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

Unlike manual transmissions, hydrostatic transmissions don’t have gears. So, you should not expect any distinct sound from the transmission indicating it is faulty. But there are other symptoms to understand the problem.

So, how to tell if hydrostatic transmission is bad? If the hydrostatic transmission is bad, you will see the following symptoms.

  1. The mower will lose power.
  2. The transmission won’t engage.
  3. The mower won’t move.
  4. The mower will wander to the sides.
  5. You will see leaking hydraulic oil.
  6. Hydraulic oil will be too hot.

These symptoms indicate that something is wrong with the hydrostatic transmission. To fix a bad hydrostatic transmission, you need to check the oil level and refill it as required. Purging the hydraulic system is also important. Depending on the condition of the belts or hoses, you need to replace the cracked ones.

This may sound like losing the trail in a desert, but I will guide you throughout the process. I have worked on hydrostatic transmissions for a long time, so I can tell you the detailed symptoms and their solutions.

Bad Hydrostatic Transmission Symptoms Chart

Check the following chart to know what can potentially go wrong when you see these symptoms.

The mower/tractor will lose power, and you may hear noise from itHydraulic fluid is low or contaminated.
Air has entered the system.
The transmission won’t engage, or the mower won’t moveHydraulic fluid is low or dirty.
Air has entered the system.
The drive belt is cracked.
Hydraulic hoses are leaking.
The mower will wander to the sidesHydraulic oil can’t reach one side.
You will see leaking hydraulic oilHydraulic hoses or seals are cracked.
Hydraulic oil will be too hotHydraulic oil is too low.

As you can see, different symptoms can indicate the same problem. I will focus on detailed symptoms and reasons without repeating anything.

How to Tell If Hydrostatic Transmission Is Bad? 5 sign with fixes

Learn how the above reasons cause the respective symptoms and how to react to these symptoms.

1. The Mower Will Lose Power

This is probably the most common problem with hydrostatic transmissions. It has two main reasons.

  1. If the hydraulic oil level is low, the system will lose pressure. As a result, the mower will feel weaker than usual, and the speed will be reduced.
  2. Air can get trapped inside if there are cracks in the hoses or seals. It is called captivation. Due to the trapped air, the system pressure will drop, and the mower will be slower.

How To Fix?

  1. You can do two things. The first thing is checking the oil level and topping it off. Remove the hydraulic oil reservoir cap and add fresh oil to the reservoir.
  2. If the problem persists, turn the mower off and disengage the transmission after lifting the rear end. Put the speed control lever in the slow position and the direction control lever to neutral.
  3. Then, push the second lever forward and hold it there for a few seconds. Pull it in reverse and hold it there too.
  4. Keep moving the lever forward and backward several times. It will purge the air from the system.

Pro Tip

  • Always choke the mower’s/tractor’s front wheels before troubleshooting.

2. The Transmission Won’t Engage

It can happen for four reasons. I have already discussed low hydraulic fluid and cavitation, so let’s focus on the other two.

  1. The drive belt transfers power from the engine to the transmission. If the belt is worn or cracked, it won’t deliver enough power to the transmission. And the transmission will not engage.
  2. As hydraulic hoses work under extreme pressure, getting loose or cracked is a common incident. Especially if the hoses are at the end of their service life, cracks can happen often. Due to the cracks, hydraulic oil will leak from the transmission.

How To Fix?

  1. Check the drive belt condition. If it is too worn or cracked, you need to replace the belt. Turn the mower off and remove the deck belt first. Then, remove the drive belt from the engine pulley and the pump pulley.
  2. Install the new drive belt. Attach the deck belt after that. The transmission should engage now.
  3. If it doesn’t, check the hydraulic hoses for leaks or cracks. Tighten the hoses if possible. Otherwise, drain the hydraulic oil first by removing the drain nut under the reservoir. Then, replace the hoses and seals.

3. The Mower Will Wander to the Sides

If the hydraulic oil from the pump doesn’t reach both ends of the drive axle, one side will lose power. This can happen if the hydraulic lines are faulty on one side. In that case, the other side will get more power and wander to the side which has low power.

How To Fix?

  1. Checking the hydraulic lines is the only solution. I provided the steps for checking the hydraulic lines in the previous section. Please refer to that solution.


  • If the mower/tractor is under warranty, it is better to contact the dealership.

4. Leaking Hydraulic Oil

The reason is obvious here. When the hydraulic seals or hoses crack or wear, the hydraulic oil will leak. If the cracks are large, you may see larger spills. Otherwise, the cracked parts may have light oil marks.

How To Fix?

  1. Check if the hose connections are loose. If so, you can try to tighten the hoses.
  2. If there are cracks in the hose, you should replace the bad hose.


  • Always drain hydraulic oil before replacing the hoses.

5. Hydraulic Oil Is Too Hot

When the hydraulic oil level drops too much, the system components face increased friction. As a result, the components become much hotter. At the same time, the oil also becomes hot.

You will notice that the hoses are very hot to the touch, and this is a symptom that the transmission can’t work properly.

How To Fix?

  1. Check the hydraulic oil level in the reservoir. The reservoir is under the seat. It has a transparent wall so that you can see the oil level from the outside. The oil level should be close to the top.
  2. Refill the reservoir with the right type of hydraulic oil. You should use 20W-50 or 15W-50, depending on what the manufacturer suggests.


  • If the remaining oil is too old or dirty, you need to drain it before refilling the oil.

Checking this video can help you:

Note to Keep in Mind to Identify Bad Hydrostatic Transmissions Quickly

Besides the above symptoms, keep the following things in mind to determine if your hydrostatic transmission is bad.

  • If there are loud noises from metal parts, the problem isn’t probably with the transmission system.
  • Any unusual motion of the mower can be caused by a bad hydrostatic transmission.
  • Hissing noise from the mower can indicate airlocks, hence a bad transmission.
  • Dripping hydraulic oil below the mower can also mean something is wrong with the system.


I listed the symptoms and their reasons to familiarize you with the situation. If you notice these symptoms, perform the necessary checks to be sure. Then, apply the solutions to make sure the transmission system is working properly. And don’t forget the routine maintenance to keep such problems at bay.

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