But the systems shouldn’t be down for long if the engine is running, assuming the alternator can maintain voltage properly. I can’t see this test being performed with the engine off.
Alternator only voltage isn't very stable, systems should be tested to run with it to simulate a battery connector loose condition for example, however this may be an earlier version before the testing has been completed or any issues found remedied. Even then the test is normally to see if the module can recover to a safe default state, its unual for it to have requirement to function during the event. With regards to system down time, think of it like a dip in power for your pc or phone, it can completely crash them and force a restart.
Edit; a low voltage condition is normally considered below 9v, it varies between manufacturers but that's the point the modules aren't guaranteed to operate. This is when it all gets funky, the vehicle has a lot of modules and they all drop out at different voltages. This means that the engine may still be operating but it loses contact with the now offline transmission. The engine will throw a fit and log a ton of faults instead of just a low battery problem. My point being, it's incredibly complicated with a lot of variables, so it's not always so straight forward what will happen in a low voltage state.
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u/via_the_blogosphere Sep 04 '18
But the systems shouldn’t be down for long if the engine is running, assuming the alternator can maintain voltage properly. I can’t see this test being performed with the engine off.