Hydrostatic lawn mowers are susceptible to a number of problems common in all hydraulic systems. Due to their nature, they can happen at any time without warning. Some of the issues can come and go, leaving you even more confused. In this article, I’ll share what I’ve learned about troubleshooting different hydrostatic transmissions and give you the solutions to hopefully get them working again.
Why Won’t My Hydrostatic Mower Move? (The Short Answer)
You may find that there are a variety of problems preventing your mower from moving. For example, a problematic belt or pulley could be limiting the amount of power delivered to the hydro pump. There may be an issue with a lack of oil or an airlock in the system. Older units could be suffering from wear. Or, very simply, the hydro motor might be disengaged.
Hydrostatic Mower Will Not Move – 6 Potential Reasons
Lawn mowers with hydrostatic transmission can fall victim to several problems that result in a loss of drive. Let’s run through the issues you may encounter and diagnose what could be at fault.
Hydrostatic Motors Have Been Disengaged
The first thing to always check when a hydrostatic mower will not move is if the transmission is engaged. At the lawn mower’s rear, you will find a release pin that allows you to disengage the transmission. This is a handy feature if you ever need to move the lawn mower without it running. For example, if you have run out of gas or it’s broken down. However, it’s not so handy when it’s disengaged without you knowing.
Slipping Drive belt
A bad drive belt is one of the main reasons your hydrostatic mower won’t move. The hydrostatic pump relies solely on power transfer from the belt. First, check the belt’s tension and ensure it’s in good condition. If you find it slack or in bad shape, you’ll have to replace it.
Lack of Oil
The transmission gets power by oil transferred from the pump to the motors. The system requires sufficient oil to create pressure. Once the oil level becomes low, it is replaced with air, providing no energy for the motors. Oil levels can be checked via the oil reservoir if you have one. Unfortunately, some manufacturers have closed systems without reservoirs. As a result, there is no way of checking the oil, and you’ll have to get a certified professional to look at it.
Air Lock in the System
From time to time, you may find air in the hydraulic system. Air can get in due to a leak in the system or from high temperatures in the oil, forming air bubbles, otherwise known as cavitation. The only way to check for air in the system is to bleed/purge the system.
Broken Hydraulic Hoses and Seals
Hydrostatic transmissions work under tremendous pressure and can cause hoses and seals to break. When this happens, the oil will be replaced first by the reservoir and then air once the reservoir is empty. You can check for broken hoses and seals by inspecting them for spilled oil.
Faulty Internal Mechanism Within Pump or Motor
If you have run through all other items in this list and still can’t find the problem, there’s likely an internal problem with the transmission. Fixing this will require the hydro system to be removed and opened up. Once you can see what’s going on inside, you’ll be able to diagnose the problem.
Try These Solutions to Fix a Hydrostatic Mower that Will Not Move
So you’ve run a diagnosis and discovered why the hydrostatic mower won’t move. Now it’s time to start your repair. Let’s go over the individual problems and see what can be done.
Have the Hydrostatic Motors Been Disengaged
This isn’t really a repair; it’s just something that can be overlooked. On the lawn mower’s rear, you will find a long pin or metal rod coming from the hydro transmission.
On some lawn mowers, this will be labeled and have a hoop that you can pull. The drive has disengaged if you can move the lawn mower with the brake off. Either push or pull the lever, and the mower should go back into drive.
Ensure you only use this when the lawn mower is switched off and the brake is on. Be careful if your mower is on an incline because if your drive is already engaged and you disengage it, the mower will potentially roll away.
Slipping Drive belt
If you have diagnosed that you have a bad or broken belt, you will need to get under the mower to fix it. Before you do, ensure that you remove the battery and apply the brake. Finally, chock the tires, so the mower can’t move.
Installing a new belt on a hydrostatic drive requires you to remove the deck belt first, since it sits above the deck pulley. Take a spring puller and remove the tension spring from the deck belt. Next, remove the deck belt from the main engine pulley next to the clutch.
