Transmission Belt Keeps Coming off Pulleys

transmission belt on pulleys

You're mowing with your riding lawn mower and it suddenly stops moving. The engine's running fine, and the mower blades are spinning, but the mower just won’t move. You wonder what's gone wrong.

A cursory inspection reveals the transmission drive belt has come off. You do the necessary maneuvers to get it back in place and once again your mower will move. A few minutes later, the same problem happens.

Now what should you do? This is one of the many frustrations of life, but can be remedied with a careful and conscientious following of suggestions in this article.

Belt Condition

Examine the condition of belt. Is it a twisted “floppy rag”, or is it stiff and straight? Floppy belts need to be replaced as they will continue to run off pulleys.

If the belt has any twists or kinks, it will need to be replaced, as when the twist or kink comes to a pulley, it most likely will jump off the pulley. A belt that is stiff and straight may be used again.

gloved hand testing engine transmission belt

Pulley Condition

Check pulleys for rotation and straightness. If idler pulleys will not rotate they most likely have seized bearings and will need replaced.

Spring-loaded idler pulleys need to be able to maintain belt tension, so be sure they can move freely on their pivots. Tension springs need to be connected at both ends to maintain belt tension.

Pulley Alignment

Carefully check the alignment of the pulleys including the motor drive pulley, any and all idlers, and transmission(s) input pulley(s). Yes, the Zero Turn mowers have two transmissions usually powered by the same belt.

Any up/down movement of pulleys will contribute to the belt coming off. If you find the pulleys misaligned, determine the cause. Once you figure out the cause of pulley misalignment, you can correct the problem.

Transmission Input Pulley

Check the tightness of the input pulley fastener as a flopping pulley will “throw” the belt. Be sure the cooling fan that is integral with the input pulley is not contacting the belt and causing it to come off the pulley.

Transmission Mounting Points and Bolts

transmission belt on gears and pulleys

Broken mounting brackets and/or missing/broken bolts will cause misalignment and that can result in the belt coming off pulleys. Carefully check for bolt tightness and tighten them as needed.

Replace any missing or broken bolts. A good maintenance practice is to check all transmission mounting bolts at each oil change time.

The vibrations inherent in riding mowers will cause bolts to loosen and the strains imposed on transmission mounting points lead to their failure. Loose bolts will add to the failure of the transmission mounts.

Loose or missing bolts will allow the transmission to tilt, and that will misalign the pulleys and cause the belt to come off the pulleys.

Engine Mounting Bolts

Check for the presence of and tightness of the bolts that fasten the engine to the mower frame. If these bolts are loose or missing, that will allow the engine to get out of alignment, and in turn, the belt can come off.

The vibration mentioned earlier can loosen any bolt on the mower, causing misalignment. It's good practice to check all bolts for presence and tightness on a regularly scheduled interval of time or use.

Engine Pulley and Belt Stabilizers

The engine pulley is held on with a bolt up through the center of the pulley and threaded into the engine crankshaft.

engine crankshaft

If this bolt gets loose the pulley will lower out of alignment and the belt will come off. This bolt can fall out of the pulley causing the pulley to come off the mower engine.

The engine pulley has some type of guide pins/belt stabilizers to aid in keeping the belt on the pulley. If these become loose or fall off they will not be able to maintain belt and pulley alignment.

Getting Things Back to the Correct Position

Follow all the steps listed here and your mower should perform well and belt should stay aligned in pulleys.