Switch on and off the alternator after start to see how much power it drains from the running engine, there will not be much.
Errrmmmm….how do I work out what the alternator itself drains, from switching it on and off?
Even if every system is switched off after start in the aircraft I fly, the battery will charge quite vigorously when the alternator is on. If I leave or switch the alternator off after start, the battery won’t charge.
Is the difference in the on/off current measurements the drain of the alternator?
The Ign switch places an earth on the relevant magneto, via the P lead, to turn the mag off.
The switch also takes bus voltage and applies it to the start contactor when the switch is in the 'Start' position. At this position there is also other switching which normally applies an Earth to the right magneto. This is via an external link screwed across the terminals.
The link can be opened if the intention is to have both mags live during the start... if for example the engine has two impulse couplings fitted.
Normally only half the spark plugs are firing during a typical piston engine start. Often the top plug on one bank and the bottom on the other.
So it can be seen that while the switch is held on 'Start' it could be that the engine may only be firing on a couple of cylinders... if the bottom plugs are fouled or flooded.
That is why they will often pick-up when the switch is released.
Example number 723 showing why it’s important to have aircraft-specific systems knowledge.
I’ve looked at the circuit diagrams for ostensibly identical aircraft. Some have 2 impulse coupling-fitted magnetos, some only 1, some have retard breakers, some have ‘shower of sparks’ and other start-assistance technology fitted to only one magneto, and some to both. 4 different ignition switch wiring and shorting link arrangements, despite the switches themselves being the same, the airframes being the same and the engine (aside from ignition systems) being the same.
Be careful: all systems on ostensibly identical aircraft may not be the same.