Some of the old alternators ran the regulator off the ignition - usually the ones with the external voltage regulator. These are great for modifying. The newer ones are all built into the alternator and after taking one apart late last year I realized how hard they are to modify plus they rely on the residual magnetic field in the rotor to energize the field windings enough to produce enough power to power itself up which if left spinning and disabled may not work as the magnetic field disapates.

The startup spike from mechanically switching the field on and off could result in alternator spikes to the battery which if not properly absorbed would end up spiking the entire electrical system of your car. Remember you could be working with a 90 amp alternator or greater - one weak connection and a current spike will produce a voltage spike. It would depend upon the feedback responce time of the regulator and the RPM that it is spinning at the time that you switch it.

Metrompg, if you want to instantly enable/disable your alternator, interrupt the field with a switch.

The brushes power the field so any wire (internally in the alternator) that leads to the brushes is a good candidate for a switch. Preferrably NOT the one connected directly to the carbon brush.

I don't think it will fry itself, and you won't have to wait 20 seconds (some alternators never fully unload once excited) or experiment with external regulators. It's the same "circuit" mod as the kill switch, only instead of the ignition coil it is the field coil.

I would suggest as cheap insurance from being stranded that you leave a scangauge readout on volts however, or check it often, at least initially. You already know how to manage a vehicle without an alternator if it stops charging for any reason.

And like randy mentions, the fan is going to make noticeable resistance at speed.

Sounds like AC clutch time again if you want to avoid that little penalty :) (interuupt the torque input instead of the field)

Not sure if this helps but I have a 1986 CRX HF and I am told that the alternator already has this feature.
It shuts down the field on aacceleration. I have never verified this.

Yes, I've read this about the VX also.

True. That's why I'm mainly interested in doing this as a winter-only mod. In the summer, I'll probably go back to unbelting it for local driving to avoid the fan losses. There are losses with an electric clutch too, which is why I'd avoid it as well.

This is similar to a response I got from someone on the EVDL: find the voltage sensing wire and feed it 14v to fool the voltage regulator into shutting down alternator output. Though he wasn't suggesting using the 14v source to actually charge the car's battery - just as an input voltage to switch the alt off, then switch to the car's actual battery voltage to resume normal alternator function.

But I want to keep it simple. I want to avoid having to bring a second battery along (even if it's just a small alkaline pack that gets me to 14v).

That sounds like the simplest approach, if I can figure out which wire it is and trace it back (or just splice it near the alt and run wires into the cabin).

If for some reason this isn't an option, then skewbe's idea of putting the switch inline with one of the brushes is the next idea. (I was surprised that the alternator even HAS brushes - I thought it was AC. Here I go revealing more of my electric ignorance! :D)

I can avoid potential problems of voltage spikes by NOT flipping the switch when the engine is running. Do that only when it's off.

Yes there are brushes in the alternator but on the low current rotor field side - the brushes are contacting slip rings instead of a series of segmented contacts like in a DC generator or DC motor. There are no gaps on the contact surfaces to cause wear. Varying the voltage to the brushs feeding the rotor winding provides the variable magnetic field in the rotor to regulate the output voltage as the demand and the speed of rotation varies.

So when people talk about a "brushless DC motor", or a typical AC motor, they are not actually brushless - it's just that the brushes are on a slip ring rather than commutator?

There are no brushes or slip rings in a typical brushless DC motor. For most brushless DC motors we make where I work, the motor drive circuitry controls phases in the stator windings and the rotors are typically permanent magnets.
Most AC motors are induction with no brushes, some are universal motors (vacuum cleaner) and they have brushes, but can work on DC or AC. There are some AC motors that use brushes and slip rings too. There are so many types of AC and DC motors that you can find all different combinations of things.

The main difference is your alternator uses electromagnets where a brushless motor uses permanent magnets (on the moving part).