Keep your John Deere 3120 running with more than 250 parts that fit. Alongside electrics and lighting, this page offers a wide choice of classic tractor parts.
An old John Deere 3120 tends to tell you about its electrics first: a slow crank on a cold morning or a charge light that stays on. The more than 250 new replacement parts on this page for the John Deere 3120 cover that and a good deal more, from GRANIT, HELLA, MAHLE, GRAMMER and LUK.
John Deere part numbers are often included in the product description, so the number on your old part or in the workshop manual is the fastest way in. The identification plate confirms that the tractor is a 3120. When matching an alternator, compare the flange and pulley, and for a starter motor check the number of pinion teeth and the direction of rotation.
Before replacing a starter motor, go over the battery, the earth strap and the terminals on the starter itself. A corroded earth connection produces exactly the same symptoms as a worn starter. With someone turning the key, put a meter across the starter terminals and watch the reading. If it drops a long way with a fully charged battery, the resistance is in the cabling. A gear reduction starter draws less current and is worth considering if the engine turns over reluctantly in winter.
If the engine is hard to start after standing overnight or dies under load, look at the feed pump. A leaking seal lets air into the fuel system, and that shows up straight away at start-up. Replace the seal or the complete pump and bleed the system with the hand primer until fuel comes through without bubbles. If the engine no longer stops cleanly, the stop cable has often stretched or become stiff. Adjust a new cable so the stop lever travels all the way.
A headlight that glows dimly often has a poor earth or a tarnished reflector. Check V-belt tension with your thumb as well.
The John Deere 4520 from the same series has its own page too, so if you run both, look up each one separately. The full list of models is under John Deere parts.