Charge Light Indicator - Electronics Help Needed

[MonzaMan]

Hello:

I am a new member here, resurrecting a 1968 widecase 250 Monza. My introduction thread is located at the link here:

http://www.motoscrubs.com/forum/viewtopic.php?f=3&t=3685

I want to wire up the charge control idiot lamp so that it works as per original. I am not hung up on correctness or authenticity but I want the charging circuit to be original. I acquired the correct 1.5W bulb and a new ignition switch (Aprilia 17121, see pictures in my introduction at the previous link). I have consulted all the wiring schematics for all years available and they all show the wiring of the charging light differently, and the schematics are confusing.

According to an old paper copy manual I have (shop manual for the narrowcase singles up to 1967). This idiot light actually did work as a charge indicator on some models. See excerpts:

https://i.imgur.com/DKqYFmJ.jpg

https://i.imgur.com/90XLfyR.jpg

Can anyone explain how this lamp works and point me to a schematic diagram? My guess is the 6V 1.5W bulb should be fed from both sides, battery on one side and Marrone on the other. If there is no ground wire and the alternator dies then current would flow through the bulb and depending on the internal working of the R/R back to ground through the stator. If this were the case the light would go on when the alternator is dead or very weak. Just like a car. Turn on the ignition and the light goes on. Start the engine and if the alternator is working the light goes out.

Any help much appreciated....


Kind regards,

David

[blethermaskite]

David, I have owned my 350mk3 widecase since new @1974 and the red idiot light does not indicate charge......it only indicates that the ignition is on,.........I have owned my 250 Desmo widecase since 2010 and the red idiot light is the same, only that the ignition is on.
Cheers,
George

[MonzaMan]

Thank You George.

I wonder if when they fitted the newer alternator (6 windings, three wires) to the widecase models, the charge light circuit changed and became a true idiot light?

Does anyone have a circuit diagram of the widecase charging circuit? There are a couple examples in the tech portion of this site, but I cannot understand how the ignition switch works the way it is shown...

Best regards,

David

[Jordan]

I should have said that I only have experience with widecase models, which don't have charge indicator lamps.
A simple "ign on" lamp can still help avoid a flat battery caused by leaving ignition on after an engine stall.

To understand more how the charging system works, read up on single phase, full wave, centre tapped alternators.
They are distinct from the more common bridge rectifier systems.

Ducati wiring diagrams are not famous for clarity.

[graeme]

Darmah used a small red box to show alternator charge.
They work as you are trying to replicate.
But they are 12 volt
Perhaps if you change to 12 V you could copy that set up ?
Pantah too I think
Graeme

[MonzaMan]

Thank you Graeme.

I'll see if I can find a schematic for the Darmah and/or Pantah. Seeing how those circuits work might give a me a valueable clue.

It is interesting to me that of all three of indicators on the bucket, the charge light is the only one that is not grounded to the chassis. Further, the side of the bulb that does not come from the ignition connects to a terminal on the board. It was intended to be connected to some wire that is not a constant ground.

Best regards,

David

[Jordan]

You like to call it a charge light - wishful thinking?

A wiring diagram for widecase singles I am looking at (450 points type ignition) does show a lamp, called "Inserted key white control bulb".

It has a wire coming from the ignition switch, in parallel to the ignition circuit (as far as I can tell from the atrocious graphics).
The other lamp wire has a ground symbol at its end (a horizontal line with short oblique lines under it), where it also says "To terminal board" - I guess there is a ground (earth) point.
Turning on the switch will make the lamp come on, and there's nothing else influencing it to go out except by turning off the ignition key.

It is likely a circuit could be designed to provide a charge indicator function.
We already mentioned that.

[Ventodue]

I'm going to try to untangle some of the confusion here ...

The problem of the source documents:
As far as I know, a full owner's manual was not issued for the wide-case 250 Monza. Instead, the model was covered by a "Annexe". And here is where things get mightily complicated ...

The first Annexe was issued in September 1967. It was published as a supplement to the 250 narrow-case manual of, first, December 1964, and then later January 1966.

In June 1968, a second Annexe was issued. But this time it was as a supplement to the wide-case 350 Scrambler owner's manual of May 1968.

However, none of these Annexes have any information about the warning lights. So we need to look at what the base manuals say ...

1. The narrow-case 250 manual (both 1964 and 1966 versions):
"The red ignition light lights up when the engine is started and revs at tick over. The light should go out and stay out all the time the engine is running faster then tickover".

Well, that sounds to me like a charge light.

2. The wide-case 350 Scrambler manual:
"The red ignition light lights up till the engine stops".

This sounds to me like the 'key inserted' function that other wide-case owners have mentioned.

So if you want a charge light function, David, it's the narrow-case wiring diagram that you need to follow. However, if what you have on the bike doesn't seem to match with that, then a simple 'key inserted' function as generally found on wide-case bikes might be the 'correct' solution.

HTH.

[veloduke]

My guess is the 6V 1.5W bulb should be fed from both sides, battery on one side and Marrone on the other. If there is no ground wire and the alternator dies then current would flow through the bulb and depending on the internal working of the R/R back to ground through the stator. If this were the case the light would go on when the alternator is dead or very weak. Just like a car. Turn on the ignition and the light goes on. Start the engine and if the alternator is working the light goes out.

I would say this is exactly how it works.

The same method is used by Electrex if you use their alternator system.

I have a repro workshop manual and what you describe is what is shown by the wiring diagram.

I had a 1963 TS125 that had a warning light, but this one stayed on when the ignition was on. It had no regulator rectifier like your bike.

[Ventodue]

<snip> Turn on the ignition and the light goes on. Start the engine and if the alternator is working the light goes out.

I would say this is exactly how it works.

And it's what the factory workshop manual* says, too (page 40):

"The headlamp warning red light should be cut out when the engine runs at 1,000-1,500 r.p.m."

And it goes on:
"If the said light goes out only when the engine is running at a higher number of revs, or it does not go out at all, you must take corrective measures as described in parag. c) at page 41."

These "corrective measures" being ... (English lightly corrected ):
"Insert an ohmmeter between the brown take-off and i) yellow No.1; ii) yellow No.2. You should obtain continuity when the lamp comes on, or insulation when the lamp doesn't come on. Inverting the probes, you should obtain the opposite."

Which suggests to me there's diodes in the reg/rec.

"Inserting an ohmmeter between the brown and green take-offs, you should read a resistance of 28 ohms +/-."

From the wiring diagram, you can see that the green take-off is the ground connection.


* "4 and 5 speed Ducati Single Overhead Camshaft Motor Cycles".