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How to Pressure Test a Tractor Hydrostatic (HST) Transmission HYSTAT Transmission

How a Hydrostatic Tractor Transmission Works

The energy (power) generated by a tractor engine is transmitted to the transmission, and ultimately to the tractor axles and wheels which causes drive. A hydrostatic transmission uses hydraulic fluid, as opposed to drive gears, to drive a hydrostatic drive motor. When the pressurized fluid enters the drive motor, the pistons of the drive motor begin to move, causing rotation which turns the axle and wheels. The speed at which the wheels turn is determined by both the amount of pressure and volume of fluid sent to the drive motor. Because a Hydrostatic Transmission of pressure and volume of fluid flowing through the


motor and transmission is critical to its performance. When a HST transmission is not operating properly, one of the very first things to troubleshoot the problem is to complete a pressure test on the transmission. The results from the test will allow you to adjust the hydrostatic transmission.

Below you will find the guide and instructions on how to successfully complete a Hydrostatic Transmission Pressure Test.

Hydrostatic Transmission Charge Pressure Test and Adjustment

A Hydro Transmission Charge Pressure Test will determine if the hydrostatic pump is producing sufficient pressure to operate the hydrostatic motor pump.

Tools and equipment required:

• Hydraulic adapter fitting

• Hydraulic pressure gauge

• Hose with coupler


Step 1. Remove the rear right wheel of the tractor

• Make sure to use a suitable lifting device.

• Make sure to support the rear axle with suitable jack stands.

• Make sure that the range gear lever is in N (neutral) position.

Step 2. Warm up the hydraulic oil

1. Start the engine.

2. Operate the hydraulic system to circulate and heat up the hydraulic oil. The hydraulic oil must be heated up to the normal operating temperature of 140 °F (60 °C).  IMPORTANT: Avoid damage! Do not overheat the hydraulic oil

3. Check for oil leaks.

4. Shut off the engine.     

Step 3. Measure the hydrostatic transmission (HST) charge pressure.

1. Remove the test port plug from the charge pressure test port (A, Fig.1) in the center plate.

2. Install the following to the charge pressure test port:

  • The adapter fitting

  • Hydraulic pressure gauge (B, Fig.1)

  • Hose with coupler


3. Perform the following procedure.

  • Start the engine

  • Run the engine at 3200 rpm

  • Read the charge pressure on the hydraulic pressure gauge

  • Pressure reading on many tractors should be 71.05 - 114.55 psi (0.49 - 0.79 MPa),   however, you will need to consult your tractor service manual, owner’s manual, or contact your tractor dealer.

  • Shut off the engine

4. Remove the following from the charge pressure test port

  • Hose with coupler

  • Hydraulic pressure gauge

  • Adapter fitting

  • Install the test port plug

  • Check for oil leaks on the hydrostatic transmission (HST)


After the test, apply the sealing tape to the plug to tighten the plug. On many tractors, the torque should be tightened to 5.8 to 8.7 fl·lbs (7.8 to 11.8 N·m), however, you will need to consult your tractor service manual, owner’s manual, or contact your tractor dealer.


When the charge pressure test reading does not conform to the required tractor specification, the hydrostatic transmission (HST) pump and motor will need to be disassembled and removed for repair. 

For more information on tractor service or tractor parts, the Yanmar Tractor Store is here to help!  Contact us today - our expert staff is firendly, and our tractor advice is free.

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