Update.
The problem appears to have been the relay.
Here is how I figured it out.
I tried the tractor again. As it had a few times before, on the first key turn, I got one click and dash lights, but no start. But subsequent key turns got nothing (not even lights).
I took the 'suspect battery' out and tried it in my other (larger) tractor. It started up immediately, suggesting the battery wasn't bad after all.
I then put an almost new battery in the tractor that wouldn't start, and it still didn't start. It did, however, also show lights and gave a click on the first turn only.
I swapped out the relay and the tractor started right away. I then put the old relay back in, and the tractor didn't start.
I put the 'suspect battery' back in the tractor, with the new relay, and it starts fine.
I got no repeat of the spark in any of this. I can only surmise that there was perhaps a loose connection at one point which got tightened up in the changing of the batteries, although I didn't do anything else differently.
Thanks to everyone for their input!
The problem appears to have been the relay.
Here is how I figured it out.
I tried the tractor again. As it had a few times before, on the first key turn, I got one click and dash lights, but no start. But subsequent key turns got nothing (not even lights).
I took the 'suspect battery' out and tried it in my other (larger) tractor. It started up immediately, suggesting the battery wasn't bad after all.
I then put an almost new battery in the tractor that wouldn't start, and it still didn't start. It did, however, also show lights and gave a click on the first turn only.
I swapped out the relay and the tractor started right away. I then put the old relay back in, and the tractor didn't start.
I put the 'suspect battery' back in the tractor, with the new relay, and it starts fine.
I got no repeat of the spark in any of this. I can only surmise that there was perhaps a loose connection at one point which got tightened up in the changing of the batteries, although I didn't do anything else differently.
Thanks to everyone for their input!