print out the wiring diagram as big as you can then colour in the wires ( pinch the derwents of the kids/ grandkids ).
Starting at the solenoid control wire going back to the ignition switch.
It makes trouble shooting a lot easier.
JD do it with most of their manuals so suddenly they are not just a mass of confusing lines
Then go to the mower, paper in hand & follow the wiring back back testing ( poke the probes hard up the back of the plugs ) the wiring all the way to the solenoid.
Don't worry too much about the numbers just keep checking for voltage as back testing does not necessarily give you a good clean contact, particularly on a mower.
When you get down to the solenoid some have a resister in them ( to protect the starter motor ) and need 9V to trigger.