I might have missed it but I did not see any mention of how old the battery is.

I have fallen prey to the 'the battery's fine' because over time, the battery gets weaker. Eventually, it might spin it over, but not fast enough to fire. If it always cranks when the battery is fully charged, I might suspect a weak battery.

If you continually drive the same car, you will not notice the gradual slow down of the cranking speed, then suddenly, one day it won't fire.

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87 300SDL - 215K Miles !!
99 F-350CC Dually PSD - 190K
86 300SDL - 189K
All on B-100