My Petrol Lawnmower has a fault with the gear box

If your lawnmower's self-propel system is not working and you have already checked that the drive cable and drive belt are correctly fitted and functioning, the gearbox may be faulty.

Common gearbox faults include:

• Internal gear wear
• Damaged gearbox bearings
• Worn drive shafts
• Damaged gearbox pulleys
• Internal gearbox seizure

This guide explains how to replace the gearbox assembly.

Before You Begin

Important: Gearbox replacement is an advanced repair that requires a good level of mechanical knowledge and the use of specialist tools.

Before starting:

• Ensure the engine is switched off.
• Remove the spark plug cap.
• Work in a clean, safe environment.
• Wear appropriate personal protective equipment (PPE).

Replacing the Gearbox

Step 1 – Drain the Mower

Drain all fuel and engine oil from the machine.

Remove the air filter assembly if required for access.

Step 2 – Position the Mower

Carefully turn the lawnmower onto its side or upside down, ensuring all fluids have been removed first.

This will allow access to the drive system underneath the mower.

Step 3 – Remove the Wheel Covers

Using a suitable screwdriver, carefully remove the plastic wheel covers.

Step 4 – Remove the Axle Retaining Springs

Detach the retaining springs from both sides of the axle assembly.

Step 5 – Remove the Rear Wheels

Using a 13 mm spanner or socket:

1. Remove the wheel retaining nuts.
2. Slide the wheels from the axle.

Step 6 – Remove the Drive Gear Components

Using circlip pliers:

1. Carefully remove the circlip retaining the drive gear.
2. Remove the drive gear, drive pin, and any plastic wheel guards fitted to the axl

Step 7 – Remove the Gearbox Spring

Carefully detach the gearbox tension spring using pliers, a spring puller, or another suitable tool.

Note: The spring may be under tension and can be difficult to remove.

Step 8 – Remove the Drive Belt

Remove the drive belt from the gearbox pulley.

This may require carefully working the belt off the pulley by hand.

Step 9 – Remove the Gearbox Assembly

Once the spring and belt have been removed:

1. Lift the gearbox and axle assembly clear of the mower chassis.
2. Disconnect the gearbox cable fitting by compressing the retaining clips using long-nose pliers.
3. Pull the cable free from the gearbox.

The gearbox should now be completely detached.

Step 10 – Remove the Gearbox from the Axle

1. Remove the retaining circlips from both sides of the gearbox bushes.
2. Remove the gearbox bushes.
3. Carefully slide the gearbox from the axle.

The old gearbox can now be removed.

Step 11 – Install the Replacement Gearbox

Fit the new gearbox onto the axle and reassemble the mower by following the above procedure in reverse order.

Ensure:

• All circlips are securely installed.
• The drive belt is correctly seated.
• The gearbox spring is securely attached.
• The drive cable is connected correctly.
• All wheel fixings are tightened securely.

Final Checks

Before operating the mower:

• Refill with the correct engine oil.
• Refill with fresh petrol.
• Reinstall the air filter if removed.
• Reconnect the spark plug cap.
• Test the self-propel function in a safe area.