NWL,
and there is power to the starter
How did you ascertain this? You measured voltage where? (And note that the presence of 12V doesn't imply that there is any power available; it just means it might be available.)
truckie36's post does apply to your 870, and the picture is close. The "always hot" wire will be a heavy gauge black wire, and the "hot when cranking" will be a lighter gauge wire which is white.
As truckie36 says, check for voltage on both wires when the key is in the spring-loaded crank position. And as truckie36 says, "If you
have power at these locations then the problem is probably the starter (assuming there are good connections at the starter). ".
If you
don't have about 12V on the white wire, then there could be a problem with a "neutral start" relay which must be closed in order to for voltage to be present at the white wire.
The "neutral start" relay is closed/energized only under "safe conditions" which involve several components. The key must be on (could the key switch be bad?) The seat safety switch must be closed, either because there's weight on the seat, or it's been defeated by tilting the seat forward and pulling up on the switch.
In addition, the seat switch must be closed for some length of time (pretty short, maybe 1 or 2 seconds, so if you hit a bump and bounce up off the seat, the tractor doesn't shut down). There's a "time delay control module" that provides for this delay.
The PTO safety switch and transmission-in-neutral switches are also involved. For a hydrostatic transmission (was it available for this machine?), the park-brake switch would also be involved.
Here's a series of tests:
0) Double check that you have PTO off, transmission in neutral, and for good measure, the parking brake set (that's normally only for hydrostatic transmission machines).
1) Then, with weight on the seat, and key in spring-loaded "crank" position: There should be 12V on the white wire at the starter. If so, some part of the starter or starter solenoid is bad (they're one unit).
2) If no voltage under those conditions, .............let me know and I'll post further instructions.