WD with magneto, convert to 12 volt

What does it take to convert a WD with a magneto to 12 volts? Is it really as simple as a battery, and an alternator? Do you have to do anything to the starter internally? What's the argument for type of alternator, I've seen three wire or one wire setups?

Thanks in advance.

I'm pretty sure you don't mess with the mag at all, the only reason I mentioned it was so people didn't try to explain coils and resistors and such. I've seen that on here and that doesn't apply to this machine...

We had a great starter and alternator shop nearby, but then it got bought out and I took a garden tractor starter generator there, they rebuilt it for $300, then I took it back a month later, and they replaced the armature AGAIN (under warranty) and it STILL DON'T WORK. So getting the generator converted is out for me, at least until I find a reputable guy to do it again.

Edited by littlemarv - 16 Apr 2017 at 8:36pm

There's nothing to change on the mag. The lights need to either be changed or disconnected.

While an alternator could be built for positive ground very few have been in the market place. That probably comes from the SAE standard about 1960 being modified to only allow negative ground 12 volt systems for cars, trucks, and farm equipment unless the cranking power couldn't be supplied at 12 volts.

The magneto is independent of the starter and light power, based purely on shaft rotation. It has an impulse mechanism that snaps the rotor past the firing position to get more voltage at cranking speeds. If 12 volts on the 6 volt starter cranks fast enough starting may not work well because of turning just fast enough the impulse mechanism doesn't work but the speed still isn't fast enough for the magneto to generate an adequate ignition voltage.

The one wire alternator requires cranking up its speed to get it to start charging, but once it starts charging it will charge at slow engine idle. It makes some sense that revving up a cold engine is not a great idea, but it only takes a second at the higher engine speed to get the one wire alternator started charging. Its not complicated to convert to three wire operation. On a tractor without battery ignition a simple option would be to add a T to the oil pressure tap and install a switch that closes on oil pressure rise to supply battery voltage to the alternator exciter terminal. No resistor, lamp, or diode needed. Those are needed when using the battery ignition circuit to supply that excitation because without them the alternator will back feed the ignition circuit and prevent stopping the engine with the ignition switch.

The voltage regulation of an alternator with built in solid state regulators is much better than the regulators for generators and that makes for a much better battery life, full change but not overcharged.

Gerald J.

Yea, the mag runs the ignition system by itself. All you need is to spin the motor over to get it to fire off. If you have a GOOD running motor that will start with  3-4 seconds of cranking, then a 12 volt battery will start it dozens of times with out a recharge. Alternator is nice and needed if your running daily with LIGHTS... but you could just install a 12 volt battery and put in on a small charger one day a month if you don't use lights.

Edited by steve(ill) - 17 Apr 2017 at 8:51pm

Yes the impulse mechanism also retards spark for cranking, hand or with starter motor. When the centrifugal advance in a distributor sticks advanced it really slows the starter.

Gerald J.