Yes Ant1845b, I perform this remedy on start wire circuits that are problematic within the wiring having excessive voltage drop on the start wire. But keep in mind, I don't do this necessarily as a "fix" for problem wiring as much as I do it for a customers benefit. When a person is paying you X number of dollars per hour to perfom a repair, this gives them a choice of A) you may have to pay me for several hours work to go through your wiring system searching for the wiring gremlin, or B) pay me for this simple, inexpensive, solution that just may last many more years of service. And I've yet to have one come back to me.
But anytime you have cranking problems, whether it's automotive or machinery, the same systematic method of troubleshooting should be followed. Condition of battery (preferrably with load test), battery terminal connections, condition of cables, all termination connections including grounds. Keep in mind, just because you find a termination connected, fasteners are good and tight, corrosion can still develop between terminal, fastener, and termination and cause a problem. Once you have determined battery and all cable connections are in good order, and you still get click click, then it's time to take a look at voltage drop at start wire.
Simple test, disconnect start wire from termination at starter solenoid. Connect a volt meter to start wire, hit start button. Start wire should have 12V +/-. Reconnect start wire to starter terminal, connect volt meter to start terminal as well, hit start button. If voltage drops to somewhere around 7 to 9 volts, likely you have problem with start wire and relay may remedy your problem, but it could be a failing solenoid coil as well pulling excessive amps. Take a small gauge jumper wire, 14 or 16 gauge, and jump from start terminal to battery positive connection at starter, if you get a good arc and still some hesitation from solenoid, likely solenoid is going south. But if it cranks solid without any coaxing, fit a relay on the start wire circuit.
As for wiring diagram, you're Case dealer should be able to provide that for you. And the fuel sending unit, I typically disconnect sensing wire from sending unit and see what position gauge is in whether it reads F or E. Then ground the sensing wire as see if gauge goes opposite direction. If it does, it's typically a bad sending unit.