Now you can move onto the clutch stop bracket and remove the bolts with a socket wrench. With both items removed, you will have clear access to the drive belt. Next, take your spring puller and remove the tension spring from the drive tension arm.
Remove the drive belt from both the engine pulley and the hydrostatic pump pulley. Before installing a new belt, inspect both pulleys the drive belt attaches to and check for any damage or wear. If they are free of damage, you can proceed to install the new belt.
Feed the new belt over both pulleys and hold it in place. Next, take your spring puller and apply tension back to the idler spring. Once there is enough tension to hold the belt in place, you can let go and use both hands on the puller. Install the spring back into its correct location, ensuring the belt remains in place.
Now you can install the clutch stop bracket onto the lawn mower, ensuring that it secures the clutch correctly. Finally, align the deck belt and reinstall your deck idler spring. Congratulations, you have successfully installed a new drive belt.
Lack of Oil
Here you just need to check the oil level within the hydrostatic oil reservoir, usually under the driver’s seat. Take a look to see if you have a fill level window on the side of the oil reservoir or an indicator with the filler cap. Make sure you top it off with the appropriate hydraulic oil, not just regular oil.
Run the engine for several minutes and engage the drive backward and forwards to work the oil into the system. Once you have refilled the oil level, the drive will get stronger and stronger until it’s back to its original power. If you are still finding a problem, you might need to purge the air from the system.
Purging Air from the Hydrostatic Motors
Air may get into the system when adding fresh oil or due to cavitation. When this happens, you’ll need to purge the system. Manufacturers know that this is a common problem and have provided a solution.
Start by raising the lawn mower’s rear off the ground so that the drive wheels are off the floor. For this, it’s best to use a jack. Next, chock the front wheels so that the mower cannot move.
Then release the bypass valve for the transaxle or both bypass valves for both pumps if you are repairing a zero-turn lawn mower. Once done, you can sit in the mower’s seat and start the engine. Next, apply forward drive and hold for a few seconds, then place it in reverse drive for a few seconds. Repeat this process several times. You will know that you have the valve bypassed because the drive wheels will not turn.
Switch the lawn mower off and check the oil level within the hydrostatic oil reservoir. It should go down a little with purging and must be topped off. Now with the hydro purged, you can go ahead and close the bypass valves and remove the jack. Finally, start the lawn mower and take it for a drive. You should notice that the drive should be good as new after a minute or two.
Repairing Broken Hydraulic Hoses and Seals
If you find oil leaking from a connection of a hose to the hydrostat, it’s likely because of a loose seal or hose.
Before you try to remove the hose, try tightening it first. Something as minimal as a ¼ turn can create a leak that will, over time, drain the system enough to cause a problem. If you find this to be the case, run through the purge process and monitor the leak. Hopefully, it will stop.
If the seal has completely gone or you have a split hose, then you will have to either replace a seal or replace the hose entirely.
To do this, take a wrench and loosen off the nut holding the hose to the hydro. With the nut off, you should be able to remove the hose. Next, inspect the hose’s end to see if the seal is damaged. If it is, you can replace the o-ring seal and reinstall the hose. Repeat the removal process on both ends if you know you have a split hose.
Before you install the hose, consider using a threadlock to prevent the nut from becoming loose in the future. Then, place the hose back into position and re-tighten the nut.
To finish, run through the purging process to remove air and fill the oil level if needed. You may need to repeat the purging process a few times since a lot of oil may have been lost and replaced with air.
Faulty Internal Mechanism Within Pump or Motor
The interiors of the pump and motor are made up of several internal parts. To fix it, you will have to remove the complete hydro system. I would categorize this as advanced mechanics, not your standard troubleshooting and repair.
Additionally, you must be very careful because this could void your manufacturer and store warranties. If you have run through all of the problems on this list and are confident it’s none of them, I suggest you consult your lawn mower supplier for professional assistance.