I agree, I've seen it over and over. Themes solder formulations make such ugly looking joints that it's nearly impossible to tell from a visual examination if the joint is good or not. The EU really has to examine this issue again. I think they made a bad call.
You wrote ". The problem was immediately apparent: several cold solder joints on one of the relays. I re-soldered every joint on the board because there was so little solder used in manufacturing that future failures were inevitable. Sure enough, the problem was solved.
"We then pulled the module on his Accord and found exactly the same problem -- exactly the same pins, even! The same remedy with the same result, the problem was resolved."
Not at all surprising. The misguided European rulings changing the composition of solder has resulted in formulations that are brittle and become more brittle with age. Electrical appliances, cellphones, cameras, music players, and cars are all failing prematurely due to this terribly ill-considered decision. I received a set-top box sent from 200 miles away, and received it with a surface mount IC (BGA) rattling around in the bottom.
I am sure you made the repairs with 60/40 or 63/37 Pb/Sn solder which should have been used in the first place.
This is not to mention the "tin-whisker" problem, also caused by the new solder formulations and suspected of causing the runaway Toyota accelerations.
Cars have used self-resetting thermal breakers instead of fuses on the headlights as early as the 1940s. There's a reason for this. On a pitch black country road on a cloudy moonless night, you don't want to be pitched into sudden darkness--and with no way to get home, just because wire insulation chafed through someplace.
You might be able to find and replace that fuse in blackness, but could your wife?
Better that you or she should drive home with blinking lights, cycling at a rate low enough to prevent fire or further wire damage but high enough for safety--to see and be seen.
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