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Fix Your Zero Turn Hydrostatic Transmission Problems (Troubleshoot)

The transmission is responsible for powering the wheels to move the mower. When you begin to experience the zero turn pulling to one side, moving slowly, or making excessive noise you may have a problem with the hydrostatic transmission.

Zero turn hydro overflow tank

Follow all safety instructions provided in your equipment operator’s manual before diagnosing, repairing, or operating. Consult a professional if you don’t have the skills, or knowledge or are not in the condition to perform the repair safely.

How Zero Turn Hydrostatic Transmissions Work

A zero-turn mower uses two independent systems to allow your mower to have a zero turning radius. You will find a hydraulic wheel motor and pump for each side of the mower.

This enables each wheel to move independently. One wheel is able to move forward while the other wheel is able to move in reverse.

The flow of the hydraulic fluid increases as you push the steering lever forward or backward. The further away from neutral you move the lever, the more fluid pressure increases in the motor, and the wheel will turn faster.

Symptoms of a Zero Turn Hydrostatic Transmission Issue

Here are some things you may experience when you develop a problem in the transmission.

  1. Pulling to one side
  2. One side moves faster than the other or is weak on one side
  3. Excessive wheel noise
  4. Moving slowly
  5. Lack of power
  6. Hot hydraulic oil
  7. Hydraulic oil leak
  8. Nothing happens when moving the steering levers. The zero-turn won’t move.
    • Ensure the transmission bypass valve is not open, allowing you to push the mower. It must be in the operating position. You will find a separate bypass valve for each side of the mower.

Troubleshoot a Zero Turn Hydraulic Issue

1. Low Hydraulic Oil

The hydraulic oil level must be kept full. A low level may not provide sufficient lubrication and cause the mower to feel weak and move slowly on the side with low oil.

Look at the overflow tanks located just behind the seat or under the seat. The oil level must be at the FULL COLD line when the hydraulic oil is cold.

Add fresh oil if needed. Check for oil leaks if the fluid level is low.

Note: Some entry-level zero-turn mowers will not have serviceable transmissions. This means you are not able to change the filters and oil because the system is sealed. Some units allow you to add oil.

2. Old or Poor Hydraulic Oil Quality

The oil quality will affect the performance of the hydraulic system. Most hydraulic failures are due to oil contamination.

This is why you don’t want to open up the hydraulic system more than needed to keep dirt and other foreign materials from getting into it.

When you do need to add oil or perform an oil change, always wipe away loose dirt from around the oil fill caps, transmission filters, and drain plug.

Change oil at regular intervals as listed in the operator’s manual. Don’t change prematurely unless the oil is getting hot (an indication of a lubrication problem) or you believe there is a contamination problem.

3. Air Trapped in the Hydraulic System

Air trapped in the hydraulic system when you don’t take time to bleed the system after a fluid change causes cavitation. This can result in excessive noise when the wheel turns and your mower may move slowly.

Bleed the transmission system:

  1. Park the mower on a flat leveled surface. Engage the brake. Confirm the PTO switch is off.
  2. Begin by confirming the hydraulic oil level is full in each transmission.
  3. Raise the rear wheels off the ground using jack stands just high enough to move the tires off the ground. (This is the best method. You can also run the mower in a large open area if needed.)
  4. Start the engine.
  5. Remove the parking brake.
  6. Engage the transmission bypass valve to open them. This allows the wheel to be spun by hand. There is one for each side
  7. Move the motion control levers slowly forward and reverse about 6 times.
  8. Close the transmission bypass valve.
  9. Move the motion control levers slowly forward and reverse about 6 times.
  10. Repeat moving the levers forward and reverse with the bypass valve open and closed as needed. You want the drive wheels to move smoothly at normal speeds forward and reverse and at a normal noise level.

4. Bad Pump Drive Belt

The drive belt transfers power from the engine to the transaxles and that’s what makes the wheels turn.

A hydro belt will wear over time and may begin to slip or come off the pulley. Check out the belt and look for worn edges, cracking, or a glazed appearance. Replace the belt that has any one of these conditions.

5. Bad Tensioner Pulley or Spring

The bearing in the tensioner pulley or the pulley itself can break. Most tensioner pulleys are made of hard plastic which can wear over time or break.

Replace a pulley that is worn or has a bad bearing. Keep the tensioner arm greased so it does not freeze up to keep tension on the belt.

The idler spring places tension on the pump drive belt. Replace the spring if it is broken or has fallen out of your mower.

6. Hydro Pump Leak

When you experience a leak from the pump the best thing to do is replace it. This is going to be an expensive part.

You may choose to attempt to repair the pump by opening up the system and using a rebuild kit, but this will be a temporary fix. I’ve seen many mechanics attempt to fix a transmission only to purchase a new transmission shortly after.

Opening up the transmission introduces foreign materials that lead to pump failure.

7. Hot Hydraulic Fluid

When you operate a zero turn with bad hydraulic fluid or low fluid, the oil is not able to efficiently lubricate the hydraulic system causing increased friction and overheating of the hydraulic fluid.

Hot hydraulic fluid can also indicate more extensive damage.

I highly recommend taking your zero-turn to your local Hustler dealership if you experience leaking from the hydraulic pump or your mower runs fine when it is cold, but stops running when it gets hot.

Hydraulic System Maintenance Tips

Here are a few tips for keeping on top of your hydraulic system maintenance. Check out this article for more information on performing a complete zero-turn tune-up.

Check Your Hydraulic Fluid Level

Check the hydro fluid level regularly to keep the transmission system lubricated for good performance.

Change the Hydraulic Oil and Filters at Regular Intervals

Perform hydraulic oil and filter changes at the intervals recommended by the manufacturer.

The interval for oil and filter changes and the type of transmission fluid used will vary depending on the transmission manufacturer’s requirements.

I don’t recommend changing the hydraulic oil before the recommended period unless you suspect oil contamination. Opening up the transmission system unnecessarily can introduce contaminants causing transmission wear.

Use the Correct Hydraulic Oil

Don’t use the wrong type of oil for your transmission system. This can lead to poor lubrication or increased heat.

Only use the oil recommended by the zero-turn manufacturer or the transmission system manufacturer.

Keep Dirt out of the Hydraulic System

Because dirt is often the main contributor to premature transmission failure, take precautions to keep dirt out of the system.

Follow these tips:

  • Wipe around the hydraulic overfill caps before opening them up to add oil.
  • Wipe around the drain plugs and the filter area before draining the fuel and removing the oil filter.
  • Use fresh hydraulic oil and don’t leave the caps off the bottles.

Clean the Cooling System

Keep the transmission cooling system clean. This includes cleaning the hydro fan cooling fins, guards, and fan.

Inspect the Belt, Pulleys, and Spring

Check the condition of the pump belt, pulleys, and spring. Ensure the belt is securely placed around the pulleys and the spring places sufficient tension on the belt.