M Starter

How do you check the armature to see if it is good? and also the fields. Thanks
Bob

There should be no connection between any commutator segment and the armature iron/shaft.
There should be connections from segment to segment across the commutator. This can be checked carefully with an Ohm meter of high quality capable of measuring in small Ohm ranges less than one ohm. On opposite sides every pair should have very close to the same resistance.
A "Growler" is of massive help. It is an elecrtical test device that uses magnetic fields to assure the electrical status of the windings.
The armature should show no differences in winding color, or solder joints at the connections to the segments. There should be no scuffing or wear on the outside of the core laminations where the armature would roll if laid on a bench.
The segments should all look the same, and be ungrooved around the circumference.
Fields should be isolated from the frame and both should have the same very low resistance,
If different, there may be internal shorts that make one have less resistance than the other.
We are talking tenths of an ohm, not hundreds of ohms. Visual checks of the connection quality where the stud mounts is also worth while. Jim

In addition to what Janicholson has posted you should also test them them with a high intensity tester. It will show any areas of poor insulation.

The force to turn the starter comes from the magnetic fields generated in the windings. As result if the insulation gets worn the wires may move either in the armature or fields coils. This means that an armature may pass all the other tests just fine but may fail when in use because the movement of the wires will allow a short to occur. The high intensity tester will find these problems by causing an electrical arc.

The last M starter I worked on had just be repaired at another shop and still would crank properly. The armature looked good but when tested with the high intensity tester it lit up in about six places. Replacing the armature cured the problems.