Alternator on during start or no?

Alternator off until your engine is running?

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James331

James331

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James331
For those so equipped, just curious what most folks do.

I've always had it off until I started up, then I'd flick the other alternator half of the switch on. Seems like outside of me and my students most people use that double switch as a single.

Just curious
 
Always used it as a single. POH never said anything other, but I've known pilots who do it like yourself.
 
It depends on the alternator. For belt-driven, I turn everything on before starting the engine.

For front-mount Continental alternators (which are gear-driven) I recently changed my procedure to alternators on after start. I have heard that with ALV-9610 alternators (which the 414 is getting right now) there have been reports of the adapters shredding when started with the alternators on. This didn't occur on the ALV-9510s apparently. It might be that the 9610s are able to push more current at starting RPM, and thus there's a stronger torque. Regardless, having an adapter shred on a front-mount Continental throws bits into the engine, which is really bad. So even if it does no good, it won't hurt anything. I have on occasion forgotten to turn the alternators on, but the 414 reminds me quickly with a "Low Volt" annunciator light.
 
I had an old CFI tell me that it was bad for the diodes in the alternator to start with the alt side of the swithc on. I figure it can't hurt to do it that way even if he was FOS
 
I used the rocker switch on the 182 as one switch during start up and shut down, on the 421 I started the engine and then turned on the alternator.
 
Logically, there is no need for the alt field to be on during engine start because the alt cannot generate power anyway and it only increases the strain on the starter. *shrug*
I think it's just a convenience thing so that us forgetful pilots don't forget to start charging.
 
My non-scientific way is I start with the alternator off.

I figure use all available battery power for the start, once started I turn the other switch on. Not sure if there is any truth to my theory.
 
I've always used it as a single switch too and the way my CFI's taught. Unless for whatever reason the battery may be flat and it might have trouble starting then I'd leave the ALT turned off to save on any amp loads, otherwise it's both at once for me.
 
When I was new I did the batt side only but I'd occasionally forget the alt switch. so.... Alt on for me. It won't create any drag until the motor spins up. Lots of years, no ill effects.
 
Stewartb said:
When I was new I did the batt side only but I'd occasionally forget the alt switch. so.... Alt on for me. It won't create any drag until the motor spins up. Lots of years, no ill effects.
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If anything always was more concerned with the extra draw on huge battery, especially if it's super cold.
 
Like I said, no iff effects. Besides, good batteries don't care if it's cold. It doesn't bother my little Odyssey.
 
Depends on usage, time cold soaked, etc etc.

I even with a well exercised sealed battery, I just don't see the benefit of starting with alternator on, minus the you can't forget to turn it on bit.
 
No, it doesn't matter. A good AGM battery will spin the engine just fine at -40, assuming the engine itself was preheated. That's why so many guys in Alaska use them!
 
If my skywagon didn't come with a brand spaankin new....gill :mad:, I'd go AGM all day long, I just can't toss a new expensive battery. Funny thing is the old owner could have gone AGM for nearly the same price.


But to my question, asides from the forgetting to turn it on, are there benefits for starting with the alt on?
 
I've always started with alternator on. I've never been taught differently. I've been flying & instructing since the mid-70's. Some of the nit-picky technical questions that arise these days amaze me. Things were much simpler before the internet. We thought we did everything correctly.

My old 182 just has a master switch & a start button so I don't have the option. It's been updated from a generator to an alternator too. I guess I could pull the alternator circuit breaker out though...if it really matters.
 
The alternator's field draws about three amps (14-volt, 60-amp unit). On a cold day, cold engine, cold battery, it may make a small difference in cranking power if the alternator is left off for start.

Someday when the drive belt is off, turn on the master and alternator and try to turn the alternator pulley. It takes a bit of torque, but not a lot. Turn the alternator off and try it again to see the difference. Again, in marginal conditions, it can make a difference.

Car alternators don't turn on until the rotor spins up and energizes the stator terminal, which turns the regulator on to feed the field.
 
Dan Thomas said:
The alternator's field draws about three amps (14-volt, 60-amp unit). On a cold day, cold engine, cold battery, it may make a small difference in cranking power if the alternator is left off for start.

Someday when the drive belt is off, turn on the master and alternator and try to turn the alternator pulley. It takes a bit of torque, but not a lot. Turn the alternator off and try it again to see the difference. Again, in marginal conditions, it can make a difference.

Car alternators don't turn on until the rotor spins up and energizes the stator terminal, which turns the regulator on to feed the field.
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Yep.
 
Dan Thomas said:
Someday when the drive belt is off, turn on the master and alternator and try to turn the alternator pulley. It takes a bit of torque, but not a lot. Turn the alternator off and try it again to see the difference. Again, in marginal conditions, it can make a difference.
Click to expand...

This. It's a minimal work difference unless you've got extreme cold or a dead/ailing battery.

In the twin, the POH calls for turning off the alternator on the dead engine side for an engine restart, however. That's probably more to do with protection of the operating electrical bus than worrying much about the starter having more work to do.
 
James331 said:
For those so equipped, just curious what most folks do.

I've always had it off until I started up, then I'd flick the other alternator half of the switch on. Seems like outside of me and my students most people use that double switch as a single.

Just curious
Click to expand...

182T POH says BAT AND ALT on prior to start, so that's what I do.

I don't quite use it as a single. I like to turn the alternator off and then back on during run-up to see a change in the ammeters.

Honestly, the alternator field coil does next to nothing while cranking.
 
neilw2 said:
My non-scientific way is I start with the alternator off.

I figure use all available battery power for the start, once started I turn the other switch on. Not sure if there is any truth to my theory.
Click to expand...

There is truth at the margin, but if the difference between starting or no is the amount of current draw by the alternator field current, your battery needs service or replacement.

-Skip
 
All the checklists I've seen just say "Master — On". If they wanted you to turn them on separately, it would say so.

A Warrior III POH, which has separate switches for the battery and alternator, has you turn both on before start.

On the older 182 where the alternator switch is completely separate, I left it off until after start, but for no particular reason.
 
My POH says "Master - ON". The only time I've used the split switch was to remove and re-enable the field voltage in-flight when I had a flaky alternator that was on its way out, but not quite completely dead yet.
 
My POH says "Master / Alternator Switch - ON"
 
add me to the start first, then alternator on club, although my POH only says master on. (69 Cherokee 140B). Learned in a 172G which only had a single switch. just kinda made sense to me to isolate field current during start, but my partners start with both sides on....go figure.
 
I seem to recall that Cessna came out with a recommendation to leave the alternator off until after start, but I don't have a source for that.
 
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