By the end of harvest most farmers are sick and tired of being around their combine. Their first impulse is to park it, forget it and move on to fall tillage, hauling grain, and the hundreds of other chores that piled up during harvest. The idea of spending a day cleaning a combine before putting it away for the winter doesn't sound like much fun and often gets delayed till mid-winter or the second week of Never.
Here are the facts: it WILL take all of a day for one man to do a thorough job cleaning a combine and prepping it for storage. The good news is that all the time and effort is not wasted. From my experience, a combine that is stored direct from the field will cost its owner an extra $200 to $500 before or during the next harvest. Much of that expense is related to corrosion/rust/water damage from rotting crop debris in or on the machine.
Some of that damage takes a long time to show up. I spent $1000 of a customer's money last week chasing gremlins through the electrical system on his combine. The pricey diagnostics eventually found a corroded electrical connector buried in three or four year's worth of rotting crop debris. Simply cleaning the combine before storage each year could have saved that guy $1000.
Cleaning combines is simple but boring and time-consuming. At a minimum, the same day harvest is finished, remove the sump covers for grain tank unloading augers and clean out rotten grain so precipitation won't accumulate in the grain tank. Open and clean the rock trap. Open the lower access doors on clean grain and tailing elevators. Use a high-pressure air hose to blow off loose, dry debris.
Some farmers avoid pressure washing their combine, due to concerns about forcing water into bearings, computer boxes and water-sensitive components. My attitude is that pressure washing does a great job of cleaning a combine with minimal risk--as long as the person running the spray wand has an I.Q. higher than a chimpanzee. Don't direct high-pressure water directly at bearings or computerized components and things will be fine.
If you're going to do maintenance and repairs on the machine within the next month, leave chains dry or spray them with WD-40, JB-80 or some other lightweight lubricant. If you won't touch the machine till next harvest, lube the chains with chain lube. The same applies to greaseable bearings and pivot/wear points.
Final step before walking away from the machine is to toss lots of mouse and rat poison into and on the machine. Clean machines are less prone to vermin damage than machines with lots of tasty crop debris on them, but even clean machines can suffer mouse and rat damage--the darned critters like to gnaw on the coatings of wires, which leads to short circuits and expensive repairs down the road.
As for problems with raccoons, 'possums and other large critters--if there's no straw or debris in a stored combine, those varmints are less apt to set up housekeeping and build a nest in a radiator shroud or deep within the separator. And if you think that having those critters nesting in a combine isn't a big problem---try cleaning up the results after you start the combine next summer and wrap a 20-pound 'coon around the clean grain auger or bust off all the engine fan blades on a 'possum that was snoozing in the radiator shroud.