Teffy wrote:

While mowing, my riding mower died. It's a Gravely G-series 16hp that
is 15 years old.

When the ignition key is turned from off to run, I hear one soft
click. The lights work in this state. If I turn the key from run to
start, I hear one loud click and nothing else.

The wiring diagram is shown he
http://home.attbi.com/~smschueler/Gr...iagram-big.jpg

The voltage (no load) across battery terminals measures 12 v.

When the key is in the start position, I measure these voltages:
8.0 v across brown wire from ignition switch to solenoid S10 terminal.
7.2 v from solenoid terminals S10 to S2 (terminals with small gauge
wires).
0.5 v from S2 to battery negative terminal.

As far as I can tell, the parts with safety switches are all in proper
position: Power Take-Off is dis-engaged, accelerator pedal is in
neutral, my butt is in the seat.

The connection from negative battery post to mower frame looks clean
and tight, and voltage from terminal to frame bolt is zero volts.

What do you think is wrong with the mower?

Thanks,
Teffy
why not just take a jumper calble for the car and jump off the big red
wire that comes from the starter to the selenoid and go directly to the
battery and see if it cranks over.. if not then you know its either the
starter or the battery or some connection inbetween... and play it from
there.. dont know what the voltage is to your battery, but if its a 12
volt one, on a car if you have under 9 volts on a 12 volt system when
crankiing its the battery thats bad....is the battery fully charged by a
battery charger???????