am I missing a Washer and Snap Ring, 743 Control Valve

I finally had good weather and could do a charge pressure test with a 200 psi gauge while driving.

At full RPM I had 120 psi that dropped to 100 psi when in motion, at min RPM it was 100 psi that stayed at 100 psi when driving.

So, I think the charge pressure is fine. Im guessing the problem is with the hydro pumps.

To test those, is the best way to take the high pressure hoses off, cap one and put a 6000 psi gauge on the other, run the machine and move the stick forward and backward?

And should I get 5000 psi at the drive motor hose?

 

[dfb]

I finally had good weather and could do a charge pressure test with a 200 psi gauge while driving.

At full RPM I had 120 psi that dropped to 100 psi when in motion, at min RPM it was 100 psi that stayed at 100 psi when driving.

So, I think the charge pressure is fine. Im guessing the problem is with the hydro pumps.

To test those, is the best way to take the high pressure hoses off, cap one and put a 6000 psi gauge on the other, run the machine and move the stick forward and backward?

And should I get 5000 psi at the drive motor hose?

My 742B had a similar problem on one side, it was the motor. I had both resealed and all worked perfectly afterward. It is worth noting that one of my motors had 2 be resealed twice to get it right, very frustrating. My machine has the valve block on the side of the chassis like yours does. I feel it worth noting, that I had been using AW-32 but switched back to the Bobcat oil. My reasoning for making the switch was I wanted peace of mind that there was no question if the oil met the correct specs. AW oils are for hydraulic systems and do not necessarily have the lubrication requirements of Hydrostatic systems. I would rather pay a little more{and the bobcat oil is not that much more} and not have to replace a drive motor because of improper lubrication.

 

[onepoket]

My 742B had a similar problem on one side, it was the motor. I had both resealed and all worked perfectly afterward. It is worth noting that one of my motors had 2 be resealed twice to get it right, very frustrating. My machine has the valve block on the side of the chassis like yours does. I feel it worth noting, that I had been using AW-32 but switched back to the Bobcat oil. My reasoning for making the switch was I wanted peace of mind that there was no question if the oil met the correct specs. AW oils are for hydraulic systems and do not necessarily have the lubrication requirements of Hydrostatic systems. I would rather pay a little more{and the bobcat oil is not that much more} and not have to replace a drive motor because of improper lubrication.

dfb, interesting. I did put new seal kits in both drive motors, I suppose something could be wrong like an o-ring moved when I reassembled it. The thing that makes me skeptical that is the problem is that after I resealed them I drove the machine all winter to plow. There was a big leak coming from the port block at that time. After I fixed that leak my drive issues started.

Before I go resealing the motors I want to get the pressure readings at the motor hoses. I just want to know the best way to do that.

My suspicion is that the hydro pump needs to be resealed and wear plates replaced.

 

[onepoket]

dfb, interesting. I did put new seal kits in both drive motors, I suppose something could be wrong like an o-ring moved when I reassembled it. The thing that makes me skeptical that is the problem is that after I resealed them I drove the machine all winter to plow. There was a big leak coming from the port block at that time. After I fixed that leak my drive issues started.

Before I go resealing the motors I want to get the pressure readings at the motor hoses. I just want to know the best way to do that.

My suspicion is that the hydro pump needs to be resealed and wear plates replaced.

Ok, I finally had time to do my drive motor hose pressure test.

What I did was remove both high pressure hoses from the motor and cap the motor side, plugged one hose and put my 6000psi gauge on the other hose. I ran the machine and moved the stick forward and backward. In my case, forward got me a reading on the gauge.

The left, as you sit, motor which is the rear pump, gave me 5000psi.
The right motor, the front pump, gave me 4000psi.

I got resistance in the opposite direction as expected.

So, what now? What does that tell us? Do I need to open the pumps?

Is there any other diagnostic that I can do to figure out what the problem is?

Hoping Tazza and you other experienced peeps can give me some direction.

I have not been able to use the machine all summer basically because of my issues and I absolutely need it to run for the winter so I can plow. I am about to bite the bullet and call Bobcat for a house call (I can't transport it to them without borrowing a truck and renting a trailer).

Thanks for any info.

 

[Tazza]

Ok, I finally had time to do my drive motor hose pressure test.

What I did was remove both high pressure hoses from the motor and cap the motor side, plugged one hose and put my 6000psi gauge on the other hose. I ran the machine and moved the stick forward and backward. In my case, forward got me a reading on the gauge.

The left, as you sit, motor which is the rear pump, gave me 5000psi.
The right motor, the front pump, gave me 4000psi.

