DIYbanter - Puzzle (of the automotive and electronic type)

I have an old Toyota FJ40 (1979) with 'transistorized' ignition. A
reluctance pickup wheel in the distributor and solid state module to switch
the coil in place of the old points.

Its starting to get a bit flakey. From time to time (and more frequently
now) the ignition just cuts out. Just like turning the key off. But it
still has +12V to the coil. After starting the replace parts until its
fixed method, I'm down to the solid state module now.

But, and here's the puzzle, one of my diagnostics for spark/no spark has
been to throw an old timing light on to the coil output. Oddly enough, that
fixes the problem within a few seconds. The engine fires up and runs fine
for a few days. Otherwise, its crank, wiggle wires, poke around with a
voltmeter to no effect.

At first, I figured that too large spark plug gaps were causing the coil
secondary (and primary) currents to decay too slowly and that this was
screwing up the module's operation (bad flyback diode perhaps?). The
addition of the timing light was like putting a gas discharge surge
arrester in the circuit, clamping the primary voltage quickly. But this
failure continues after having changed plugs, cables, cap, and rotor. New
coil too. So everything should be within spec. And when it runs, it runs
beautifully.

I'm pretty sure its the solid state module that's got to go*. It's the only
old part left. But I'd like to understand the cause behind the failure
while I fix it. And this is an interesting puzzle. BTW, the module is
potted, so I'm can only guess about its innards.

Any thoughts?

*A replacement is $350. I'm pondering building a DIY unit from scratch. With
a few test points brought out as well. I've found a few DIY projects on
line (including one with the obligatory uC).

--
Paul Hovnanian
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