John Deere Hydrostatic Transmission Problems (Resolved)

Issues with hydrostatic transmissions are relatively common. As hydrostatic transmissions use multiple mechanical parts including a drive axle, a differential and hoses, it is necessary to disengage the John Deere hydrostatic transmission before working on it. If the zero mower only turns one way it may be due to gear transaxle problems

Fortunately, you can troubleshoot and resolve the hydrostatic transmission problems at home. Below we troubleshoot the transmission problems that are commonly seen in the following John Deere models: 100 series, D130, D110, Z655, Z425. 

If your John Deere is having hydrostatic transmission problems e.g. the tractor won’t go forward or reverse, then start by purging the system to remove any air. If the problem persists, check the condition of each of the hydraulic components and repair any that are worn. 

Troubleshooting Issues With John Deere Hydrostatic Transmission 

When troubleshooting issues it is important to check your operator’s manual so you have access to information that is specific to the make and model of tractor you are using. We can only offer general advice that will apply to most, but not all, situations. 

How To Purge A Zero Turn Mower Hydraulic Drive System 

Air in the system, also known as cavitation, is a common cause of hydrostatic transmission problems. Purging the system is a good way to sort the problem.  

Step 1. Park the Mower

Before you begin purging the mower it is important to park it on level ground and apply the parking brake. Once safely parked, put the rear onto jackstands.

Step 2. Check the Oil 

Secondly, you need to check the oil reservoir. Ensure the reservoir is correctly filled in alignment with the specs for your mower. The 100 Series models often have issues with low oil levels so check this before continuing. 

Step 3. Disengage the Transmission 

If you have never disengaged the transmission before, check the operator’s manual on how to do this. Here is the operator’s manual for the John Deere 100 Series including the D130 and D110.

Step 4. Start the Engine

Now that the transmission has been disengaged you can start the engine. Once the engine is running, move the throttle control to “slow” and then move the motion control levers to N (neutral) before disengaging the clutch/ brake pedal. 

Step 5. Move the Motion Control Levers

Push the motion control levers forward as far as they go and hold this position for five seconds. Then, pull the same levers back to the full reverse position and hold for a further five seconds. 

Step 6. Repeat

Repeat step 5 three times. This will purge the air from the hydraulic transmission system. 

Step 7. Put Levers in Neutral Position

With the system now purged you can return the control levers to the neutral position. Turn the engine off and make sure the parking brake is on. 

Step 8. Check the Oil Levels

Now check the oil and top it up if needed.

Step 9. Engage the Transmission 

You can now engage the transmission. Once engaged, remove the jackstands, start the engine and disengage the brake.

Step 10. Carefully Roll the Mower Forward Then Backwards

Finally, set the control levers to forwards and roll the mower 5 feet forwards. Then, put the control levers in reverse and slowly move backwards the same distance. Return the lever to neutral and then repeat this step three times. Once complete, the system is free from air and the mower is ready for use.  

Old Hydraulic Fluid 

Hydraulic oil can be old or overused, in which case it is time to change the oil. It is worth considering whether this is the cause of the sluggish operation and transmission issues you are experiencing. 

Hydraulic fluid doesn’t go bad as such but it can become contaminated and overused. It will degrade over time, largely due to exposure to oxygen, contaminants and excessive temperatures. To add to this, John Deere says that premature hydrostatic transmission failure can be the result of operating outside of the recommended oil air temperature range. 

How To Change Hydraulic Oil 

Changing hydraulic oil on a John Deere tractor is relatively straightforward and is similar to the method of changing engine oil. We recommend you check the hydraulic fluid levels every 50 hours of use and top it up as necessary. As a rough guide, the hydraulic oil needs changing every 1200 hours. 

Step 1. Start the Engine

Firstly, it is necessary to start the engine and move the tractor around to warm the engine. This step gets the fluid and any settled contaminants moving to make it quicker and easier to flush the system. 

Step 2. Position Buckets Under The Oil Plug

Secondly, position large buckets under the rear hitch where the oil fill plug is located. The size of the buckets needed will depend on the size of the tractor. The buckets are necessary to catch the hydraulic oil

Step 3. Remove the Plug

With the buckets in position ready to catch the oil, the drain plug on the bottom of the transmission can now be removed. The hydraulic fluid will drain from the system. The amount that drains will be around the same amount that needs to be replaced.

Step 4. Clean the Transmission Sump Screen 

Remove and clean the transmission sump screen.

Step 5. Replace the Filter

Replace the old hydraulic filter with a new one. Once the filter is in position, put the drain plug securely back. 

Step 6. Refill the Oil 

You can now refill the hydraulic oil. You will need to replace around the same amount as you removed.  Before you add the hydraulic fluid, make sure you are using the right type for your mower/ tractor. SAE 20W-50 is the hydraulic fluid recommended by most manufacturers. Don’t use Type F transmission fluid and bio-based oils as these are advised against by John Deere themselves. 

Step 7. Start the Engine

Start the engine and idle for around 30 seconds. As modern hydraulic systems are self-bleeding the tractor will now be ready for use. Turn the steering wheel both ways to ensure the power steering is functioning as normal. 

