Hydrostatic Mower Won’t Go Up Hills

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Does your hydrostatic mower cut your flat lawn just fine but suddenly develop a problem when it reaches a hill? This situation can be quite frustrating and as well prevent you from cutting the hilly areas of your lawn.

In this article, I talk about the common reasons your hydrostatic mower won’t go uphill and simple troubleshooting steps to help you diagnose and fix the problem.

Reasons Why Your Hydrostatic Mower Won’t Go Up Hills

Some of the reasons your riding mower might have a tough time going up hills include:

  • Insufficient drive belt traction
  • Worn out or glazed drive belt
  • Damaged tension spring and idler bracket
  • Hydrostatic transmission with an airlock
  • Lack of hydrostatic fluid in the transmission system 

Tools You Need For This Job

Some tools you may need for this job include:

  • Work gloves
  • Water hose
  • Screwdriver
  • Small scraper
  • Jack

Hydrostatic Mower Won’t Go Up Hills: Causes And Fixes

1.  Insufficient Drive Belt Traction

The first reason your hydrostatic mower won’t go up hills is a lack of traction on the drive belt

As you climb a hill, the pump has to work harder to generate more pressure to enable the mower to move up successfully. 

Grass or dirt buildup on the belt and pulley system can reduce the traction on the drive belt. The hydrostatic pump’s energy supply will be reduced greatly if something manages to get inside the pulleys. 


Check the deck, pump, and pulley system. If you notice grass or dirt build-up, it needs to be cleaned.


  • Clean the dirt buildup by using a water hose to blast off as much dirt and grass as possible
  • Next, use a small scraper or screwdriver to remove any baked dirt in the hard-to-reach places
  • Start the mower again to expose more dirt and repeat the cleaning process

2. Worn Out/Glazed Belt


A worn-out or glazed belt won’t give the pump enough force that it needs to be able to go uphill. This could be the reason your mower experiences problems going uphill.

Belt glazing happens when friction heat melts the belt’s surface area. The belt face will harden and smoothen as this surface area cools, making the belt unable to grip and turn pulleys effectively. The glazing situation could get worse the longer a belt slips.

The mower may simply be slipping under load if the belt is worn out or glazed, in which case you will need to replace the worn belt.


Examine the belt that rotates the pulley on top of the transaxle. If it appears to be worn, glazed, or contaminated with oil, it is damaged and could be reducing traction.


Replace the belt if it is slacked, worn out, oil-stained, or glazed.

3. Damaged Tension Spring And Idler Bracket

If the drive belt appears to be in good condition or was recently replaced, the idler bracket and tension spring that maintains this belt in place may be at fault. 

Look for bent or broken belt idler brackets, and a broken or worn idler tension spring, and perform any necessary maintenance.


Examine the tension spring. If it is compressed, it is damaged. Look for any damage or broken parts on the idler bracket.


Replace any damaged or broken parts.

How To Replace The Drive Belt Tension Spring

Using a spring-pulling tool is the most straightforward method of replacing the spring. 

  • Pull the tension spring away from its mount by placing the spring puller over its end. 
  • Simply use your spring puller to install your replacement spring to the mounting. 
  • Make sure the new spring is put correctly aligned when you replace it. 
  • The hooks on the ends of the springs often come in various diameters. It is simpler to install if the smaller hook is put on first.

4. Hydrostatic Transmission With An Airlock

Another reason your hydrostatic mower might refuse to go uphill is that air has entered the transmission system and an airlock has occurred.

The pump’s pressure and the oil’s movement are what support hydrostatic drives. If air gets inside the hydrostatic motor, it will not be able to move uphill because air has no force and reduces traction. 

Air could be entering the hydrostatic motor through a leakage or loose connection. Oil cavitation may also create air bubbles that build up during high temperatures and result in an airlock in the hydrostatic system.


The only way to check for air in the hydrostatic transmission and remove it is to purge the system.

How To Purge The Transmission System

  • Lock the non-powered wheels at the front and hoist the drive wheels up off the ground using a jack
  • Open the bypass valve on the transaxle
  • Start the engine and advance the throttle to normal operating speed
  • Disengage the brake if activated
  • Slowly move the directional control to forward, neutral, and reverse directions, holding each direction for a few seconds each
  • Repeat this process about half a dozen times. You will know when you have opened the bypass valve because the drive wheels will not turn 
  • Next, shut off the engine 
  • Check the oil level and top it up if needed 
  • Finally, close the bypass valve and test the engine to see if it runs fine 
  • Repeat the purging process if necessary

5. Lack Of Hydrostatic Fluid In The Transmission System 

A lack of hydrostatic fluid or oil in the system can also cause difficulties when going uphill in your hydrostatic mower. 

