I was working on a D90 the last couple of days, a '94 NAS truck so 3.9 V8-powered. I noticed right off that it had the cheap aftermarket blue plastic distributor cap so we yanked that off and reinstalled the old Lucas cap that he had fortunately kept as a spare, as it was still in good shape and just need a little cleaning/de-carbonizing of the contacts. He had tossed the rotor and I didn't have any spares with me, so I pulled the genuine Lucas rotor off my RRC, gave it a quick cleaning and swapped him.
Drove my RRC about 8 or 9 miles around the are where he lives, and then last night headed for home in the pouring rain. Got about 6 miles from his house when my engine shut off as if I'd just turned off the key. Worked on it for a bit and then threw in the towel, he came down and towed me back to his house where I worked on it again this morning. Checked (again) for spark from the coil, looked good, had fuel, could hear the pump running, checked all wires and even pulled the distributor, checked timing chain as well as I could to see if it had jumped time (didn't really think so as it has a double-roller timing chain with all steel gears, not the stock chain with the fiber cam gear.)
Even tried to start it with ether. wouldn't even pop so it had to be ignition-related. Finally, out of desperation I swapped his old rotor back into his D90 and took out my Lucas unit, and his 90 suddenly wouldn't start. Put my Lucas back into my RRC and lo and behold, it fired right up! Ran a crappy at first as it was pretty flooded from all my attempts to start it, but it settled down and smoothed out to it's usual purr after a bit.
So here's the question for you electrical engineer types (and other people who are way smarter than me!)...how can a part as simple as a rotor, with no moving parts and consisting of a piece of plastic (or is it Bakelite?) and a simple strip of brass who's only job is to conduct spark from the center electrode of the distributor cap out to the various terminals leading to the plugs, suddenly fail and lose all conductivity? I can see it getting dirty and building up resistance, but this one was freshly cleaned. There's no visible reason for it not to work...I'm still scratching my head in wonder and frustration.