Hydrostatic Transmission Slipping When Hot (How to Fix)

A frustrating issue with hydrostatic transmission is losing drive after a few minutes of starting up your mower. Unfortunately, this is one of the common issues of hydrostatic drives as they get older or when they are neglected. So, why is your lawn mower’s hydrostatic transmission slipping? Let’s take a look.

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Hydrostatic Transmission Slips When Hot (The Short Explanation)

Slippage of a hydrostatic transmission when hot is most likely due to damage to the cylinder block and the valve plate. When the hydrostatic fluid heats up, the fluid viscosity decreases and can leak between the cylinder block and the valve plate. The leak through the damaged area diverts the pressure of the fluid away from the pistons resulting in a loss of power.

Why Your Hydrostatic Transmission May Be Slipping As It Heats Up

One thing to remember when dealing with a hydrostatic transmission slipping when hot is the fact that the problem only occurs when the transmission is heated up. If your transmission is slipping when it is both hot and cold, then it is likely caused by different issues, such as a lack of fluid or an airlock.

So, why does the slippage only occur when the transmission is hot? Well, as the transmission fluid heats up, the viscosity decreases and becomes thinner, allowing the fluid to flow more easily. In addition to easier flow, the fluid is able to move through smaller spaces.

Tucked inside your hydrostatic transmission is a hydrostatic pump that converts the engine’s power into moving fluid, which drives the hydrostatic pump pistons. The point in which this process can fail is the seal between the hydro pump cylinder block and the valve plate. So you would think you could just swap out the old seal for a new one, and the problem is solved. Unfortunately, it’s not that simple.

The seal between the cylinder block and the valve plate is created by both surfaces of these two components being so perfectly flat that the gap is smaller than the size of the oil film. Now, without getting too technical and getting into oil film pressure breakage points, let’s just say that as the transmission fluid heats up and thins out, it can work its way between this gap through any imperfection. When the fluid finds a new path, the pressure on the piston reduces, resulting in the loss of power.

So, why is this now a problem? Well, the transmission fluid not only travels through the pump, but it also moves through the transmission itself. Several clogs inside the transmission are all moving against each other and wearing each other down. The result is tiny metal filings work their way through the pump and into the gap between the cylinder block and the valve plate.

These filings damage the seal, allowing the transmission fluid to escape rather than drive the pistons. There are a few magnets and filters in the transmission designed to catch these metal filings, but eventually, some filling will work into the pump and cause irreparable damage.

The only way to tell if the cylinder block and valve plate are damaged is to strip down the transmission and inspect the components for wear.

Fixing a Hydrostatic Transmission That Slips When Hot

You have to follow the same steps when both inspecting and repairing a hydrostatic transmission or pump. The only difference is that for the repair, instead of returning damaged parts to the transmission/pump, you replace them with new components.

Now, I say transmission/pump because some lawn mowers, like tractor lawn mowers, have transmissions with the pump inside, whereas zero-turns have the pumps separate from the transmission. So regardless of what hydrostatic drive system design you have, it’s still the same two components at fault.

So let’s look at the standalone pump design that you will typically find on zero-turn lawn mowers.

Steps to Repair a Hydrostatic Cylinder Block & Valve Plate

Replacing the cylinder block and valve plate is no quick fix, and I recommend it for people who are confident about pulling engines and lawn mowers apart. So, to make things a little easier, I have broken the repair down into five different stages:

Hydrostatic Pump Removal
Cylinder Block & Valve Plate Removal
Installing a New Cylinder Block & Valve Plate
Installing the Hydrostatic Pump Back Onto the Lawn Mower
Bleeding the Hydrostatic Transmission

Let’s look at each of these five stages in more detail.

Hydrostatic Pump Removal

Jack up the rear of the lawn mower so that the rear wheels are off the ground.
Remove the rear wheels and set them to one side.
Remove the hydro lines from the drive motors of the wheel and catch the hydro fluid in an oil pan.
Remove the hydrostatic drive belt from the hydro pulley.
Remove the hydro pulley.
Remove all the hydro lines connected to the hydro pump.
Remove the handlebar linkage from the hydro pump.
Remove the pump mounting bolts.
Lift the hydro pump from the lawn mower.

Cylinder Block & Valve Plate Removal

Remove the bolt securing the top of the pump housing.
Lift off the top of the pump housing to gain access to the inside of the pump.
Lift off the valve plate from the bottom section of the pump housing.
Lift out the cylinder block.
Lift out the pistons and springs.
Thoroughly clean the inside of the pump, making sure to remove metal filings.

Installing a New Cylinder Block & Valve Plate

Place your new cylinder block with the pistons and springs in position on a flat surface with the pistons facing upwards.
Take the lower main section of the pump housing and turn it upside down so that the pump shaft is facing downward.
Thread the pump shaft through the cylinder block.
Lift the cylinder block so that it firmly rests inside the cylinder housing. Make sure to keep the cylinder block in an upright position until seated so that the pistons and springs do not fall out.
With the block installed, turn the pump housing over so that the cylinder is facing upward.
Install the valve plate, making sure the plate locks into place.
Place the top of the pump housing back onto the pump.
Install the bolts to secure the top of the pump back into place.

Install the Hydrostatic Pump Back Onto the Lawn Mower

Lower the pump into position.
Install the pump mounting bolts.
Connect all the hydro lines.
Reattach the hydro pump pulley.
Connect the hydro belt.
Reattach the handlebar linkage.
Reconnect the hydro line to the hydro drive located at the wheel.
Reattach the wheel.

Bleeding the Hydrostatic Transmission

Loosen the pump bleed bolt.
Disengage the transmission.
Fill the hydrostatic fluid reservoir.
Start your lawn mower.
Set the RPM to low.
Push the handlebar fully forward, then fully back, and repeat 10x.
Top off the reservoir.
Repeat the back-and-forth motion of the handlebars 10x.
When fluid starts to leak from the bleed bolt, tighten the bolt.
Engage the drive.
Test the drive function.
Repeat the priming and bleeding process until all the air is removed from the hydrostatic system and the drive is fully restored.
Lower the lawn mower off the jack.
Test the lawn mower on open ground
Repeat bleeding and topping off of hydrostatic fluid as required.

Tools & Parts to Replace a hydrostatic Cylinder Block & Valve Plate

Jack
Socket Wrench Set
Wrench Set
Oil Catch Pan
Hydrostatic Fluid
Oil Funnel

Can You Just Replace the Hydrostatic Transmission/Pump?

If all of this sounds like too much work or it’s not something you are comfortable with, then you could go ahead and replace the entire transmission/pump. But keep in mind that a replacement cylinder block and valve plate will set you back about $50-75, whereas a replacement transmission/pump could be anywhere between $500-$1000.

So, I recommend that you have a conversation with your local lawn mower repair shop and ask them how much parts and labor would be to repair your hydrostatic transmission. I know it won’t be cheap, but it shouldn’t be anywhere near the cost of completely replacing the entire hydro transmission.