I got resistance in the opposite direction as expected.

So, what now? What does that tell us? Do I need to open the pumps?

Is there any other diagnostic that I can do to figure out what the problem is?

Hoping Tazza and you other experienced peeps can give me some direction.

I have not been able to use the machine all summer basically because of my issues and I absolutely need it to run for the winter so I can plow. I am about to bite the bullet and call Bobcat for a house call (I can't transport it to them without borrowing a truck and renting a trailer).

Thanks for any info.

Those are some pretty good pressure readings, the relief valves should be set to 5,000 PSI, so the lower one tht was 4,000 may just need shimming.
I don't believe that will be giving surging issues while driving. You have done everything i'd have done, and more.
The most common cause of issues is worn linkages, but you have fixed that already. Are you able to get any video or audio of the noises at all? if you look up my profile, feel free to email me.
These systems are very simple, i'd like to think the issue is something simple.

 

[onepoket]

Those are some pretty good pressure readings, the relief valves should be set to 5,000 PSI, so the lower one tht was 4,000 may just need shimming.
I don't believe that will be giving surging issues while driving. You have done everything i'd have done, and more.
The most common cause of issues is worn linkages, but you have fixed that already. Are you able to get any video or audio of the noises at all? if you look up my profile, feel free to email me.
These systems are very simple, i'd like to think the issue is something simple.

Tazza, thanks for the response.

The shims you are talking about are where? Are those the shims on the pump itself (I see a shim kit as part of the rebuild kit in the overhaul manual for the TA1919) or is it a shim on the relief valve? Is the relief that kick in to keep the pressure at 5000psi the cross port valve on the middle part of the pumps? The big nuts that have the poppets inside and a yellow spring? How would I adjust the one side to be 5000psi instead of 4000?

I will do some video so you can hear the sound. The machine is blocked at the moment so I can do that and then I will put it on the ground and take some video of driving it, hopefully you can see better and I think it sounds different under the load of the vehicle.

Thanks a lot.

 

[Tazza]

Tazza, thanks for the response.

The shims you are talking about are where? Are those the shims on the pump itself (I see a shim kit as part of the rebuild kit in the overhaul manual for the TA1919) or is it a shim on the relief valve? Is the relief that kick in to keep the pressure at 5000psi the cross port valve on the middle part of the pumps? The big nuts that have the poppets inside and a yellow spring? How would I adjust the one side to be 5000psi instead of 4000?

I will do some video so you can hear the sound. The machine is blocked at the moment so I can do that and then I will put it on the ground and take some video of driving it, hopefully you can see better and I think it sounds different under the load of the vehicle.

Thanks a lot.

The relief valves are the springs and poppets under the big nuts in the centre part of the pump. You put a shim between the hut part and the spring to give the spring a little more pressure on the poppet. Even thin washers may be enough of a shim to do the job, but i don't know what the rule is for how much is needed to add 1,000 PSI, sadly i can't tell you which one is for which port. To make things more difficult, there are 2 on the top that is easy to access, but there are 2 under the pump that are not so easy.

 

[onepoket]

The relief valves are the springs and poppets under the big nuts in the centre part of the pump. You put a shim between the hut part and the spring to give the spring a little more pressure on the poppet. Even thin washers may be enough of a shim to do the job, but i don't know what the rule is for how much is needed to add 1,000 PSI, sadly i can't tell you which one is for which port. To make things more difficult, there are 2 on the top that is easy to access, but there are 2 under the pump that are not so easy.

Tazza,

Ok. I finally had time to make the videos.
The first video is of the machine on blocks so you can hear the sound when the sticks are moved forward and backward:
https://www.youtube.com/watch?v=n4VLNIpsFks

The second video is of me driving the machine. You can see it hopping around. This is happening when I am barely engaging the sticks. When I go straight and get the sticks full out it runs smooth, when I back off to make a turn you can see how much the machine starts to hop and I have to stop because it feels like it is getting out of control. I up the RPM and drive a little more but the hopping continues any time the sticks are in maybe the first 10-15% of movement either direction.
https://www.youtube.com/watch?v=U0hfGDX_ld0

 

[Tazza]

Tazza,

Ok. I finally had time to make the videos.
The first video is of the machine on blocks so you can hear the sound when the sticks are moved forward and backward:
https://www.youtube.com/watch?v=n4VLNIpsFks

The second video is of me driving the machine. You can see it hopping around. This is happening when I am barely engaging the sticks. When I go straight and get the sticks full out it runs smooth, when I back off to make a turn you can see how much the machine starts to hop and I have to stop because it feels like it is getting out of control. I up the RPM and drive a little more but the hopping continues any time the sticks are in maybe the first 10-15% of movement either direction.
https://www.youtube.com/watch?v=U0hfGDX_ld0

