In the parts catalogue the John Deere 8400 belongs to the 8000/8000T range. Here you get more than 300 matching parts, including lighting and electrics.
The John Deere 8400 spends its working life on heavy draft jobs, and the parts that wear are the ones that keep it cool, keep the driver comfortable and keep implements attached. This page brings together more than 300 parts that fit the 8400, supplied by GRANIT alongside HELLA, Wabco, ROCKINGER and GRAMMER.
On the engine side there are fan clutches, thermostats, engine sensors and a hand pump for bleeding the fuel system. Seals, sealing rings and gaskets come in many forms, including a gasket for the exhaust gas recirculation valve. For the cab you will find air conditioning compressors and tractor seats such as the air suspended GRAMMER Maximo Professional, plus LED work lights from HELLA and others. At the rear there are drawbars and drawbar frames from ROCKINGER, Wabco connector heads for trailer air brakes and PTO shaft ends.
If your 8400 runs hot in summer even with a clean radiator, the fan clutch is a likely suspect. With the engine cold, turn the fan by hand. It should move with light resistance. After a run, with the engine at working temperature, that resistance should be clearly higher. If the fan spins just as easily hot as cold, or there is oil around the hub, the clutch is no longer engaging properly and replacing it is the only real cure. Since the thermostat sits in the same cooling circuit, it makes sense to change it at the same time.
Start with the serial plate to confirm you are working on an 8400. After that, the quickest route is the reference number on the old part, moulded into a gasket or printed on the housing of a sensor or compressor. Some items fit several John Deere tractors and say so, such as sealing rings listed for the 7R, the 8R and the 8000 range. When in doubt, hold the old part next to the photo and description before you order.
If you also look after another model from this range, it has its own page showing only the parts listed for that tractor, which keeps you from ordering for the wrong machine.
Every other model from the brand is listed under John Deere parts.