John Deere LA-120 Hydrostatic Transmission Repair Mod #1

The photo's below 1 thru 6 first set are the LA-120 John Deere riding mower, a homeowner model. The transmissions are hydrostatic and manufactured by  Tough Torque, many other homeowners riding mowers have the same transmission that also fail within a time of 240 engine hours to 600 hours. All failures have been from the same source and can be detected early. By installing an oil filter kit and repairing the damaged transmission we have testimonials of local individuals that have gotten 700 hours more from the modified transmission. This is the third year of modification experimenting with these transmissions and the past year they have been performing very well.


The transmission is a tough torque 46 and is used in many John Deere mowers as well other manufacturers use them in their products.

The work completed in the above pictures took 40 minutes. This was a casual pace and the tools needed are shown. The work started and includes positioning the tractor onto stands to remove the transmission. Also the axle shafts had rusted the wheel to the axle shafts and they had to be hammered off with the large hammer shown. There are two rods, a pin on the topside of the transmission, two frame side straps, and a long bolt in the front right of the transmission that holds a bracket that were also removed with the tools shown.

The top case needs to be cleaned and the surfaces that seal the transmission case together need to be cleaned also. Use hot water and powder Tide laundry detergent to clean the case and gears and use a single edge razor blade to clean the sealing surfaces. This is the secondary cleaning, primary cleaning was done with a knife and screw driver to get all the oil and dirt without using a liquid cleaner. This secondary clean is to make the cases ready for the modifications for the Filter Kit. The valve block shift control in the #3 picture and the rotating piston blocks have been machined for assembly. I cannot stress the cleaning enough, this needs to be as clean as possible and free of any oily surfaces.


This portion of the work to clean the case top, bottom cover pan, and the parts inside the transmission was one hour. The valve block and the the rotating members cost 100 dollars to machine or one and a half hours at Hawk Eye Machine Workshop and 25 dollars in grinding materials. This is necessary due to it is the reason the transmission quit operating but it is not the cause of the problem. Clean-Clean-Clean and then re clean as well as the bolt holes and threads inside the top case. There are two tiny steel balls and two springs (straightwound) in the LA-120 valve block that controls the direction of the hydrostatic pump fluid flow. There is also a very small pin inside the valve block that pushes the hydrostatic drive motor rotating member off the seat surface of the aluminum direction control valve block so you can easily push the tractor when the engine is off.

Picture 1 above shows the aluminum block with holes, this block provides the passages and surfaces that cause the hydraulic fluid force from the pump to the motor that drives the rear wheels of the tractor. The white pvc tube is the addition to the system that directs inlet oil flow into the hydrostatic oil pump inlet. Picture #2 shows the existing screen system in the stock transmission.


Picture #3 is of the top cover, the rubber plug is replaced with a dorman freeze plug and the right hole where the one way breather was leaking oil is replaced with a brass elbow and not the straight tube fitting as seen in this picture. Picture #4 shows the placement of the brass elbow and the freeze plug.


Picture #5 shows the bottom oil pan cover with the added inlet fitting and picture #6 is the outside of that pan.

The above pictures are to example the assembly process as well as a result of overheating of the hydrostatic pump oil. The hydrostatic oil valve and rotating members are not installed in this set of pictures due to this assembly is to example the order of assembly and the components associated with the assembly.


Picture #1 is to show the hollow compartment in the lower right of the transmission case, this is the oil pump/motor/block assembly compartment and is surrounded by 7 case tapped holes. These holes are for the sealing of the transmission oil pan to the surface of the transmission top cover. The 9 parts shown install in this order, 1- the two rectangular shaped strips of metal with a hole centrally located in each piece. These are wear surfaces called "Plane Bearings" and locate onto the two pin bosses in the top case. 2- is the metal piece directly to the right of the two bearing plates and pivots on the two bearings installed in the case, this is called the pump "Swash Plate". With the case in this position the shaft is 3- slid upward into the case from underneath into position as well as 4- the ring clip "Snap Ring" that prevents the pump shaft bearing from sliding back out of the bore. The three rings on the right side of the parts layout install from bottom ring  5- with larger inside diameter first in the swash plate bore, 6- the ball bearing ring, 7 the ring with the smaller inside diameter hole. The seal below the Timken seal box does not install at this time.


Picture #2 examples the parts installed first assembly, (Swash Plate/Pump Shaft). Picture #3 shows wear on the surfaces of the axle drive gears. The larger diameter thinner gear is the final drive gear and is driven by the thicker smaller diameter drive gear. Both gears have wear on the surfaces that is the cause of the hydrostatic pump failure. These gears are installed opposite of their removal. The book states in fact to install the gear in a specific way. We do it backwards and modify a spacer, this allows the reverse direction gear faces to mesh when in the forward direction after modification. Reverse surfaces are usually not damaged and it saves 80 dollars for new gears.


Picture #4 shows gears and assembly of the final drive parts.

In the pictures 1 to 3 above they are simply what you will end up with after the transmission gears, pump and motor assembly are installed. The most critical factor for this entire upgrade is clean the inside parts and case with hot water and powder Tide, wash it down until you could eat out of the case. These systems do not tolerate any dirt or debris getting into the pump suction, one small particle will instantly wipe out the pump and no more drive.


There will be a detailed parts list provided in the X-300 transmission upgrade page of this site as well as differences in the two transmissions. More details and final costs in labor and raw materials will also be available on the X-300 Transmission page.