Rinker 280 with Volvo Penta GXi-H won't crank

hookup1

This is a new boat to me. Been sitting in storage for 10+ years. I have turned engine over by hand. Belt off, water pump off.

Rinker 280 with Volvo Penta GXi-H won't crank. Turn key to ON - voltage gauge indicates 13 volts. Turn key to START - voltage drops to 0 - no starter solenoid or starter noise.

New battery, starter removed and bench tested, Jumping starter wire will engage starter. Inspected neutral start and kill switches - everything looks good. For some reason key won't engage starter.

I have looked all over for a wiring diagram and color codes for Rinker key switch wiring and for the engine.

Formulaben

You say you "Inspected neutral start and kill switches - everything looks good."  What exactly does that mean?  Did you put a multi-meter on them and check for continuity/operation, or just put eyes on them and see no obvious damage?

Here are your wiring diagrams:
280 service diagrams.pdf

Here is a generic key switch.  Definitely check that you don't have a bad switch with thee multi-meter; you can alternatively "jump" the terminals with 2 alligator clips to see if the switch is bad.