# Need Knowledge on Hydro Pumps



## [email protected] (Jun 2, 2021)

Hi:

I made a very bad purchase 1 month ago. My Simplicity Zero Turn has a terrible jerk to it. We have been told the hydraulic pumps need replacing (500 hours on it). Can anyone tell me how to know if it is the hydraulic pumps or the hydraulic motor?


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## HarveyW (Sep 15, 2014)

Hello pat, welcome to the tractor forum.

Your mower's steering control arms have dampers on them that control the rate at which you can move them. I don't even notice this when operating the mower. They get some slop in them due to wear. Not expensive. Replace them. I think this will fix your problem. 

Another possibility is your pump drive belt may be slipping giving radical acceleration. Replace the dampers first...


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## Bob Driver (Nov 1, 2017)

[email protected] said:


> Hi:
> 
> I made a very bad purchase 1 month ago. My Simplicity Zero Turn has a terrible jerk to it. We have been told the hydraulic pumps need replacing (500 hours on it). Can anyone tell me how to know if it is the hydraulic pumps or the hydraulic motor?


Kind of confusing when you say "My Simplicity Zero Turn has a terrible jerk"..... I've got a few customers with zero turns that a "terrible jerks", but their mowers seem to run fine.😁 Check out the link below and try "purging" the transaxles. The previous owner may have changed oil recently an got air into the system. Check the fluid level and condition of the oil. Those are also some of the easy fixes when the t-axles aren't smooth.

Try those couple of things and let us know if that didn't fix it. If not, we can get into the more complicated troubleshooting. 500 hours on a z-turn drive system is nothing as far as wear. Don't panic and start with the simple stuff first.....

Hydro Gear Purge Procedure


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## [email protected] (Jun 2, 2021)

HarveyW said:


> Hello pat, welcome to the tractor forum.
> 
> Your mower's steering control arms have dampers on them that control the rate at which you can move them. I don't even notice this when operating the mower. They get some slop in them due to wear. Not expensive. Replace them. I think this will fix your problem.
> 
> Another possibility is your pump drive belt may be slipping giving radical acceleration. Replace the dampers first...


We did replace the control arms - those were fairly inexpensive ($72).


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## [email protected] (Jun 2, 2021)

Bob Driver said:


> Kind of confusing when you say "My Simplicity Zero Turn has a terrible jerk"..... I've got a few customers with zero turns that a "terrible jerks", but their mowers seem to run fine.😁 Check out the link below and try "purging" the transaxles. The previous owner may have changed oil recently an got air into the system. Check the fluid level and condition of the oil. Those are also some of the easy fixes when the t-axles aren't smooth.
> 
> Try those couple of things and let us know if that didn't fix it. If not, we can get into the more complicated troubleshooting. 500 hours on a z-turn drive system is nothing as far as wear. Don't panic and start with the simple stuff first.....
> 
> Hydro Gear Purge Procedure


The hydraulic fluid/filter has been changed. The oil/filter has been changed. We were told to make sure the air is out of the lines which we followed those instructions. We put the control arms on it and it worked great for the first 1/2 hour then went right back to the jerking as you turn. I'm sorry - I'm not explaining this real well!!!


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## [email protected] (Jun 2, 2021)

If the mower does have air in the lines, would that make it lose power or turn hard?


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## BigT (Sep 15, 2014)

Yes, of course you would lose power with aerated fluid. I have a Kubota hydrostatic mower that had a suction leak. It would aerate the fluid in about 10 minutes and get to the point it would barely move. When your mower is acting up take a look at your hydraulic fluid on your hydraulic fluid dipstick for tiny entrained bubbles in the fluid.


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## Bob Driver (Nov 1, 2017)

[email protected] said:


> The hydraulic fluid/filter has been changed. The oil/filter has been changed. We were told to make sure the air is out of the lines which we followed those instructions. We put the control arms on it and it worked great for the first 1/2 hour then went right back to the jerking as you turn. I'm sorry - I'm not explaining this real well!!!


