The International Cessna 170 Association

Recently purchased a 170B with an alternator, and I have a couple of questions for the electrical gurus. I've searched through several threads and don't see the questions addressed, so forgive me if these has been asked and answered.

1.) When should the alternator switch be placed ON? Prior to engine start, after start, or does it matter?

2.) When should the switch be place OFF during the shutdown sequence or does it matter? I'm currently turning off all the radios, shutting the engine down, and then turning the alternator to OFF.

Thanks for your replies.

Most early installations have a regular 2 amp circuit breaker on the field circuit, it sounds like you have a circuit break switch instead. The later installations have the double rocker switch which control the field and master power. I think you would be safe to leave it on if is in fact a circuit breaker switch and not just a switch (i.e. you should have 2amp protection on the field circuit).

I will turn the field switch on after the engine is running and open the field prior to shut down. If you don't have a raido switch make sure you have all of your electrical items off before you turn the field on and off.

I'm in the "doesn't matter" camp. (I'd turn 'em on/off simultaneously. The only reason to have them switched seperately is in order to continue essential electrical operations (such as fuel pumps, radios, flaps, lighting, etc.) with Master ON should the alternator fail, in which case the alternator would be switched OFF.)

turn it on and off when there's no load, even though I think the normal coupling for these installations isn't the "elastomeric" type that is supposed to break loose to prevent gear damage in case of an alternator seizing. Our company has several Senecas with alternators with that type coupling and I teach the guys to never touch the alternator switches, just leave them on always. If you switch off the alternator when there's a good electrical load on it it'll jerk a bit and that's when you lose those breakaway couplings.

I think you'll have the regular type coupling like is used with the generators, and that is more robust. But it's easier on things if you just go on-off with no loads.

I turn it on after the engine is running, I like to see the Amp meter go from - to +