John Deere Hydrostatic Transmission Will Not Engage [Fixes!]

If the hydraulic fluid is overused or the level is low, the John Deere hydrostatic transmission will not engage. Cavitation or air in the hydraulic line can also prevent the transmission from engaging. Sometimes, broken hydraulic hoses prevent the pressure build-up, so the transmission won’t engage. Bad drive belts or faulty hydraulic pumps can also cause the problem.

To solve this problem, you need to do the following things.

  1. Check the hydraulic fluid level and condition and refill it.
  2. Purge the system to eliminate air from the system.
  3. Check hydraulic hoses for signs of cracks and replace them.
  4. Check the drive belts for cracks and replace them.

Knowing the exact reason behind the problem and the right solutions is crucial. Let me tell you every possible solution to the hydrostatic transmission problem in your John Deere lawn tractor.

Read: 5 Reasons John Deere Mower Deck Stuck In Up Position

6 reasons why John Deere Hydrostatic Transmission Will Not Engage

Check this quick-fix chart to find the solution right away.

Common ReasonsEasy Fixes
1.Hydraulic fluid is old, or the level is lowCheck the level and condition of the hydraulic fluid. Drain the old fluid and refill the reservoir.
2.Cavitation or air in the systemPurge the system to remove the airlock.
3.Hydraulic hoses or seals are crackedReplace the cracked hoses or seals.
4.The drive belt is crackedReplace the cracked drive belt.
5.The hydraulic pump is faultyGet the hydraulic pump replaced.
6.Clogged oil filtersClean or replace the oil filters.

Hydrostatic transmissions have a simpler working principle than manual transmissions in vehicles. So, the reasons behind transmission problems are also limited. Take a closer look at the reasons and their solutions.

1. Low or Overused Hydraulic Fluid

Having enough good-quality hydraulic fluid is a must for hydrostatic transmissions. Check how the fluid level or quality affects the transmission.

  • Low Hydraulic Fluid: If the fluid level is low, the transmission won’t get the necessary power from the pump due to low pressure. As a result, the transmission will not engage. As the tractor ages, fluid lines will develop cracks or leaks. Due to this, the fluid level can drop drastically.
  • Expired Hydraulic Fluid: When the hydraulic oil has been used for too long, it gets dirty and too viscous. So, it becomes difficult for the pump to transfer the oil to the motors. So, you will face transmission problems.

How To Fix?

  1. You need to check the fluid level and condition.
  2. The fluid reservoir is under the seat. Remove the seat and check the level to see if it is visible from the outside.
  3. Remove the reservoir cap. See if the oil is too dirty or milky.
  4. If the level is close to the bottom or the oil is too dirty, drain the oil using the drain plug under the reservoir. Then, refill it with new hydraulic oil.
  5. Also, check the transmission fluid lines for cracks or leaks. If the fluid lines are faulty, replace the lines. First, drain the fluid completely by removing the drain nut. Then, remove the fluid lines and install new ones. Finally, refill the fluid.

2. Cavitation or Air in the System

If there is a leak in the system, air can intrude into the hydraulic line and reach the pump. Leaks generally occur when the tractor gets older, and the hydraulic lines become brittle due to extreme heat and pressure.

Then, the pump won’t be able to generate enough pressure. This situation is called cavitation. The transmission doesn’t get enough power due to cavitation, so it will not engage.

How To Fix?

  1. First of all, you should check the system for leaks or cracks. Replace all the cracked hydraulic lines before purging.
  2. You need to purge the hydraulic system to eliminate air from it. For this, lift the rear end of the John Deere mower/tractor with a car jack.
  3. Make sure the transmission is disengaged. Then, start the mower.
  4. The throttle should be set to low. Put the direction controller in neutral and release the brake.
  5. Put the direction controller forward and keep it there for five seconds. Then, do the same with the direction controller in reverse.
  6. You may need to repeat the previous step 5-7 times to completely purge the air. Then, put the transmission in neutral and turn the mower off. Now, you can engage the transmission, and it should work properly.


  • After purging, drive the mower forward and backward slowly a few times. It helps an even spread of the fluid.

3. Cracked Hydraulic Seals or Hoses

Hydraulic hoses work under immense pressure. Over time, these hoses can become brittle and develop cracks. Extreme temperature and pressure also cause these hoses to be prone to cracking.

In that case, the oil may spill through the cracks, and the pressure inside the system will drop. So, the hydrostatic transmission will not engage.

How To Fix?

  1. You should visually inspect the seals and hoses for signs of cracks or loose connections.
  2. If the connection is slightly loose, you can tighten it. But if the cracks are too big, you should drain the hydraulic oil first. Then, replace the hose with a new one.


  • After replacing the hose, purge the system to remove any air that may have entered the system.

Read: How To Tell If Hydrostatic Transmission Is Bad? [5 Sign + Fixes!]

4. Cracked Drive Belt

Drive belts ensure the hydraulic pump can get power from the engine. If the belt is cracked or worn, the pump won’t get enough power to generate oil pressure. So, the transmission will not engage.

How To Fix?

  1. Shut the mower/tractor off and apply the brake. Then, get below the frame to find the engine pulley. Remove the deck belt first and then the drive belt. If the belt is under tension, use a spanner or wrench to pull the spring closer.
  2. Next, pull the drive belt out of the pump pulley. Take a new drive belt and attach it to the pump pulley first. Then, attach it to the engine pulley, maintaining the spring tension.
  3. Finally, attach the deck belt and release the brakes. The transmission system should now work fine.


  • Make sure to align the belt grooves and pulley grooves to avoid hassle.

5. Faulty Hydraulic Pump

The hydraulic pump is responsible for pumping the hydraulic fluid to the transmission. If the pump goes bad, the transmission won’t engage. The pump has many different components that can be faulty. Fluid contamination is the biggest reason behind faulty hydraulic pumps.

Debris in the fluid causes unnecessary wear on pump components. Once the components are worn, they should be replaced to make the pump work again.

How To Fix?

  1. This isn’t DIY troubleshooting because the pump has many components that aren’t visible from the outside.
  2. I suggest calling an experienced mechanic to get the pump fixed or replaced. Repairing or replacing the pump may cost $1,000 or more.


  • If solving other problems doesn’t make the transmission right, the problem is probably with the pump.

6. Clogged Oil Filter

Over time, oil filters get clogged due to the debris in the hydraulic oil. As a result, the oil can’t pass efficiently, which causes a pressure drop. So, the transmission doesn’t engage in your John Deere mower.

How To Fix?

  1. Remove the oil filter from the housing and check if it is dirty or clogged.
  2. If the user manual recommends cleaning, you can clean the filter with compressed air.
  3. Otherwise, replace the filter with a new one. It will cost less than $25.


  • Clean the filter carefully so that you don’t damage the filter.

This video can help you to solve the problems:

Expert Advice to Solve John Deere Hydrostatic Transmission Problems

Here is some advice from my experience of working with hydrostatic transmissions.

  • You should always use the recommended hydraulic oil by the manufacturer. Using other hydraulic oils can cause issues with the mower.
  • Never use engine oils or other types of oils. Only use dedicated hydraulic oils.
  • Don’t try to troubleshoot the mower/tractor right after using it. Allow it to cool down properly.

Besides, I provided effective solutions to tackle the problems. If you apply these fixes correctly, the transmission problems should go away, and transmission will engage normally. You should also ensure routine maintenance of your John Deere mower, such as checking oil levels, hose conditions, etc.

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