I have a 1996 721D with the 21 horse Kubota diesel. I bought it in 2000 and it had 1600hrs on it. The engine runs great and the machine is still in good shape but the drives were weak. I asked on this site and people in general about rebuilding the gemini transmissions and no one seemed to know how to do it , I was mostly told that I could buy new ones for about 650.00 a side. I was about to do that when I decided to take one apart and see if I could find anything wrong with it. With assistance of 4-H Equipment in Denham Springs, La. I was able to replace some O-rings and back up rings on some pistons and replace the check valve assembly that is attached to the dump valves. It made a big difference in the performance of the machine and I only spent about 150.00 in parts. I only share this cause I was lead to believe that the only fix for the Gemini was to replace the drives. Hope this helps some one else. Later, 22R
Grasshopper owners with weak Hydro units read this !!
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22R
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I have a 1996 721D with the 21 horse Kubota diesel. I bought it in 2000 and it had 1600hrs on it. The engine runs great and the machine is still in good shape but the drives were weak. I asked on this site and people in general about rebuilding the gemini transmissions and no one seemed to know how to do it , I was mostly told that I could buy new ones for about 650.00 a side. I was about to do that when I decided to take one apart and see if I could find anything wrong with it. With assistance of 4-H Equipment in Denham Springs, La. I was able to replace some O-rings and back up rings on some pistons and replace the check valve assembly that is attached to the dump valves. It made a big difference in the performance of the machine and I only spent about 150.00 in parts. I only share this cause I was lead to believe that the only fix for the Gemini was to replace the drives. Hope this helps some one else. Later, 22R
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4,477 Posts
Thats great that you did that. I've never heard of anyone taking a GH hydro apart. When I talk to my local dealer, they act like going into a hydro is taboo. I've got a 1997 & 1999 GH. They both have close to 1500 hours. The 97' had the left hydro replaced last year at the tune of $800. GH makes work manuals for their machines. I bought 2 on eBay. Good luck.
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224 Posts
My dealer never ever considers working on the motors. It's easier for them to replace them. If any of you decide to tear a motor apart, contact the tech guys at Grasshopper. They will go through step by step to help you. They have saved me lots of time and headaches.
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33 Posts
I've never heard of these gemini hydro units everyone is talking about here. Every Grasshopper I've ever seen has hydros made by Eaton just like Snappers do (both machines were designed by the same people BTW). Is this some new unit they're using now or what?
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33 Posts
Ok, now I understand. I knew they were made by Eaton but never heard the Gemini part.
I can add one thing that will save someone from throwing one of these units away. If you disassemble one of these units, you will find some big ball bearings. DO NOT SWAP THEM AROUND IN THE HOLES! These balls are measured with some type of laser micrometer (can't remember the name of it right now). When you see the measurements written down, there are about 10-12 digits to the right of the decimal point. The balls are fitted in their respective holes to exacting tolerances. If you mix the balls up, when the unit comes up to operating temp the balls will start to stick in their holes causing a loss of pressure and driving problems. Without the machine the factory uses to measure the balls and holes, you would almost never get the balls back in the right hole. I did this very thing a few years ago (mixed up the balls) and then found out you can't do that. Had to put in a box to use it for parts later maybe. To buy a new set of rotors with balls already in the right holes would cost almost as much as the whole hydro unit would. I guess sometimes it doesn't pay to play when you don't know what your playing with.
I can add one thing that will save someone from throwing one of these units away. If you disassemble one of these units, you will find some big ball bearings. DO NOT SWAP THEM AROUND IN THE HOLES! These balls are measured with some type of laser micrometer (can't remember the name of it right now). When you see the measurements written down, there are about 10-12 digits to the right of the decimal point. The balls are fitted in their respective holes to exacting tolerances. If you mix the balls up, when the unit comes up to operating temp the balls will start to stick in their holes causing a loss of pressure and driving problems. Without the machine the factory uses to measure the balls and holes, you would almost never get the balls back in the right hole. I did this very thing a few years ago (mixed up the balls) and then found out you can't do that. Had to put in a box to use it for parts later maybe. To buy a new set of rotors with balls already in the right holes would cost almost as much as the whole hydro unit would. I guess sometimes it doesn't pay to play when you don't know what your playing with.
