One solution is to put a diode from the purple ign wire towards the alternator to provide the "jump" when the ign is turned on. You need a diode, not a jumper wire, to keep the alternator output from flowing directly to the purple ign wire. I show this on my website.

What you're doing is flashing the field of a generator. It restores the residual magnitism. You don't have to do this on an alternator, it uses the battery to excite the field. A generator uses the residual magnitism. If you have a one wire (and a ground ) alternator it has a generator built into it that generates a D.C. current to excite the alternator instead of using the battery. If you have a 120 volt generator ( like a Homelite or whatever ) with no battery, the secret is to plug in to the 120 volt output while it's running and flash it with a battery. A flashlight battery is all you need to restore the magnitism, and then it will put out whatever voltage the alternator is supposed to put out. ... Now if she would only put out so easily....
..Ed

I have heard some pretty weird stuff from shops, and I'm not sure I believe this.

Alternators don't have "residual magnetism". At least not as a design feature. The field magnet (the part that rotates) is supposed to be magnetized by the field coil, which is driven through the brushes. Without any field current, there may or may not be some output on the windings, depending on whether there is indeed "residual magnetism" in the iron. But it's not guaranteed by any means, and you shouldn't need it anyway. The field current is supposed to come from the battery.

If you have an external regulator, then the Field terminal should go hot as soon as you turn on the ignition switch. This provides field from the battery.

If you have an internal regulator, then the power for the regulator, and thus the field, comes from the hot terminal. If this is connected to the battery, there is no problem.

What happened to me was that each of my single-terminal alternators was driving several batteries through a diode splitter. Current could not go backwards from the battery through the diode to the alternator, so there was no power to the alternator to run the field. The alternators did not start.

This was particularly distressing since the tach drive is from the alternator, since I have no points, and so I would be starting the engines and not able to see the rpms.

At some rpm, around 1500 or so, the alternators would "light up" and I would get both charge and a reading of engine revs. After that they would work. This, of course, was caused by the "residual magnetism" in the iron that created enough output to provide field, after which all was OK.

But this is not guaranteed, and in fact one alternator had better iron than the other and therefore started at a much higher engine rpm.

The cure is to provide field power from the ignition. But, if you just hook the ignition to the alternator output, it will try to charge the battery through that tiny wire. A diode lets the field current flow from the battery, but stops any current from the alternator to the ignition and battery.

..."Without the diode or connection to the aux term the engine will stay running when the key is turned off because current will feed off the field "

And the battery will run down if the field stays energized after the key is turned off--as I have learned to the detriment of at least one battery, possibly two.

From now on I shut off both battery switches before leaving the boat, and click them off for a few seconds after arriving at an overnight destination. That opens the field circuit IF the diode is not working as it should.

The only thing that stays energized with the battery switches off is bilge pumps and my radio/ CD player--I'm sick of reprogramming the darn thing!

Jeff

It might, but I have one wire alternators.

Jeff

On any car or truck the alternator output goes right to the battery. So there is always voltage there.

Not so when using diode splitters to drive two or more batteries from one alternator. Without the voltage on the output, my alternators would not self-excite until a considerable engine speed was reached - 1500 rpm or more, which is a lot when redline is 2800. Diesels should be started and idled until they warm up a bit, and I never knew for sure that the second engine had started.

With the diode, the alternators get voltage as soon as the ignition is turned on. The tachs work immediately, and I can see when the engine is up to idle (450 rpm). Happy camper.

That was my experience. Today, we can get devices that short the batteries together only when charging, and I would not use diodes to charge multiple batteries again.