Need clarification about Generator Light necessity

[Enforced_Leo44]

So, I noticed that my alternator system is not charging the battery at all, still have to properly diagnose the problem but before doing any testing I wanted to ask about the necessity of a generator light, because I remember reading about the fact that not having one would make the alternator NOT charge the battery

Keep in mind that I my bike has been scramblerized (majority of the work has been done by the previous owner) and doesn't have the original dashes (which means no generator light as well), if I wanted to try and see if that's the cause of the alternator system not charging properly, how would I go about testing this?

Thanks a lot in advance, this forum has been extremely helpful.

[Kurt in S.A.]

What you read is correct. The light becomes bright when current is flowing through the bulb. This current flows through the coil windings to give it an initial voltage/charge...I'm not great with these details. Once the bike starts and the alternator is up and running, it is self-supplying. Without that initial rush of voltage, the alternator won't work as you discovered.

On Snowbum's website, he discusses how to add a resistor to the circuit so that charging can return to normal. The idea of this resistor wasn't to actively bypass the bulb in the speedo, but rather to keep the bike working should the bulb actually burn out. The bulb usually lasts a long time. But if you're out on the road and the bulb does go out, you're in a bit of trouble. Having the resistor in the circuit affords the possibility that you'll get home OK.

Kurt

[barryh]

Yes the bulb is needed. With ignition on a small current flows from the battery through the gen light and then via the brushes and slip rings through the rotor winding to earth in order to energise the rotor winding so that it will produce a magnetic field. Once the alternator is producing output there is battery voltage on one side of the bulb and generator output on the other side so no current flows through the bulb and it is extinguished.

Without a gen light you might get lucky if there is a bit of residual magnetism in the rotor and you rev the engine high enough the rotor might kick start alternator output but this is not something to rely on.

The bulbs are very reliable. I have every reason to believe mine is still the original after 43 years. Not surprising given they are only illuminated for a few seconds every time the engine is started.

[Rob Frankham]

So, I noticed that my alternator system is not charging the battery at all, still have to properly diagnose the problem but before doing any testing I wanted to ask about the necessity of a generator light, because I remember reading about the fact that not having one would make the alternator NOT charge the battery

Keep in mind that I my bike has been scramblerized (majority of the work has been done by the previous owner) and doesn't have the original dashes (which means no generator light as well), if I wanted to try and see if that's the cause of the alternator system not charging properly, how would I go about testing this?

Thanks a lot in advance, this forum has been extremely helpful.

The first thing to ascertain is what has been done to the system to get around the need for the generator light. That is, presuming the system has been working satisfactorily before. Has the PO fitted a resistor? Does the bike still have a stock alternator and associated items?

Once you know this information, it will be possible to trouble shoot the system and see why it isn't charging.

As Barry and Kurt have said with the stock system, the charge light bulb is an important part of the charging circuitry. Without it, reliable charging is next to impossible. It's also worth mentioning that it needs to be a filament bulb of a wattage value close to or higher than the lowest value specified by BMW (1.2 Watts). A LED replacement bulb will not work in this circuit without extra circuitry... it doesn't carry enough current.

Rob

[RPGR90s]

As the others have mentioned, the GEN light is essential to the circuit. It is why when you turn on the ignition on an Airhead, you first make sure the GEN light comes on.

When my group of buddies were all riding Airheads years ago, one of the bikes stopped charging and we figured out the GEN bulb had opened up. Simple fix, to swap one of the other bulbs in the instrument cluster and make it home.

[hal]

Would it be possible to connect a reisistor instead of the bulb? If the bulb is 1,2w and P=U*I and U=I*R that makes R=U*U/P, 12*12/1,2, 120ohm. So a 120ohms resistor should do the work. Or am I missing something here?

[Kurt in S.A.]

Pretty hard to attach a resistor to the thin electrical film inside the speedo pod. If you want to add a resistor for the same effect, do it as Snowbum describes on his website.

Kurt

[hal]

Was just thinking, as he allready had no instrument cluster. Just academicly interested

[Rob Frankham]

Would it be possible to connect a reisistor instead of the bulb? If the bulb is 1,2w and P=U*I and U=I*R that makes R=U*U/P, 12*12/1,2, 120ohm. So a 120ohms resistor should do the work. Or am I missing something here?

It's perfectly possible to add a resistor in place of the lamp and 120 Ohms should do it but bthere are a few of things to bear in mind.

A 120Ohm resistor will still dissipate 1.2Watts so you realistically need a 3 watt rated resistor and...

Eveb a 3 watt resistor dissipating 1.2 Watts is going to get warm so the bigger the better and it should be mounted where it can get cooling air and...

You lose the other (main) function of the lamp which is to give you a rather useful indication that the alternator is actually working.

Rob

[spo123]

It is called an EXCITER Lamp....