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30 Posts
IS there a seal or o-ring on the output side of the shaft. My left hand wheel has fluid leaking in it. Can it be taken apart aND REPLACED?
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I have a late 90's Woods Mow'n Machine 6225 with the same Eaton Gemini hydro pump/motor...as you said the pumps can be replaced for $650 per side, but some people dont have that kind of $$$ at this time of the yr. I am going to fix the thing myself....thats right, i said fix (not attemp to fix). there isnt much that i have not experienced in fixing. It seems to me that the ones with bunches of money...doesnt have the brains...or maybe find it much easier the just "buy new". Im not knocking on the ones who have money to do it that way more power to them, i just believe in doing the old fashined way bustin knuckles and gettin greasy!!! There has got to be a way of marking the steel balls when you disassemble it.....use different colored paint pens or something. Just my two cents worth. 
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112 Posts
The reason for the resistance to repair a GH hydro is because of the tight tolerances. How many numbers to the right of the decimal can you measure?
As long as there are no contaminants in the oil a GH will last almost forever. If you start to have trouble chances are that contaminants have pumped all throughout the trans and this has been happening long enough to "wet sand" ALL the moving parts out of tolerance. Yes there is an oil filter, no it doesnt catch everything. So every part in the trans needs replaced. This can be done but at what cost? I'm not saying dont try but the chances of repairing one economicaly are slim.
As long as there are no contaminants in the oil a GH will last almost forever. If you start to have trouble chances are that contaminants have pumped all throughout the trans and this has been happening long enough to "wet sand" ALL the moving parts out of tolerance. Yes there is an oil filter, no it doesnt catch everything. So every part in the trans needs replaced. This can be done but at what cost? I'm not saying dont try but the chances of repairing one economicaly are slim.
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168 Posts
I'm having the same problem. Still goes back, very weak forward. From various readings there is a check valve that may not seat properly. Thus when you are going forward, the fluid leaks through the open check valve instead of turning your motor. This is a cheap repair. Takes a special wrench to tighten them. And only to 10 ft lbs.
I'm hoping it's that.
Failing that I'm looking at a new (used) hydrostat on that side. 2-3 grand. Not made anymore. Surprised on one makes aftermarket adapter kits to mount newer units in older mowers.
I'm hoping it's that.
Failing that I'm looking at a new (used) hydrostat on that side. 2-3 grand. Not made anymore. Surprised on one makes aftermarket adapter kits to mount newer units in older mowers.
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168 Posts
I think the 771. Why? Grasshopper 721D 1995
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14 Posts
Here is a link to the illustrated parts manual
http://www.eaton.com/ecm/groups/public/@pub/@eaton/@hyd/documents/content/pll_1623.pdf
Here is a link to the repair manual
http://www.eaton.com/ecm/groups/public/@pub/@eaton/@hyd/documents/content/pll_1620.pdf
If you need parts, go here
http://www.the-mower-shop-inc.com/transmissions/transmissions-770_771_series.aspx
http://www.eaton.com/ecm/groups/public/@pub/@eaton/@hyd/documents/content/pll_1623.pdf
Here is a link to the repair manual
http://www.eaton.com/ecm/groups/public/@pub/@eaton/@hyd/documents/content/pll_1620.pdf
If you need parts, go here
http://www.the-mower-shop-inc.com/transmissions/transmissions-770_771_series.aspx
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Was you able to find a repair kit or seal parts etc?
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168 Posts
Somewhere else i read that not even Eaton will warrant a rebuilt gemini drive unit. This comes up often enough on this and other forums, that I'm surprised that no one has an after market kit to replace eaton drives with something a little less fiddly. Why would not an off the shelf hydraulic motor work for this. You then need at worst a custom spool that gives you proportional control.
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