[Mogasaurus]

I have even seen a few of these that had the W terminal for the Tach output.

But, probably not the same number of poles as the original. The tach would probably not read accurately.

 

[Mogasaurus]

That's a really good question and I have not heard a definitive answer, but...

Very simply (and Bob did say it), when the ignition is OFF (0 position), the alternator has no load - it's driving an open circuit. The voltage can (might, could, ect) build to a point where it destroys the regulator (internal or external) or arcs the winding.

Any AC generator could possibly destroy itself in a no-load situation - your truck, your home back-up generator, your power companies big units, Hoover Dam, 3-Mile Island, any of them...

Kinda like the 'knowledge' that setting a battery on concrete will ruin it. That was true decades ago when battery cases were made of hard rubber which became porous, absorbed acid, and conducted, thus shorting itself into concrete. That's no longer a problem.

I always wondered why my Dad told me this - being a 1970's educated electrical engineer, it never made sense to me. I always guessed it had something to do with the cold concrete floor somehow screwing up the temperature stability of the battery under charge. Thanks Bob - now I can remove the various pieces of plywood I always sit the batteries on when they're out of the vehicles !!!

 

[Mogasaurus]

Well, thank you all for the feedback on the alternator and my misunderstandings of overloading it. So, would running it in the off postion also fry lights, and other electrical things? or would the fuse system stop that?

IF the switch system connected the alternator output to the DC accessories when in position -0-, running with the lights on would actually provide the load desired. But, seeing it's position -0-, it does not connect anything

Here's an example of a proper, but unexpected, alternator load - MY truck is set up with a master battery disconnect. Running the engine with that switch open (no battery) denies the alternator it's load and could possibly harm it. BUT, leave that switch disconnected (open), turn on all the lights and the alternator would be happy and harm free. That 20A (or so - I have all aftermarket high energy bulbs) load simulates the battery.

(edit) BTW, a simply fuse system will NOT reliably protect anything from high voltage spikes - only (extraordinary) overcurrent conditions.

 

[Mogasaurus]

Also, how are these spikes created? and, if the key was in the off position, then now worries about currents running through the trucks electrics, right?

The generator windings create a electromotive force (EMF) as they rotate, through the principle of electromagnetic induction. This is the fundamental of all rotational voltage generation as discovered by Faraday/Lorentz/Maxwell in the early 1800's. When there is no place for this force to dissipate, it can grow larger then the designer of the generator intended. If it gets too large, damage to sensitive electronics can occur.

In a modern alternator equipped vehicle, this damage could occur to the regulator semiconductors. In older, generator equipped vehicles, the possible damage could be to any connected electronics, indicators, coils, mechanical regulators, ect. The potential peak voltages from a automotive generator probably wouldn't be too great since it's so small and limited in rotational speed - most probably less then 100V.

In a Mog, any damage would probably be limited to the alternator or generator itself unless you had some kind of ultra-modern, cheap (no protection circuits) gauge, GPS or other consumer device attached to the DC line. If the key was -OFF-, yes, no worries as nothing is connected. If there WAS something connected, then we probably wouldn't need to talk about damage as there would likely be enough of a load to prevent the root cause in the first place