The sound of it when blocked sounds normal to me.
As for the jumping around. Without a bucket on, it is very light on it's feet, so easy to do wheel stands.
The jerkyness, when turning on "sticky" surfaces like bitumen or concrete, it's very normal for it to jump around. The Tyres side wall will flex till you loose traction on the ground, slide then repeat, it makes the whole machine wobble, mine does the same thing on surfaces that grip to the tyres, in the dirt, no issues.
Honestly, your machine seems good. If it can turn like that on bitumen, the pump and morors are good. I had machine that wouldn't turn, even on dirt.

 

[onepoket]

The sound of it when blocked sounds normal to me.
As for the jumping around. Without a bucket on, it is very light on it's feet, so easy to do wheel stands.
The jerkyness, when turning on "sticky" surfaces like bitumen or concrete, it's very normal for it to jump around. The Tyres side wall will flex till you loose traction on the ground, slide then repeat, it makes the whole machine wobble, mine does the same thing on surfaces that grip to the tyres, in the dirt, no issues.
Honestly, your machine seems good. If it can turn like that on bitumen, the pump and morors are good. I had machine that wouldn't turn, even on dirt.

Tazza,

Sorry, I didn't get a notification of your response.

So, this is normal? I feel like it jumps around even when I use both sticks in the same direction. For instance when I try to back it into the garage, it hops so much I'm afraid I'll hit the top of my door jam, that's why I have it in the driveway for now. Also, with the bucket on it acts pretty much the same.

When I drove it in the winter, when it had a leak at the port block, it was smooth to back it into the garage, it also didn't jump around that much when turning, but maybe as you said, it was because I was on snow or slippery ground.

I just feel like it drove smoother before I fixed the leak.

Do you think that there is any possibility that it could be something I did when I resealed the drive motors (as someone else mentioned)? Or maybe some debris did some damage to them when I fixed the port block leak?

Thanks, for you help.

 

[Tazza]

Tazza,

Sorry, I didn't get a notification of your response.

So, this is normal? I feel like it jumps around even when I use both sticks in the same direction. For instance when I try to back it into the garage, it hops so much I'm afraid I'll hit the top of my door jam, that's why I have it in the driveway for now. Also, with the bucket on it acts pretty much the same.

When I drove it in the winter, when it had a leak at the port block, it was smooth to back it into the garage, it also didn't jump around that much when turning, but maybe as you said, it was because I was on snow or slippery ground.

I just feel like it drove smoother before I fixed the leak.

Do you think that there is any possibility that it could be something I did when I resealed the drive motors (as someone else mentioned)? Or maybe some debris did some damage to them when I fixed the port block leak?

Thanks, for you help.

Sorry for the delay, I don't get on the forum as much as I used to.
I really don't see a drive motor or port block could cause the issue. I'd like to think that the slippery driveway would have helped make it smoother.
Have you tried running it on grass or dirt to see how it acts? Even wet your floor and see if it bounces around less?

 

[onepoket]

Sorry for the delay, I don't get on the forum as much as I used to.
I really don't see a drive motor or port block could cause the issue. I'd like to think that the slippery driveway would have helped make it smoother.
Have you tried running it on grass or dirt to see how it acts? Even wet your floor and see if it bounces around less?

Thats cool. I actually went out and got new tires and rims.
I had planned to when the machine was running well, but seeing as there are no leaks, the pressure readings all seem good, and you think the tires are the problem I invested.

I ended up getting BKT Sure Trax HD tires, I also forgot to mention that the tires that came on the machine were I think a DIY foam fill job, so they were worn and heavy as hell. The rear tires looked under inflated but no way to inflate the foam fill.

So, after wrestling with getting the new tires on the rims, inflating, and installing on the machine, I'm happy to say it drives better.

I tried to be really conscious of keeping the sticks stable as I moved around. I locked my hands against my legs because it seems any bit of minor bucking can easily grow to bouncing around since its hard to keep the sticks stable when your arms are just floating in the air.

This brings me to my new question. I know the later models had steering dampers, do you think it would be possible to install dampers on the 743?
I can't seem to find detail on how they are installed. I think they are on the 463. But is the stick neutral position set where the damper is extended 50%?

I feel like if the sticks were stabilized by the dampers then driving the machine would be way smoother since it would be harder to move the sticks inadvertently when you hit some bumps.

But I am a bit embarrassed I didn't know about the tires being an issue. But I learned a lot doing all my diagnostics so I'm probably better for it.

