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Yeah, they were naive using a 16V cap.
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It's not the voltage value that's the issue. A typical rule of thumb for any design engineer would be to use a capacitor with a voltage rating at least 25-30% higher than the system calls for. Add to this the fact that a 16V cap can withstand overvoltage conditions for a period of time without sustaining damage (I'd be surprised if we would ever see 25-30 volts DC with huge amounts of ripple current anywhere in these cars no matter what conditions exist). The problem is the dielectric material. Large capacitance values (in this case 100uf) require a dielectric substance that will provide a high amount of capacitance in a small amount of space. The only type of capacitor that will provide this is an electrolytic capacitor. If we tried to use film, the capacitor would be prohibitively huge and expensive. While the upside of an electrolytic is large, cheap capacitive properties in a small space, the downside of is that the dielectric material dries up over time, and the capacitor simply fails, as is the case in these clocks.
For awhile I suspected that certain weird CIS problems may be caused by drifting or failed capacitors in the control box. Since most of the caps in the control box are film, I suspect the likelihood of that being the issue is lower than if the caps used were electrolytic. Some day I'll reverse engineer the circuit just for kicks.