John Deere mower decks are attached to the chassis with tiller tine pins. A belt comes off the engine and engages the mower blades, which are attached to spindles or a bearing-and-pulley assembly that transfers the power from the belt to the blades. If something goes wrong anywhere along the line, you may notice that the mower deck is not cutting right.
Mower Cuts at an Angle
Raise the deck up all the way.
Inspect the deck to see if it hangs lower on one side than the other. It will be very noticeable. If it does, a tiller tine pin is missing. These pins hold the deck onto the chassis and keep it level.
Search for the missing pin on the deck. An empty mounting hole on the chassis will correspond to an empty mounting hole on the deck. But because the deck will be hanging lower, the holes will not initially be lined up.
Raise the deck to align the holes and slide in the pin. Secure it with the cotter key that came with the mower.
Check for a belt problem first.
Locate the mower belt directly under the chassis. It should be tight.
Pull on the belt. If it pulls right out in one long piece, it is broken and will need to be replaced. If it pulls out but is not broken, the belt is good, but it has jumped a pulley and needs to be rethreaded.
Check for a jam between the belt and the mower pulley if the belt is tight but the blades don't spin. If there is a jam, such as a branch or twig, remove it.
Reach beneath the mower deck and grab hold of each individual blade. Turn the blades to see if they move. Watch the pulley that the blade is attached to, to see whether it also moves. If one of the pulley spindles either does not turn or wobbles as you turn the blade, the bearings are bad, and the entire spindle needs to be replaced.