Thanks!

 

[Tazza]

Thats cool. I actually went out and got new tires and rims.
I had planned to when the machine was running well, but seeing as there are no leaks, the pressure readings all seem good, and you think the tires are the problem I invested.

I ended up getting BKT Sure Trax HD tires, I also forgot to mention that the tires that came on the machine were I think a DIY foam fill job, so they were worn and heavy as hell. The rear tires looked under inflated but no way to inflate the foam fill.

So, after wrestling with getting the new tires on the rims, inflating, and installing on the machine, I'm happy to say it drives better.

I tried to be really conscious of keeping the sticks stable as I moved around. I locked my hands against my legs because it seems any bit of minor bucking can easily grow to bouncing around since its hard to keep the sticks stable when your arms are just floating in the air.

This brings me to my new question. I know the later models had steering dampers, do you think it would be possible to install dampers on the 743?
I can't seem to find detail on how they are installed. I think they are on the 463. But is the stick neutral position set where the damper is extended 50%?

I feel like if the sticks were stabilized by the dampers then driving the machine would be way smoother since it would be harder to move the sticks inadvertently when you hit some bumps.

But I am a bit embarrassed I didn't know about the tires being an issue. But I learned a lot doing all my diagnostics so I'm probably better for it.

Thanks!

You did a great job getting the foam filled tyres off the rims, i have never tried, but heard it's a nightmare.
It's always possible to addapt shock absorbers to it, but i'd like to think it's not needed, but it may still help. I found my 743s were never out of control, but as you noticed, when it starts to buck you around, it just gets worse.
I'd give it a go with replacing the centering spring, more load on it pulling back to neutral may help things.

 

[onepoket]

You did a great job getting the foam filled tyres off the rims, i have never tried, but heard it's a nightmare.
It's always possible to addapt shock absorbers to it, but i'd like to think it's not needed, but it may still help. I found my 743s were never out of control, but as you noticed, when it starts to buck you around, it just gets worse.
I'd give it a go with replacing the centering spring, more load on it pulling back to neutral may help things.

OK, its been a while but I've been hard at work and figured I'd report my findings.

The issue I had with my washer and e clip not being on my aux spool turned out to be the culprit with much of my drive issues I think.

I tried to just put the washer and clip on the back of the spool and see how it worked. What happened was the entire aux control was seized up. I couldn't move the stick. So, my guess is that at some point in the past someone got the aux detent stuck and as a work around they pulled the retainer clip off the back of the spool and then they could move the spool back to "center" and lock the right stick with the bolt. When I went to run my tests after I had fixed my hydro leaks and replaced drive motor pistons earlier in the summer I took the aux bolt out to see if it functioned. That is when my drive issues started acting up.

So, what I did was unhook and plug my main hydro tank to port block line. I removed the port block entirely (again) and went to work on the spools. I resealed them all, rebuilt all the ball detents with new springs and ball bearings, cleaned all the caps, replaced the centering spring on the aux spool, put new boots on and generally cleaned everything I could and greased everything up really well. Also, I actually torqued the spool screws down, some of them were very loose when I took it apart.

I then replaced the suction hose from the port block to the pump because the old one was dry and stiff and I had a hunch that it was sucking a little air there but couldn't prove it. I also replace the pump to control valve hose because the one I had was routed over the top of the suction line and the cab seat bottom would actually press down on it and I thought that could cause the suction hose to be pulled off the barb a bit. In the process I found that my right side drive motor hose was loose at the fitting on the pump, maybe the o-ring was holding up but also maybe that is where I was losing pressure in my tests 5k vs 4k on the right side. That is tight now.

Anyway, got it all back together, added hydro oil and started to work the air out. The hydraulics are now super quiet, the arms go up and down so smoothly and quietly it is amazing. Also, since I greased up and tightened all the foot controls I have really fine control over the bucket and arms. The drive is better now too, the straight line jerking that I experienced is gone, the aux spool self centers when I move the stick left but it sticks if I move it right (not in detent). I think it is because there was some scoring and damage to the detent spool cap but I couldn't get a replacement. The arm float detent is much easier to engage now too.

It is amazing how all these little problems add up to stuff not working right and when you knock out all the maintenance issues the machine improves so much. The only thing I haven't done is rebuild the transmission/hydrostatic pumps. I think next summer I will pull the engine and transmission and clean everything out, reseal the pumps and replace or lap the wafer plates and any wearable items. Then I will basically have a fully refurbished machine.

So, thanks for everyone's help, especially Tazza who always seems to have the answer or gets things focused in the right direction. Maybe my finding will help someone else in the future.