Most John Deere lawn mowers are equipped with an electric PTO (Power Take Off). The PTO is the mechanism that turns on the blades located under the mower deck. The PTO works off of a 12-volt system. The electric clutch is operated by a switch located on the dashboard of the lawn mower. When the lawn mower is running and the switch on the dashboard is engaged to the "on" position, the blades will start to turn. If they don't, you have to troubleshoot a portion of the electrical system.
Park the lawn mower on level ground and set the parking brake.
Open the hood of the lawn mower. Directly behind the engine is the battery. Take your volt meter and clip the positive lead (+) of the meter to the positive (+) battery post. Take the negative (-) lead of the volt meter and and clip it on the negative (-) battery post.
Turn your volt meter to the 12-volt setting. The meter should read 11-12 volts. If it reads less then 11, replace the battery.
Check the battery to see that it is between 11-12 volts. If it is, your battery is charged. Start the lawn mower while the volt meter is still attached. Your meter should read between 13-14 volts while the lawn mower is running. If it reads higher than 15 or lower then 13, the voltage regulator (alternator) needs replacement.
Check to see that the volt meter reads between 13-14 volts. If it is, your voltage regulator (alternator) is working fine. Remove the volt meter leads from the battery. Turn off the lawn mower. The electric clutch is located under the front of the frame and is bolted to the crank shaft of the engine. On the top of the clutch there is a connector. Remove the wire harness connector, place the flat head screw driver under the tab of the connector and lift the tab and pull connector free.
Look inside the wire harness connector for two metal tabs. Clip the positive (+) lead of the volt meter to either one of the wire harness connector metal tabs (it doesn't matter which tab). Take the negative lead from the volt meter and clip it to the other tab. Start the lawn mower and engage the blades by pulling up the PTO switch located on the dashboard. If the volt meter reads 12 volts, replace the electric PTO.