Safety first: wear insulated gloves, disconnect the battery cables, and ensure the tractor is on a level surface. The John Deere 4020 originally uses two 12‑volt batteries wired in series to provide a 24‑volt supply for the starter, with each battery powering one side of the electrical system. Keeping both batteries evenly charged can be challenging, so converting to a single 12‑volt battery with a modern starter simplifies maintenance and improves reliability.
Detach the battery cables and remove both batteries. Unbolt the generator from its mounting bracket, then disconnect the drive belt from the fan pulley. Remove the fan and its mounting hardware, followed by the voltage regulator. This clears the path for the new alternator.
Mount the 12‑volt alternator in the generator space. Secure it with the alternator tensioning bracket and four 3/8" threaded rods. Attach the alternator drive belt to the water pump pulley, ensuring proper tension as per the manufacturer’s specifications.
Run a length of #10 gauge wire from the alternator’s battery terminal to the tractor’s ammeter. Use a short section of #16 gauge wire to connect the alternator’s “2” terminal to the battery terminal, establishing the charging circuit.
Remove the 24‑volt starter. Mount the new 12‑volt starter onto its bracket and secure it with the 1" tension bands and 3/4" bolts. Connect the starter’s positive and negative leads to the corresponding alternator terminals, ensuring polarity is correct.
Attach the ground lead to the negative terminal of the single 12‑volt battery. Connect the starter’s power cable to the battery’s positive terminal. Verify all connections are tight and free of corrosion.
If your tractor still uses 6‑volt bulbs, replace them with 12‑volt variants before powering the system. The new alternator will deliver full 12‑volt output, ensuring all accessories operate correctly without risking bulb damage.
