Lawn Mower: Won't Drive or Self Propel

One of the most problematic parts in a self-propelled lawnmower’s drive system is the drive belt. The drive belt transfers power from the engine to the transmission. There is a simple clutch, controlled by a cable, that tensions the belt to engage the transmission. The belt will begin to wear and stretch over time. Usually there is a tensioner built into the drive cable that can be adjusted to compensate for belt stretching. Eventually the belt will stretch to the point that the extra length can no longer be adjusted for, at which point it will need to be replaced. The belt will wear thin with years of use. It will eventually wear thin enough that it will begin to slip on the pulleys. When worn too thin, the belt will often break and require replacement. Because belts are made of rubber, they will harden and become brittle over time. This is especially true if the mower isn’t used much and sits for long periods of time. The belt will form cracks as it hardens. Once cracks form, the belt can easily break. The belt on your mower should be inspected at least yearly to check on its condition. If the belt is worn thin or has developed cracks, it is wise to replace it right then rather than waiting for it to break while in use, leaving you stuck with a half-mown lawn. Replacing the belt is usually a pretty straightforward repair. Start by tipping the mower on its side so you can access the underside of the deck. Make sure you tip the mower with the carburetor facing up so fuel doesn’t leak out of it. Remove the blade as well as any belt shields to access the belt. Usually a second cover will need to be removed from the top of the mower’s deck or at the rear of the mower to access the belt at the transmission. There is typically a belt guide around the transmission pulley that will need to be loosened or removed so the belt can be accessed. Replacing the belt is a simple task of placing the belt around the pulleys, replacing the covers and blade, and then using the drive cable adjuster to tension the belt.

The transmission in a self-propelled lawnmower is an important part of the drive system. It connects to the engine by way of the drive belt, it changes the direction of drive rotation from vertical to horizontal, and it has gears that set the maximum drive speed and torque. Modern lawn mower transmissions are relatively trouble-free and are often maintenance-free as well. As with any moving part, with enough use it will wear out and need to be repaired or replaced. Extreme conditions, such as steep hills or a very dusty environment, will cause the transmission to wear out faster. Common symptoms of a worn or failed transmission are: grinding gears, starting and stopping or jerky operation, and the transmission pulley rotating while the wheels do not. Some transmissions have parts available and can be rebuilt, others must be replaced when they fail. Rebuilding a mower transmission is a more complicated repair so repair time, degree of difficulty, and overall cost should be weighed when deciding whether to rebuild or simply replace a faulty transmission. Replacement will require varying amounts of disassembly depending on the model. Typically, you will start by removing the drive belt and the drive wheels. The drive shafts will have a clutch on both sides of the mower that will need to be removed. There will be bearings or bushings where the drive shafts pass through the mower’s deck. Once the old transmission is removed you will want to carefully inspect the drive belt, the clutch assemblies, and the bearings or bushings. Replace any of these components that are worn. It is much easier to service these parts while you already have the drive broken down rather than waiting for them to fail in the future. Install the new transmission and the other components of the drive system. The parts breakdown for your mower is a valuable reference while reassembling complicated components such as the wheel clutches. Refer to it to help you make sure you install each part in the correct order. Once reassembled use the adjuster on the drive cable to properly tension the drive belt as needed.

The drive wheels of your self-propelled lawnmower are the last component of the drive train and are frequently the cause of a mower that won’t self-drive. The drive wheels will have a gear either molded into them or attached to them. The drive wheel gear mates to the drive shaft gear. It is common for the drive wheel gear to be plastic and the drive shaft gear to be metal. Over time, the plastic gear on the drive wheel will wear down and eventually start to slip. Faster wear will occur if the mower is used in dirty conditions, on steep hills, or if the drive is abruptly started and stopped frequently. The drive wheel gear on most mowers should not be lubricated. The gears are exposed to dirt and other debris. If the gears are lubricated the dirt will stick to the grease and form an abrasive paste that will quickly wear down the gears. Replacing the drive wheel is typically a very easy repair. On most models, you will begin by removing a hub cab from the wheel to expose the wheel nut. Next remove the wheel nut and the wheel will slide off the drive shaft. Once the wheel is removed take a minute to clean the drive shaft gear. Install the new drive wheel by sliding it onto the drive shaft and aligning the drive gear with the wheel gear. Secure the wheel with the wheel nut and replace the hub cap.

The drive cable connects the drive bail to the transmission. It allows you to engage the mower’s self-propel drive and in some cases, varies the speed of the mower. The cable attaches to the bail and to the transmission using a cast connector or a metal hook on either end. There is usually an adjuster on one end of the cable that allows you to adjust the tension of the drive belt. Drive cables wear over time and eventually fail. Common types of failure are the cast connectors or metal hooks breaking or the cable itself snapping. If the mower is left out in the weather the cable will rust and break much sooner than a mower that is stored out of the elements. A mower stored in the weather is also susceptible to having corrosion forming inside the cable’s sheath. This will make the cable stick and not operate smoothly. Accidents occasionally happen that can damage the cable. The cable can be caught on a shrub or low tree and kinked. If the cable is allowed to rub against a fence or wall it can be worn thin and eventually snap in two. If you suspect the drive cable might be the reason your mower no longer self-propels, there are a couple of things you can check. In normal operation, the drive bail will spring back to the stop position when you let go of it. If it doesn’t, the cable might have broken in two. Take a look at the connectors on both ends of the cable and make sure they have not broken. Replacing a broken drive cable is a pretty easy repair. Start by removing the connector from the drive bail at the top of your mower’s handle. Cut any cable ties and remove any fasteners that secure the cable to the handle. Next, you will need to access the cable at the transmission. Usually this is done from underneath the mower or sometimes by removing a panel at the rear of the mower. Then remove the cable where it attaches to the transmission. The new cable is installed in the reverse order. Make sure you replace any cable ties that were cut when you removed the old one so the cable doesn’t snag on shrubs or trees. Last, you will need to adjust the tension on the drive belt using the adjuster on the drive cable.