All done!

I'm all done, and it looks like the failed splice was the root cause. I removed the throttle/FNR assembly, cut the remaining wires and splices, soldered and shrink wrapped everything, and then strain relieved them with Ty-Raps to the assembly itself. I put everything else back together properly and when we went for a test drive, all was well.

I inserted a quick disconnect in the lead coming from the "throttle guardian" so that if I ever encounter this problem again I can remove it from the circuit quickly without turning off the engine.

More research on my part reveals that the "throttle guardian" is a specialized rev limiter controlled by the throttle/FNR assembly. Mercury's service manual says that the engine should be prevented from revving over 2000 RPM when in neutral. Apparently they sell this module to serve that purpose, but they sure don't bother to mention it in the manual or document it in the schematics.

Since the throttle/FNR assembly prevents the throttle from moving out of idle when the FNR lever is in neutral, the "throttle guardian" is completely redundant. I was sorely tempted to leave it out of the circuit, but decided to put things back the way the manufacturer intended - just better in the sense of making things electrically and mechanically more secure. I never want to have to look under there again.

The switch to which the broken wire connected appears to be an enclosed reed switch. A magnet on the throttle lever itself approaches the switch to indicate "idle". Both are mounted on threaded members, presumably so the max idle in neutral can be adjusted. I left them where they were.

I wonder how much time and money I would have spent at a typical dealer to have them diagnose and fix this broken wire...?