"We were told to make sure the air is out of the lines"....... Huh, makes you wonder why the seller went out of his way to tell you that? When I service the oil in drive units, I take them for a 10 minute test ride. Perhaps the reason he decided to sell it is because he couldn't get it to purge? Wrong oil (20w50), suction leaks, belt slipping, any of the numerous reasons listed in the Service Manual. The seller could have just been laying the foundation for an excuse when you inevitablly found out the drive system was weak

You said "Lines"..... To me, that means you might have a full blown hydraulic drive unit with a pump/two hydraulic drive motors, and not Hydro Gear, or Tuff Torq, units with left/right transaxles. Could you post a picture(s) so we can see what type of system you have on the machine?

That might be a good thing. You can come up with a "test tool" for a full blown hydraulic system for less than $50.









The test tool for a Hydro Gear, or Tuff Torq unit runs $800 retail


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## [email protected] (Jun 2, 2021)

Bob Driver said:


> "We were told to make sure the air is out of the lines"....... Huh, makes you wonder why the seller went out of his way to tell you that? When I service the oil in drive units, I take them for a 10 minute test ride. Perhaps the reason he decided to sell it is because he couldn't get it to purge? Wrong oil (20w50), suction leaks, belt slipping, any of the numerous reasons listed in the Service Manual. The seller could have just been laying the foundation for an excuse when you inevitablly found out the drive system was weak
> 
> You said "Lines"..... To me, that means you might have a full blown hydraulic drive unit with a pump/two hydraulic drive motors, and not Hydro Gear, or Tuff Torq, units with left/right transaxles. Could you post a picture(s) so we can see what type of system you have on the machine?
> 
> ...


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## [email protected] (Jun 2, 2021)

Good to know on the test kit. After watching the video on purging the air in lines we think we may have told to do it wrong. We are trying that first this weekend. We will then check the fluid in the drive motors. I will also take some pictures and let everyone that is trying to help know how it goes.

Thanks!!


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## [email protected] (Jun 2, 2021)

Okay - another question - We can't find the bleed off valve like the one in video link above. The wheels are turning - no way to put it into a neutral position to complete the purge process. Ideas? Pictures attached















.


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## Bob Driver (Nov 1, 2017)

Those are Hydro Gear HGM 12 drive motors. That means you have a full-blown hydraulic drive system and not transaxles. Look under your seat and follow the hoses up from those drive motors until you see two Sunstrand drive pumps (one for each side). They'll look like the picture below. Notice in the picture, on the lower right corner of the pump, there is a large hex nut with a hole through it, it's to the right of the yellow plug in the hydraulic port. That is the "by-pass" valve. Sometimes there is a little sliding lever through that hole, otherwise you use a 5/8" wrench and back the valve out (rotate counter-clockwise). It only takes a 1/2 turn on to unseat the valve. This will put the pumps/motors into "by-pass" mode for the purging procedure. This is also what you need to do to be able to tow, or push, the machine. Once your done, turn the valve back in until you feel resistance. Don't over tighten the valve, it just needs to seat.


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## [email protected] (Jun 2, 2021)

We found those and opened those and sat on the mower to do the purge procedure and the wheels keep moving. Not sure what to think. What are we doing wrong?


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## Bob Driver (Nov 1, 2017)

[email protected] said:


> We found those and opened those and sat on the mower to do the purge procedure and the wheels keep moving. Not sure what to think. What are we doing wrong?


Back the by-pass valves out a few more turns and make sure your control arms are at neutral- center. The wheels may be turning from residual flow.... Be careful, but see if you can stop them from turning with hand pressure. A wheel turning when it's jacked up is usually an indication the arm RTN (Return to Neutral) is out of adjustment. As a matter of fact, that's how you normally adjust for RTN is adjust both sides to where neither wheel turns with it jacked up

You're making progress.... Hang in there and you'll get it


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