The same problem that air-lock causes also arises when there is inadequate fluid/oil. When the amount of oil in the system is too low, air will start to replace it. 

The power output of the wheels will be decreased as this air makes its way to the motors. This will cause your mower to go either uphill very slowly or not be able to move up at all.


Check the transmission fluid level. You can view the fluid level if your mower has a viewing glass model. If not, use a dipstick to check the fluid level and determine if it is low.


Top up the hydrostatic oil but if your transmission system is sealed, you should contact your supplier or local repair shop. Don’t attempt to top up or change the oil yourself.

If your system isn’t sealed, make sure you use specific hydraulic oil and do not exceed the oil level indicated in your owner’s manual. 

Also, purge the system after changing the oil to get rid of trapped air that might have gotten in during the process to prevent airlock.

How To Remove And Replace The Drive Belt Of Your Hydrostatic Mower

The following steps will show you how you can remove and replace the drive belt of your hydrostatic mower.

Tools You Need For This Job

  • Work gloves
  • Safety goggles
  • Spring puller
  • Socket wrench
  • New drive belt

Removing The Hydro Drive Belt 

  1. Take off the left belt cover.
  2. Remove the Power Take Off (PTO) idler spring hook from the anchor bolt using a spring puller or equivalent instrument while wearing safety goggles.
  3. Remove the PTO belt from the clutch pulley at the unit’s back.
  4. Remove the mower deck transport lock and adjust the cutting height to the lowest setting.
  5. Remove the hydrostatic drive idler spring hook from the anchor bolt with a spring puller or similar tool.
  6. Remove the hardware that holds the clutch stop bracket to the frame and the clutch stop bracket itself.
  7. Disconnect the clutch’s wire harness.
  8. Disconnect the hydro drive belt from the left or right transaxle pulley, then push the loose belt length toward the interior of the mower.
  9. Disconnect the hydro drive belt from the engine drive pulley and the clutch.
  10. Remove the hydro drive belt from the remaining transaxle pulley, then discard the belt.

Hydro Drive Belt Installation 


Before installing the new belt, ensure that the pulleys and surrounding area are clean, then carry out the following steps:

  1. From either the left or right side of the unit, insert the hydro drive belt toward the interior from above the transaxle. 
  2. Install the hydro drive belt around the remaining transaxle pulley after passing it over the clutch and engine drive pulley.
  3. Use a spring puller or similar instrument and safety goggles to re-engage the hydro idler spring around the anchor bolt. Make that the hydro belt is taut and positioned correctly in each pulley.
  4. Reattach the clutch’s wiring harness.
  5. Make sure that the clutch stop bracket is centered between the two holes in the frame and the clutch’s slot. Use the two original tapping screws to fasten the bracket to the frame.
  6. Around the clutch pulley, reattach the Power Take Off (PTO) belt.
  7. Align all of the mower deck’s spindle and idler pulleys with the PTO belt.
  8. Reattach the PTO idler spring around the anchor bolt using a spring puller or a similar tool.
  9. Make sure the PTO belt is taut and positioned correctly in each pulley.
  10. Put the belt cover back on.

If My Hydrostatic Mower Goes Slowly Uphill, Can I Improve Performance?


Yes, you can improve the performance of your hydrostatic mower if it moves slowly uphill. I advise taking the above actions if you discover that your hydrostatic transmission is very slow up hills despite working elsewhere. 

Every one of the preventive and repair techniques needs to be a part of your regular maintenance schedule to keep your mower in the best condition and to prevent similar problems from occurring in the future. 

Your mower will continue to ride like new if you maintain your hydrostatic system properly.


Your hydrostatic mower experiencing difficulties when going up a hill could be a result of lack of traction, dirt buildup, insufficient hydrostatic fluid, hydro airlock, damaged drive belts, or tension springs.

Follow the recommendations shared above to diagnose and fix your mower going uphill problem. But if the problem persists, it is best to seek the help of an experienced technician.