There are two service publications that are "must have's" for most John Deere Two-Cylinder tractors: Electrical Systems and Hydraulic Equipment. (Click on the diagrams to see them full size.)
The Electrical Systems manual (SM-2029) contains just about every piece of electrical information you could possibly want for a John Deere Two-Cylinder Tractor. It contains (or at least used to contain) the following sections:
The magneto section contains information on how magnetos work, along with information on servicing them. This appears to require several special pieces of test equipment for testing the magneto condenser, the magneto coil, and determining the polarity of the magneto rotor.
The wiring section contains wiring diagrams for all sorts of configurations of the following tractor models: M, B, A, G, D, R, 40, 50, 60, 70, 80, 420, 520, 620, 720, 820, 430, 530, 630, 730, 830, 440, and 840. It also has diagrams of the 92, 145, and 217 series power units. I have included one diagram here so you can see the kinds of things they show. I will NOT send copies of these diagrams to anyone. For one thing, they are copyrighted. For another, they are difficult to scan using my current set-up. Finally, for about $30 you can buy the whole book.
Section A of the Service Data chapter consist of tables which show the test readings that should be obtained if these various items of electrical equipment are in proper working order. The tables also contain certain other specifications, such as brush tension and point gaps, which are involved in proper adjustment of the components. These components include generators, regulators, cranking motors, magnetic and solenoid switches, distributors, condensers, and magnetos.
Section B contains a list of all John Deere tractors and engine-driven machines and indicates the generator, regulator, cranking motor, distributor, and magneto used on each.
Section C contains a list of electrical components and shows the John Deere tractors or engine-driven machines on which each is used.
I also have some links to other electrical articles that may help you out.
If you own or work on John Deere H, B, A, G, R, 50, or 60 that has either Power Lift or Powr-Trol, then the Hydraulic Equipment Service Manual (SM-2011) is required reading. This manual covers all the principles of operation for these systems, as well as detailing how to service the hydraulic units, pistions, pumps, and other components of the system. I have included two of the diagrams from the book as a teaser. I will NOT distribute copies of other diagrams in the book for the same reasons as listed above. This book is also about $30, and worth every penny.
The table of contents is as follows:
You can't go wrong with this manual, either.
As you can probably guess, I have both of these myself, and I have referred to them extensively. If you own or work on tractors that either of these manuals apply to, I'm sure you will wear them out as well.