3wire alternator

[jhypnar]

I have a 1968 250 I just finished restoring and it's ready to go except for connecting the 3 wires that exit the case from the alternator. This bike will not be riden on the street and therefore the only thing requiring electricity is the 2 6v coils, 2 spark plugs and I have a 6v battery (no lights, turn signals, horn, etc). My question is where do the 3 wires connect to? Thanks for any help.

[Paddy]

They go to the Reg/Rec.

There are some basic wiring diagrams here
http://www.motoscrubs.com/Duc_Technical.htm

[ducwiz]

I presume it's a wide case model, so the wire colors should be 2 x yellow, 1 x red.
2 plugs and 2 6V HT coils require a 12 V system, as the coils must be connected in series. If you don't want to run the bike off the battery (12 V!) alone, you also need a 12V regulator-rectifier, to convert the AC power from the alternator into a DC voltage, for battery charging. This R-R has to be a very special one, the original 6V type does not work, but may be electronically modified to suit. I. e., you can use any of the newer 12V R-Rs from an L-twin with two-wire DUCATI alternator.

cheers Hans

[jhypnar]

Thanks Hans. My 250 is a narrow case. Does that make any difference in what you wrote? Also this 250 is going to be used in vintage roadracing. Does that make any difference? Why can't I connect the points to each 6v coil separately and connect a 6v battery to each coil separately? I assume if I want to run the alternator I will need a R/R.
Jon

[ducwiz]

Jon,

it makes a difference, indeed.
The w/c alternator has 6 (or 5) coils in a 2phase configuration, and 3 wires color coded as I wrote before.
Different alternators were used for the N/C models. AKAIK there are at least 2 different 3wire stators, both with only 2 coils. These are arranged as 2 different power circuits. The older one was used in the 250SCR and Mk3 until a certain engine number. It has one coil for the lighting circuit, another for the ignition. Both are AC driven, requiring a special AC ignition coil, but no battery.
The later alternator also comprises 2 coils, but for lighting and charging, respectively. The ignition is a DC type, needs the appropriate DC coil, and of course a battery.
Connecting two 6V coils in parallel to the single point in parallel may not work, especially in case of the AC ignition system, because they consume twice the power, so will overload the alternator coil and produce a weak or even no spark at all. Running them from a battery also will draw double power, but may work or not, due to the unpredictable interaction of the coil's primary windings and the single capacitor. I never made this experiment, always implemented two series connected coils, either in 3 or 6 Volt version, or a dual HT output coil.
For reference you should read through the extract from the workshop manual http://renato2.bplaced.net/DUCATI/electrical_system-NC.pdf.
Do you remember the type of your stator (2 or 6 coils)?

Hans

[jhypnar]

Hans, Thank again for all your help. The stator is a 2 coil type.

[ducwiz]

OK.
So, if you like to identify your system, check this: http://renato2.bplaced.net/DUCATI/Duc%20flywheel%20info2-web.pdf. The number stamped onto the rotor and starting with 31. tells whether it's an AC/magneto or a DC/battery system.

cheers Hans