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How Do You Fill a Hydrostatic Transmission on a Lawn Mower?

Hydrostatic transmissions are commonly found on lawn mowers, tractors and on other machines and vehicles. The purpose is because they allow you to mow through a wide range of speeds, which is useful for different types of terrain and cutting heights. 

This type of transmission works by using a hydraulic pump with a hydraulic motor to increase or decrease the pressure in the system, which changes how quickly the wheels turn and where that power is transmitted to within the machine. 

It’s recommended to keep this fluid fresh and clean so it can create a tight seal with the other parts in your transmission system.

Hydrostatic lawn mower on a lawn
Hydrostatic lawn mower

Steps to fill a hydrostatic transmission on a lawn mower

You can follow these steps to efficiently fill a hydrostatic transmission on a lawn mower.

Allow the engine and transmission to fully cool

You should allow the engine and transmission to fully cool before filling the transmission with oil. The engine and transmission of your lawn mower should be allowed to cool down before you fill it with oil. 

A lawn mower in a garage
Allow the lawn mower to cool down

This is because the oil will expand and make it difficult to get out of the bottom of the engine or out of your lawn mower’s transmission if it is too hot.

Gather old transmission oil

To fill a hydrostatic transmission on a lawn mower, the old transmission oil needs to be gathered to prevent spillage. The old oil can be collected by placing a container under the drain plug and loosening the plug.

Remove dirt and debris

To get the dirt and debris out of the hydrostatic transmission, you need to remove the filter element that is usually bolted to the top of it. If necessary, you will need to replace or clean your air filter. 

Dirty transmission of a lawn mower
Remove all dirt from the transmission

This happens more often on older mowers that have been run for many hours or if they have been sitting for an extended period without being used. 

It’s also important to keep your spark plug clean so that it provides enough power for your engine to run efficiently.

Allow air to enter the oiling system

You should lose the hydrostatic transmission oil reservoir cap to allow air to enter the oiling system. Pour in the required amount of transmission fluid into the reservoir. Tighten the cap back down.

If it is too tight, air can’t enter, and the system will not work properly.

Drain out all transmission oil 

Unscrewing the drain plugs from the bottom of your mower hydrostatic transmission should allow all its transmission oil to flow out onto a suitable surface.

Maintain the transmission oil properly

Next, it is essential for the transmission oil to be maintained properly for it to run smoothly. The hydrostatic transmission has a drain pan at the bottom of the filter and when it gets too full, it will need to be changed. 

To unscrew the transmission oil filter from the transmission oil filter housing, you should insert a wrench into the aperture of the cover and turn it counterclockwise. Please note that you must screw at a right angle or else it will break.

Apply a little clean transmission oil

Put a drop of clean transmission oil onto the gasket and thread the new oil filter onto it to replace the new transmission seal. The drain pan can then be moved to one side and underneath the transmission.

Fill reservoirs with new transmission oil

Next, fill the hydrostatic transmission oil filter reservoirs with new transmission oil. Next, pour the transmission oil into the reservoirs and check that it has risen to the level of the filler neck. 

It’s important that you don’t overfill it because this could cause fluid leaks and damage the transmission. You can fill the oil reservoir to the full line that is embossed on the side of the reservoir tank. 

You may need to check your owner’s manual for specifics, but usually you can simply fill up your tank until oil comes out of the hole on top.

Close the reservoir 

Screw the transmission oil reservoir caps and start the lawn mower engine. Check for transmission oil leaks before adding any oil to the reservoir or to the engine crankcase, which can lead to costly repairs. 

Drive the lawn mower in your lawn forward and backwards for a while to check for good transmission operation. 

Start the mower and test

Park the lawn mower on a level, flat surface. Shut off the lawn mower engine and check the level of the hydrostatic transmission oil. Check if the riding mower is running slow or normal. If it needs more transmission oil, finish adding it up to the ‘full’ line on the transmission oil reservoir.

A person is riding on a riding mower
Start the mower and test


Hope this guide has showed you to replace the transmission on a lawn mower. Now you have clear idea about filling a hydrostatic transmission on a lawn mower. Still, you have trouble, you should get help from professionals.

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