The John Deere 4020 tractor was manufactured with a pair of 12-volt batteries wired in series. Both batteries are wired to the 24-volt starter, with all other electrical systems being divided down the middle. One battery runs all the electrical systems on the left side of the tractor, while the other battery runs all the systems on the right. However, it can be difficult to keep both batteries evenly charged. Therefore, it may be preferable to convert the tractor to run from a single 12-volt battery using a modern 12-volt starter.
Disconnect and remove the batteries. Remove the generator by disconnecting the generator belt from the fan and unbolting the generator from its mount. Remove the fan along with its mount and the voltage regulator.
Install a 12-volt alternator on the generator mount. Use an alternator tensioning bracket to hold the alternator in place and secure it using four three-eighths threaded rods. Connect the alternator to the water pump using the generator belt.
Run a length of No. 10 wire from the battery terminal on the alternator to the ammeter. Connect the battery terminal to the "2" terminal on the alternator using a short length of No. 16 wire.
Remove the 24-volt starter. Place the 12-volt starter on its bracket and secure it using a pair of 1-inch tension bands with three-quarter-inch bolts. Connect the positive and negative leads from the starter to the corresponding terminals on the alternator.
Connect the ground lead to the negative terminal of one of the 12-volt batteries. Connect the starter cable to the positive terminal of the battery.
If your tractor was outfitted with 6-volt bulbs, replace them with 12-volt bulbs before connecting the battery. The alternator will provide a full 12 volts to the sockets, and will burn out 6-volt bulbs in short order.