So your riding mower will cut the flat lawn just fine, but there’s a problem when it reaches a hill. The revs of the engine stay up, the blades keep cutting, but your riding mower struggles uphill. In my experience, if you’re slow going uphill, your hydrostatic drive needs TLC. So here’s my guide to getting your hydrostatic transmission working on hills and what fixes you can carry out.
Why Won’t My Hydrostatic Mower Go Up Hills? (The Short Answer)
Once the drive system is put under pressure from an incline, underlying problems will show through. Insufficient drive belt traction, hydro air-lock, or lack of hydro oil will cause your hydrostatic drive to have a tough time going uphill.
3 Potential Causes of a Hydrostatic that Won’t Go Up Hills
You’ll start to notice problems in your lawn mower any time a drive system isn’t 100%. A good test that will let you know your drive system is not in tip-top shape is driving your riding mower up a hill and seeing it struggle. If anything, you should be thankful for the hill because it’s giving you a heads up that there’s a problem.
Here are five potential causes that might explain why your riding mower is floundering when going uphill.
Lack of Traction on the Drive Belt
You’ll first want to check to see if the belt and pulley system is free of dirt and grass build-up. Anything that works its way into the pulleys will reduce the energy provided to the hydrostatic pump. Going uphill puts more demand on the pump to produce more pressure. This is why you probably don’t notice a problem on flat ground.
Additionally, a worn or slack belt will not provide sufficient power to the pump. You will know how it feels if you have tried driving a lawn mower with a hydrostatic transmission on low revs.
Hydrostatic Transmission with an Air-Lock
Hydrostatic drives rely on the pressure created by the pump and the movement of the oil. If air works its way into the hydrostatic motor, it cannot move, as air provides no force. Again, this may not be noticeable on flat ground, but when the hill appears, problems begin.
Now you may not be getting air into the system, but oil cavitation will produce air bubbles that will collect together and cause this very problem.
A Lack of Hydrostatic Oil in the System
A lack of oil results in the same issue as an air-lock, just that this time the problem is coming from insufficient oil and not oil cavitation. Once the oil in the system drops low enough, it will be replaced with air. This air will then work its way to the motors and reduce the power output of the wheels. As a result, you’ll either experience slowness going uphill or be unable to make it up at all.
What You Can Do to Fix a Hydrostatic Mower That Won’t Go Up Hills (5 Things)
Now that you have some idea why your hydrostatic mower won’t go up hills, let’s consider some improvements to get the mower’s hydrostatic transmission working 100% on hills again.
Getting Traction Back to the Drive Belt
The first thing you can do to get traction back to the drive belt is to give it a real good clean. Over time, dirt and grass will build up, and traction will gradually fade. So let’s look at the best approach to cleaning the drive system.
Cleaning out the Drive Belt Area
I find the best way to start is to grab a powerful hose and blast off as much of the dirt as possible. You’ll find this makes a good start, but it will not get in all the hard-to-reach places.
Next, take a small scraper and work off the hard, baked-on dirt. Pay close attention to the tensioner; if dirt builds up, it will reduce the contact with the belt, and you’ll lose traction.
When you think you have finished cleaning the drive belt area, take the mower for a quick ride and get the parts moving. Then, repeat the cleaning process once again. You’ll find that once the parts have moved, they will expose more dirt.
- Power Water Hose
Checking the Belt for Tension
You can confidently conduct further inspections now that you have cleaned the belt and eliminated the potential of dirt being the root of the problem. The last thing you want to do is change out a belt because of a lump of dirt on a tensioner. The belt can be slack due to either insufficient tension or because the belt is stretched out.
If you inspect the idler spring and it’s compressed, then the belt would be stretched. As the belt becomes slack, it will start to show wear on the sides as it’s able to move in an up and down motion as well as around the pulleys. This basically thins out the belt and makes stretching occur much quicker. Once you have established the cause, you can either replace the spring or the belt.
Replacing the Drive Belt Tension Spring
The easiest way to swap out the spring is to use a spring pulling tool. Place the spring puller over the end of the tension spring and pull it from its mount. Simply take your replacement spring attaching it to the mounts using your spring puller. Ensure that when installing the new spring, it’s installed with the correct orientation. Usually, the hooks at the ends of the springs are different sizes. Putting the smaller hook on first makes for an easier install.
