A riding lawn mower that has a bad starter can be difficult to diagnose. A bad starter can manifest itself in a cranking noise without engine turnover, a clicking when the ignition button is pressed, or a mower that simply does not respond to attempts to start. An indication of a bad starter motor is the absence of other electrical problems that can be more easily tested.
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A Functioning Battery
Test your riding lawn mower's battery. Check your manual or the battery itself to ascertain how many volts it should be producing, and use a voltage tester to make sure the battery is not to blame for your starting issues. If the voltage being put out by the battery is less than the mower requires, recharge or replace the battery and attempt to start the mower.
The negative and positive cables that attach to the battery need to have clean leads to make good electrical contact. If the battery posts or the cable clamps are dirty or corroded, detach the battery from the mower and use a wire brush with a wooden or plastic handle to clean the contacts.
Clean Spark Plugs
Remove the wires running to each of your mower's spark plugs and check the contacts for signs of fouling or wear. If the spark plug ends are oily, carbonized or otherwise fouled, this may be the cause of your starting issues. An oily plug may indicate worn piston rings, however, which means you will face the issue again even if you replace the spark plug. Replace any worn or damaged spark plugs and attempt to start the mower once more.
Functioning Starter Solenoid
The starter solenoid lies between the battery and the starter. It is a switch that, when thrown, supplies power to the starter to start the motor. Jumping the starter solenoid by turning the key or pressing the ignition button on the mower and simultaneously bridging the gap between the two bolts on the solenoid can test for a bad solenoid. If the mower starts, the solenoid is bad and needs to be replaced. If the mower still does not start, your starter is likely to blame.