Imagine my surprise...

Red wires are not supposed to turn black.  The vibration of the unit caused the conductor to flex where it rested on the raw edge of the conduit ending. It took 42 years to break.

Imagine my surprise when it WAS a broken wire.

I got the junction box open courtesy of penetrating oil and patience.

Everything was fine in there.

Taking resistances from the junction box to the mechanical contactor in the unit showed OL on the red wire.

Disconnecting the wire and wiggling it did not reveal any limp stretches.

And then I looked where the wires came out of the feed conduit. Zowie! Signs of heat.

According to my friends who are industrial electricians, 99% of the breaks will be at the lug, i.e. terminals. This was the one percent that was otherwise.

In an industrial setting there would often be strain-reliefs or "grips" (locally called  Callems after a brand name) to distribute the flexing.

I guess I will be heading to the big city after church to buy some 8-gauge, multi-strand wire. I might as well replace all three wires. I think the $45 for the wire will still be cheaper than a service call.,

Gratuitous picture of the disconnect that was in the junction box. I filed the ends of the blades to make them bright, shiny copper in case that was the problem.  Obviously, it did not fix the problem.