- Spring Puller
- New Tension Spring
Replacing a Drive Belt
To replace the drive belt, you must remove the deck belt first. Start by removing the tension spring on the deck using a spring puller. With the tension removed, you can go ahead and lift the belt off of the clutch pulley. Next, take a socket wrench and remove the bolts holding the clutch alignment bracket in place. Finally, you can disconnect the power to the clutch. Once these four tasks are complete, you can address removing the drive belt.
Retake your spring puller and remove the tension spring for the drive belt. You can remove the belt from the drive and hydrostatic pump pulley now that it’s slack.
Before installing the new belt, take a look around the pulleys and ensure that they are clean and in good condition. Now, place the new belt over the two pulleys with the correct orientation. Ensure that the inside of the belt is on the inside and that it hasn’t twisted.
With the belt over the two pulleys, you can move the tensioner back into place and install the tension spring with your puller. Double-check that the belt is seated correctly and that it’s sitting within the pulley flanges properly. You may want to spin the drive by hand to check this.
Next, take the clutch alignment bracket and reinstall it, making sure that the bracket holds the clutch in the right position. Once you have the clutch aligned, you can install the bolts. Finally, reattach the clutch power to complete the installation of the new drive belt.
The last job you’ll need to do is install the deck belt. Align the belt in the pulleys and reinstall the tension spring to the idler using your spring puller.
Tools & Parts Required
- Spring Puller
- Socket Wrench
- New Belt
Purging Air From a Hydrostatic Transmission
Cavitating oil is a common problem with hydrostatic motors. Because of this, manufacturers have incorporated a simple purging system within the drives as a release valve. This is the same release valve that you use to disengage the drive. Let’s look at how to use the release valve to purge the air.
Purging Air Using the Release Valves
To work the air out of the system, you’ll first need to raise the drive wheels of the lawn mower. I like using a trolly jack or car jack to get the wheels a few inches off the ground.
Start by blocking the front wheels of the lawn mower so that it cannot move when lifted. During this process, you will not be able to use the parking brake as the wheels will not be on the ground. Then lift just the drive wheels off the ground, ensuring the mower is stable. Next, release the bypass valve to the hydrostatic transmission/hydrostatic motor. If you are doing this with a zero-turn, you will have two release valves, one for each motor.
Follow this by starting the lawn mower and engaging the drive. Slowly increase forward drive, making sure the wheels are not turning. If they begin to turn, then you have not disengaged the drive. In this case, stop the engine and release the bypass valve correctly.
With this done, start the engine and put the drive into forward. Hold the drive in full forward motion for a few seconds and switch into reverse drive. Again, hold it in reverse for a few seconds. Repeat this process several times, working the air out of the system. Once complete, you can stop the engine and close the bypass valve.
The last thing you need to do is repeat the process with the drive engaged—this time, you need to be very careful as the wheels will be turning. So, ensure that the mower is jacked up securely and that there is no way the mower can come off the jacks. Once you have completed the process in both drive and bypass, you are done.
- Strong Jack
- Wheels Blocks
Topping Off the Hydrostatic Oil Level
If you are one of the lucky ones, you’ll be able to top off the oil. Unfortunately, not all lawn mowers give you this ability. Some manufacturers use sealed transmissions that do not allow for adding or changing of oil. If you have this type of mower, you’ll have to contact the supplier or local repair shop. For you guys that can, you’ll just need to locate the reservoir.
Adding Oil to the Oil Reservoir
Start by locating the oil reservoir for the hydrostatic drive. You’ll probably find it under the seat. Next, look for the level indicator. Once you know how much oil you need to use, you can add the oil. Make sure you use specific hydraulic oil and not just regular engine oil. Once you reach the correct level, you can run through the purging process. This will ensure that the oil is worked through the system and any air is removed.
Tools & Parts Required
- Strong Jack
- Wheel Blocks
- Hydraulic Oil
If My Hydrostatic Mower Goes Slowly Up Hills, Can I Improve Performance?
If you find your hydrostatic transmission on hills works but is very slow, then I recommend following the above steps. Each of the methods should be part of your routine maintenance plan. Proper maintenance of a hydrostatic drive will keep your mower riding like new.