An audible test of the ignition system

After replacing the ignitors on the engine they must be tested. The safest way to do this is to leave them in the engine and listen for the "bangs". The spark these ignitors emit is literally lethal. The engine is first dry cycled (ran without fuel) to ensure no residual fuel is left in the engine that may ignite.

There a two ignitors per engine, each has it's own High Intensity Electronic Unit, or HIEU, sometimes called an exciter. These contain big capacitors which are charged and then release the electrical energy pretty much instantaneously down the High Intensity Ignition Lead to the ignitors. The ignition system must be left for at least five minutes after power is removed so that the energy may dissipate from the capacitors before any work may be carried out.

For each engine:
There are 4 available 115v AC buses:
1. The Left AC main bus
2. AC standby bus
3. The Right main AC bus
4. AC standby bus

They then go through relays in a power management panel so they can be switched. These are then fed into the engine EEC (electronic engine controller) ignition circuitry, removing ignition power does not remove power from the EEC.

The EEC has 2 channels, A and B.
A is powered by 1 or 2 above,
B from 3 or 4.

Each channel of the EEC can provide power to either/both of the exciters. So even if there was only one AC bus powered, both exciters/ignitors could operate.

As always, please feel free to ask any questions and I'll do my best to answer them.