One side dies on runup

In my (limited) experience, faults with the ignition modules are either:

1. Will not start a cold engine but are OK when running.

2. Ignition switch cannot turn off ignition.

3. Ignition module does not work at all.

Although intermittent faults with the module are possible they are less likely.

Find the wiring diagram in the heavy maintenance manual and check the resistance of the ignition switch as seen at the connector to the modules. It should be open circuit when the switch is in the 'on' position and short circuit to ground when it is 'off'. There could be a fault in the wiring so I wouldn't condemn the module just yet.

It could also be the generator coil or its wiring, you can also measure the resistance of that, the limits are in the manual or you can compare it with the other module's generator coil.

We would up pulling the modules and sending them to Lockwood for hence testing.  Module B has significant problems and needs to be replaced.  Knowing that when one of a pair goes bad the other is probably not far behind, we’re replacing both and banking A for an emergency spare.  Thanks to everyone for their help.

Any update on this? Having exactly the same issue. Ignition B was borderline at runup recently and then I just tried to investigate further and this exact thing happened. No drop from ignition A and engine shutoff when ignition B only. 

Additional info: my last landing the engine was running rather rough, but runup was in spec 30 minutes earlier on takeoff. On the same approach the CHT indicated 3000+ for about 15 seconds and then normal just before touchdown. 

Lastly, I sumped the fuel at the filter and had air coming out. 

I had the exact same behavior happen to my 912ULS.  Wound up replacing both modules, now have a spare in my kit.  But switch leads first, as previously suggested.  It;s pricey, but a simple switch out,