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Find and Solve Hydrostatic Transmission Problems in the Cub Cadet

If you have an older Cub Cadet mower, you may develop a Hydrostatic Transmission issue sooner or later. With exposure to air and contaminants, this zero-turn mower transmission is contaminated.

It may cause harmful effects as well as poor performance. Cub Cadet lawn mower hydrostatic transmission problems are common in Cub Cadet lawn mowers that have a hydrostatic transmission. 

You may experience contamination of air, fluid contamination issues, the hydraulic system getting too hot, cold hydraulic system problems, and system failure. 

Cub Cadet lawn mower hydrostatic transmission problems are not easily diagnosed, but the following information should help you determine if you are having transmission troubles.

Cub Cadet Lawn Mower
Cub Cadet Lawn Mower

Problems with the Hydrostatic Transmission in Cub Cadet

The hydrostatic transmission is the system that allows you to shift gears or move the mower forward or reverse. It’s like a regular transmission on your car. If you have issues with your hydrostatic transmission, you’ll need to troubleshoot the problem at hand. 

The most common issues with this type of transmission are contamination of air, fluid contamination issues, hydraulic system gets too hot and cold hydraulic system problems

It can also result in the fluid pressure loss and temperature rise of the hydraulic system. The hydraulic system of a tractor will get too hot when its fluid does not flow at a high-velocity rate.

Air contamination

Air contamination occurs with a problematic Hydrostatic Transmission in Cub Cadet. 80% of hydraulic failure is caused by air contamination. When air gets way into the hydraulic system of the mower by any fault or crack, as a result the hydraulic fluids get contaminated. 

Air contamination will take place in two ways: aeration and cavitation. Both aeration and cavitation will result in serious damage if it’s not taken care of. The solution is to purge your hydrostatic transmission.

Here are steps to pure the hydrostatic transmission:

If you don’t check for leaks, you could have major problems later on down the road. With this in mind, here are steps to take when it comes to purging the hydrostatic transmission on a Cub Cadet lawn mower:

Park The Machine On A Level Ground

Set the equipment on a flat surface. There should be enough oil in the system so that there is no need to add more oil until this point is reached. So make sure there is enough oil in the tank before beginning to work on this portion of your machine.

Disassemble The Transmission

Disengage the system by pulling out the key from the ignition. Remove any fuel from your tank and set it aside so that it doesn’t spill or run out onto your ground. Allow all of your components to cool down. 

Don’t attempt to remove them while they are hot because you might burn yourself and hurt yourself and others working with you. When everything has cooled down, remove them from your machine and place them on a table or other flat surface where they can be easily accessed.

Get Rid Of The Air

Get rid of air from the pressure line by pushing forward on each leveler until it is fully extended. Push back on each lever slowly to make sure no air remains in the line or hose. 

Now, push forward on each lever until they lock into place and hold it for one minute. After holding them in place for one minute, push back on each lever slowly to ensure that all of the air is removed from them. 

If any air remains in them, repeat until all of the air has been removed.

Fluid Contamination

Fluid contamination occurs when fluids are mixed together to form one substance. This happens when there is a leak in the hydraulic system. The contaminants are usually water and oil, but acid and other substances can also contaminate the system if present in the fluid.

Whether it is air contamination, impurities or a compromised hydraulic fluid, the result is the same: a system that cannot perform to the manufacturer’s design specification. 

The causes for fluid contamination are many, but they all lead to one outcome: poor system performance and sometimes even cold running issues or failure.

There are several signs you can look for to determine if you have an issue with contaminated transmission fluid:

  1. Poor shifting
  2. Slipping in forward or reverse
  3. Vibrations or noise in the transmission area
  4. Issues with rise of temperature in the system

If a Cub Cadet lawn mower is having problems with steering or braking, this can also be caused by contaminated fluid. 

From small accumulations of oil and dirt, to larger scale leaks from seals or other moving parts in the system, contaminants can affect nearly every part of a vehicle’s mechanical systems. 

Oil Leaking from Breather Tube
Oil Leaking from Breather Tube

The effects can range from reduced performance to dangerous failures and serious accidents. Ensuring that all systems throughout your vehicle are free of contaminants will ensure the best performance and longest life possible.

Hot Hydraulic System

A hot hydraulic system occurs when the fluid in the system gets too hot. While hydraulic systems are designed to operate at a certain level and temperature, a hot hydraulic system will damage the components of the system. 

Hydraulic systems are designed to have heat dissipated from them. Heat build-up requires the fluids in these systems to have extra additives to help keep them at an ideal temperature.

There are several reasons why your hydraulic system could be running too hot. If you are constantly working on heavy equipment that requires major use of your hydraulics, you will notice a higher rate of temperature increase. 

The most common cause of this is having your pump and tank not insulated correctly. This can be done by either having no insulation or having inadequate insulation. 

Insulation is important for keeping the heat away from the fluid inside the tank and pump. If you think your tank or pump may be too hot, you can place an infrared thermometer on it to see if it’s reading too high of a temperature. 

Generally, any temperature above 180 degrees Fahrenheit is considered too high for a hydraulic system.

Cold Hydraulic Fluids

Cold hydraulic system causes a problematic hydrostatic transmission in cub cadets. The cold hydraulic system not only makes the mower unable to drive smoothly, but also threatens the safety of the driver and the passengers. 

If you have such a problem, you should know how to solve it. The fluid becomes thick as it gets cold and can’t move through it’s lines well. This is especially noticeable when you go to turn the mower on for the first time and you have a hard time getting it to start. 

You may also notice that it doesn’t seem to be shifting correctly. As the fluid continues to get colder, your hydrostatic transmission is unable to dissipate heat and damage will occur. 

The best way to avoid this problem is to keep your mower in a garage or shed where temperatures are warmer, but if that isn’t possible there are a couple of other options for you. 

Another option is to pick up some oil additives that are designed for this type of situation. These additives will help thin out the oil so it flows better in low temperatures.


This article has covered the more serious symptoms of transmission failure in the Cub Cadet and the solutions to the Hydrostatic Transmission Problem. Unfortunately, hot fluid contamination, fluid contamination, and air contamination will damage your transmission beyond repair. 

Sometimes, the only solution to these problems is completely replacing your transmission. As we have found, the most common cause of hydrostatic transmission problems is air contamination. 

In this case, numerous symptoms might be pointing to a clogged filter or a restricted valve. Both of these can easily be remedied by changing the oil and/or servicing the transmission. 

Should changing the oil not solve the problem, it is highly recommended to remove and replace the transmission. When troubleshooting this issue, it is important to thoroughly read through the owner’s manual procedures before conducting any repairs.

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