Fluid Disposal 

When you replace the hydraulic fluid in your tractor you will be left with the old fluid. When you dispose of hydraulic fluid it is essential to do so carefully and correctly. Firstly, be sure to collect all of the old hydraulic fluid and store it in a labelled, leak-proof, water-proof container. There are strict regulations relating to hydraulic fluid disposal so it’s important to store it correctly in a secure location and recycle the fluid at home. 

System Failure

If you have purged the system but you are still experiencing issues, it’s necessary to take a look for what is causing the system failure. 

Start with a visual check of the John Deere hydrostatic transmission parts: 

  • Check the oil level and top it up if needed.
  • Inspect the hoses and connections are ensure they are free from damage, leaks, or other wear and tear.
  • Check the cooling fins are free from damage and clean them with a cloth or compressed air.
  • Make sure the parts are compatible – if you have replaced parts make sure they are designed for use with your tractor model.

If any of the parts are damaged or need to be repaired, check if they are covered under the John Deere Warranty Protection Plans

Cooler Hose Leaking

A broken or damaged cooler hose could be the cause of the hydrostatic transmission problem. If the oil has been replaced and the problems are continuing, check whether the transmission cooler hose leaking is the cause.

Signs of a cooler line leak include:

  • Leaking coolant 
  • Low coolant levels
  • Overheating engine
  • Broken or collapsed cooler hose

Repair HST Cooling Hose

Step 1. Examine the Hoses

Firstly, examine the cooling hoses to check for damage and leaks. Make sure the engine is cool as these hoses can get hot after use. The hoses should be firm but flexible and free from leaks, cracks or swelling. 

Step 2. Locate the Cause of the Leak 

Secondly, locate the damaged hose that is leaking. If the issue is small then you may be able to temporarily repair it. However, if the hose is badly damaged then replacement is necessary to prevent damage to the tractor. 

Step 3. Fix the Damaged Hose

A quick way to temporarily fix a cracked hose is to apply insulating tape. Wrap the tape tightly around the area that is leaking. You could also use duct tape if you do not have an insulating tape. The tape will stop the leak while you get the tractor to the workshop for repairs. 

Fuel Pumps

Another potential issue could be something with the fuel pumps. Pump related problems are usually characterized by: 

  • An increase in noise level
  • An increase in heat
  • Difficulty or inability to develop full output
  • A decrease in the cylinder or hydraulic motor speed
  • Erratic operation of the cylinders
  • System failure

John Deere Hydrostatic Still Not Engaging?

If you have worked through each of the issues we have discussed and the hydrostatic transmission will not engage then it is necessary to troubleshoot the system. Follow these steps to see if you can find the cause of the problem:

  1. Inspect all internal parts. Look for signs of damage and wear and tear. Any parts that are not in good condition should be taken out and replaced.
  2. Check the settings are all as they should be and make adjustments where necessary.
  3. Check the viscosity of the engine oil. Make sure it is proper for the temperature. 
  4. Check the oil reservoir and fill or change the oil as necessary. 

Hydraulic Fuel Leak 

If the lines are leaking oil then it can cause jerky or sluggish movements, oil patches on the ground and a slight burning smell. Check the gasket seals, oil lines and gaskets to identify which part is responsible for the leak. Then repair or replace the part. 

Are Hydraulic Transmissions Reliable?

Yes, hydraulic transmission is considered reliable. It is also considered to be efficient, economical and compact. The automatic transmission, gear-free driving and instant forward and reverse are some of the key advantages to this type of transmission. A tractor with a hydraulic transmission should last at least 3000-5000 hours but regular servicing is a must.

Although there are a lot of advantages to this transmission type, there are some disadvantages too:

Overly Responsive 

Driving a vehicle with hydraulic transmission needs to be approached with care. This is because light pressure on the acceleration pedal can cause the vehicle to move forwards uncontrollably. 


The system can be difficult to check for faults and is prone to experiencing leaks. Hydraulic transmission systems have a lower efficiency compared to sliding gear transmissions and also have a reduced lifespan as less efficiency means higher stress on the engine, higher maintenance costs and more general wear and tear. 


Hydraulic transmissions require a lot of oil to work properly. The engine needs to be well lubricated and frequent oil changes are necessary. This increases the overall running cost of these systems. 

How To Make A John Deere Transmission System Last Longer

Regular maintenance is essential if you want your transmission system to stand the test of time. Remember that wear and tear is standard but handling any issues as they arise will help the system last longer. 

Keep the System Lubricated

It is essential to keep the system properly lubricated as this keeps the fast moving components of the transmission in good condition. The lubrication offsets both the heat and the friction of the moving parts. 

Regular Servicing

Hydraulic transmission systems need to be regularly serviced and maintained to keep them running well. The systems are prone to general wear and tear as well as leaks so regular servicing will help pick up on issues quickly. 

Final Thoughts

When you repair lawn mower transmission problems it is best to start by purging the system as air getting in is one of the most common causes. If problems continue the issue could be caused by old hydraulic fluid, system failures or leaks. Once you have dealt with this problem once you will find the maintenance of the hydraulic system much more manageable.