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One of the common leak point of the model 15U Sundstrand hydraulic pump is the input seal. Below outlines information on how the pump works and one of the many ways you can change out the input seal.

On the 300 and 400 series Deere Garden Tractors the drive shaft from the engine goes to what is commonly referred to as the transmission. The transmission is then coupled to the rear differential.

The hydraulic flow originates from the charge pump located right at the input shaft. The charge pump is powered by the turning of the driveshaft. The charge pump itself can actually be held in the palm of your hand. The bulk of the transmission is for the variable displacement piston pump that is controlled by the hand lever on the dash. This pump is responsible for direction of movement for your tractor. The variable displacement pump is fed by the charge pump on the front of the transmission.

Let’s look at your problem. This is what a leak from the input seal looks like. This leak should not be confused with a leak from the check valves located on top of the pump. One method to test for leaks is to use baby powered (thanks to Rubadub for this technique).

Courtesy of mdchamb

Once you have established the seal on your charge pump is leaking now is the time to replace it. Personally I would plan for this task to consume most of a Saturday or Sunday (depending on your mechanical skill set).

Your first order of business is to decide how you want to replace your seal. To replace the seal the driveshaft MUST be removed from the pump. This can be accomplished one of three ways.
1) Remove the driveshaft from the engine, at which point you will have access to the pump. This method will generally require removing the engine or at minimum moving the engine forward.
2) Remove the rear end from the tractor
3) Remove the engine with the driveshaft

The model you have will most likely determine your plan of action. Onan powered machines (316, 318, and 420) will most likely qualify for removal of the engine over removal of the rear end. Yanmar powered machines will almost certainly qualify for driveshaft removal or rear end removal.

For the purpose of this post the machine was a 318. To begin with we removed the entire engine. This was done to provide the most access to the working area and to clean the frame.

Steps involved in preparation for replacing the charge pump input seal in a 318:
1) Remove the fender pan
2) Remove the side panels and hood
3) Remove the engine (for safety of fingers it would be best to completely remove the engine over sliding it forward, your fingers will thank you later!)

Once you have decided HOW you want to approach your seal replacement get on and order your parts. At minimum you will need #’s 18 and 14 from the parts breakdown below. M85508 and M45375. Real seals may need to be replaced as well. Order parts according to your situation. NOTE: These parts are only for the transmission in the later 300 series tractors, not the earlier closed frame tractors.

Once you have your parts and you have access to your transmission the fun can begin. Take plenty of pictures along the way to help put everything back together. It may be easier to remove the two lines on top of the transmission to access the charge pump vs. leaving the lines in place.

Once you feel that you have enough area to work in you will need a 5/8 wrench to remove the cap bolts on the charge pump. Remove the charge pump and carefully take the gerator off the shaft paying close attention to not lose the pin.

This video will show the removal of the charge pump on a transmission from a 317 as well as some of the steps to replace the seal.

Once the charge pump cap bolts are loose the charge pump “should” come of the transmission with minimal effort. Pay attention to the direction of the charge pump, one side will be flat compared to round. If the pump is reinstalled in the wrong direction your pump will not work.

Dig the old seal out using a screw driver, be careful not to damage the bearing inside the charge pump. We used a long bolt and washers to install the new seal square with the opening. Follow the directions outlined in TM1590 should you have any concerns about the seal placement.

Once the seal has been replaced, you can prepare for reassembly. Install the pin on the shaft, place the o-ring in the groove and put the charge pump on the shaft. Lining up the gerator and the pump housing may take a little work. Then put the cap screws on and torque to 52 ft lbs.

Then reinstall the driveshaft and put the tractor back together. Crank it up and test out your handy work!

Good Luck!!

Other helpful threads:

Relevant TM1590 Pages:



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