GEN Bulb Minutiae

[montmil]

I've been following and participating in a discussion on the Boxerworks forum regarding an Airhead owner who is not getting a charge to his battery. Not an uncommon problem with our classic ride of choice.

A 'splinter' line of commentary has developed within the thread regarding a GEN bulb that faintly glows but yet the battery is still being charged. How's this so?

A couple of forum members, living on opposite sides of the world, brought up the issue of the "small" diodes (you may find three or five depending on model year) that occupy the interior space between the diode board's two metal plates. I looked into the two spare diode boards I have and spotted in each five small diodes -electronic switches that allow electricity to pass in only one direction.

Should one of these small diodes fail -that info hasn't been revealed- it will permit about 1/2V to pass through the GEN bulb filament causing a very faint glow; yet the battery will still be charged but less than at peak system capability.

The GEN lamp is a helpful diagnostic tool as has been previously discussed here in the R65 asylum. Also, clean connections and terminals throughout the charging system are vital to charging performance.

A good article discussing this issue and also how the BMW Airhead charging system works: http://www.buchanan1.net/charge.shtml



 

[nhmaf]

One of those smaller diodes can fail - at least partially-and charging can still occur as you point out.  It can also sometimes be the result of gritty/compromised terminal connection at the voltage regulator.  If the terminal at the voltage connector causes a  small voltage drop between the bulb and the regulator's sensing input (or if the voltage regulator's ground reference isn't a clean connection), it may still enable the alternator to output charging voltage, but again less than its full capacity.  Again, a small voltage drop will be present across the bulb, and very dim glow might appear.

Be sure to check and keep clean your wiring connections on all those undertank parts!

[davidpdx]

Will the Gen bulb still function in the same way for diagnostic purposes if a resistor has been installed in parallel with it? I was planning on doing that modification this winter.  

[Bob_Roller]

Yes it will, the resistor only comes into play if the filament in the bulb breaks .

Also, worn slip ring brushes or weak springs that push on the carbon brushes can cause the light to glow dimly .

My '87 Guzzi, which has the same Bosch charging system as the airhead bikes, still has this problem after replacing the diode board and voltage regulator .

It's only visible if it's dark when I leave for work in the morning, in daylight it's not visible .

It's been too hot to dive into this any further, the charging system works and keeps the battery charged, I'm deferring this until cooler weather in October ( at least hopefully cooler !!!).

[nhmaf]

It will as long as the resistor isn't too low in value.  If too small a resistance value, the combined parallel impedance will not permit enough voltage for the bulb to glow very brightly (but will still glow).  The resistor just allows you to be able to keep charging and get home after you notice that your GEN light is no longer working.

[Bob_Roller]

The resistor that I got from Motorrad Elektrik, I believe is 340 ohms, or there abouts .

[nhmaf]

330 would be a very common, standard value.   I'd stay above 200 ohms and under 500 ohms myself.  Below 200 you'll likely make the bulb dimmer and and generate more waste heat in the resistor, requiring you to pick a larger wattage package.   Over 500 and you may run into an issue with  restricting the current into the voltage regulator such that it doesn't adequately excite the winding in the alternator to start the charging process.

[donbmw]

I have had the Gen light on dim on both my bikes and still have the system charging for years at time. On the R90 the brushes were worn and installed new brushes but no change. Next removed all wire plugs at Alternator, diode board and regulator. Cleaned connections and this has fixed the problem. Did all this on the 82R65 never did go away until installed the 450 watt alternator and newer diode board. After that no more dim Gen light.

Don

[montmil]

Here's how to build and install a solid-state alternator back-up for less than two bucks:

Available at Radio Shack, pick up a five-pack of 330 Ohm, 1/2 watt resistors for just over a buck. A bit of wire, shrink tubing and a zip tie completes the shopping list.



Solder a wire to each end of the resistor leads. I used about one foot each of blue and green/blue as this is the color of the bike's wires you'll tap into. Use a heat sink clip on the resistor leads to protect the resistor from overheating. Cover the soldered joints with small diameter shrink tubing.




Finally, trim down the zip tie and insert it into the larger diameter shrink tubing. The zip tie stiffener will prevent any potentially damaging flexing of the resistor leads.



The install is quite simple: The green/blue wire attaches to the green/blue wire terminal on the ignition coil. Attach the blue wire to the blue wire found in that big, white nylon connector block under the fuel tank. That's it. I always feel better if I detach the negative battery cable when ever I'm doing any kind of wiring chores on my Airheads.

[Bob_Roller]

I've always wondered why one of the wires from the resistor would go to the ignition coil a powered terminal, instead of a ground ?

The light bulb in the tachometer goes to a ground circuit there .

Anyone have an explanation for this

[montmil]

I've always wondered why one of the wires from the resistor would go to the ignition coil a powered terminal, instead of a ground ?

The light bulb in the tachometer goes to a ground circuit there .

Anyone have an explanation for this

Rick Jones at Motorrad Elektrik used to sell this product for about five bucks. I had him toss in one with a larger order I placed with him. The wire hook-up is from his instruction sheet.

ADDENDUM: The following is verbatim from the Motorrad Elektrik Charging Light Back-Up Circuit instruction sheet.

CONNECTIONS: The green end of the bypass resistor wire connects to the ignition coil terminal that has green or green/blue wires already connected to it. Or, if it's more convenient, connection to any other solid green wire on the motorcycle (such as found inside the headlight shell) will be OK. Connection must be to a switched, unfused 12v current source. This provides an alternative power supply for the rotor exciter current since it usually comes through the bulb.

Locate the light blue wire from the charging light D+/61: this wire connects to the voltage regulator, the D+ terminals on the back of the diode board, and one side of the charge warning light, all  part of the D+/61 circuit. Tapping into this circuit at any point will provide the desired connection.

[georgesgiralt]

Hello !
The charging (GEN) bulb gets a POSITIVE in the tacho, and finds it's ground through the voltage regulator and the alternator's stator.
It's purpose is to supply +12V to the rotor in order to start the charge.
If you put a ground in the tacho's end of the bulb, there will be ground on both sides of the bulb. It won't come on but the alternator won't charge either ;-)
The lighting of this bulb depends upon the voltage of the battery. It turns off when the alternator's output voltage exceeds the battery one, reducing the current seen by the bulb to zero. This is why it may come off early (around 1000 RPM ) or late (around 1700 RPM).
Hope this helps and I'm clear and bright enough ;-)

[Bob_Roller]

Thanks Georges, I need to look at a copy of the wiring diagram for the charge light in the tachometer, the copy I have, I enlargened it on a copy machine and the tachometer area is unreadable .

Your  explanation clarifies what I didn't get before .

[Bob_Roller]

Alright, looked at the owners manual wiring diagram and I understand this now !!!!!!

The charge light gets power from a blue/green wire in the tachometer .

This wire comes off the same terminal on the kill switch as the blue/green wire that goes to the ignition coil .

Learned something today, I think I'll take the rest of the day off .

[marcmax]

When I removed my tachometer, and the resulting indicator lights, I created a small dash with new indicators. Not being sure the new generator light would be enough I followed Monte's instructions and it works flawlessly.

On a side note several years ago I had a faintly glowing gen light on my 82R65LS. It was only noticeable when I was on a country road away from street lights. A new voltage regulator solved the problem for me.