Do yourself a massive favour go to the JD web site and get a copy of the technical manual for your mower.
They may come as a digital download, CD or hard copy.
While they are not cheap, and nothing that is good is ever cheap, in reality the price is less than a dealers labour charge for a routine service so it is not expensive.
What is particularly good is each section starts off by telling exactly how things work then goes through a set of simple, easy to follow testing proceedures.
They put in diagrams showing exactly where everything is hidden on the mower.
The electrical section has both a schematic diagram and a loom diagram so you know what every pin on every plug is, where the wires go & what they do
And a really big plus is they have an engine service section for every one of the engines that were fitted new which is way better than what Kohler, Briggs or Kawasaki put out.
Don't bother scouring the web for cheaper pirated copies or Deer Service compilations CD's / DVD's as most contain the same bad, incomplete or useless manuals that some keyboard pirate found.
I started doing this but ended up with things like all the words but empty holes where the photos would go, or the 2 page fold out wiring diagrams as a JPEG that can not be read.
On top of that I get an extra $ 100- $ 200 ( AUS ) when I sell one with the technical manual included thus making a 100 % ( or more ) profit on the manual alone.