GEN Lamp Circuit Mods

[montmil]

Zach, my Soldier son and R65 forum member, is doing a few modifications and upgrades to his '83 R65. Since adding the R100S to my small herd and running out of shop space, I gave the '83 to Zach. They both now reside in Austin, Texas.

Couple calls and emails and Zach shared that he plans to install an Acewell digital instrument pod. This will entail removing the OEM instruments and the forward "panel". New headlight brackets will be as are on my '81 R65 andd the '78 R100S. Plus the Acewell will need a custom mount prior to the wiring being plumbed.

We have had a couple discussions regarding the GEN lamp and its function in keeping the alternator supplying a charge to the battery.

Options right now include: 1) Possibly using one of the indicator lamp positions in the Acewell as a GEN lamp; 2) Fitting a GEN lamp in the hole at the top of the steering stem nut; 3) Delete the GEN lamp altogether and fit an auxiliary resistor circuit to act as the bulb's filament; 4) maintain a GEN bulb plus a GEN lamp back-up circuit

Maintaining the GEN lamp as is, a back-up circuit will keep the battery charging in the rare instances should a GEN warning bulb fail. That little bulb's filament plays a critical part in the exciter circuit of the Airhead charging circuit. Bad bulb. No charge.

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.


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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.


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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.


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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.

[Barry]

What did we do before heat shrink - it's wonderfully satisfying stuff to use.  Neat trick with the cable tie.

[nhmaf]

Yes- nice write-up Monte and a good idea with the cable tie stiffener.   Maybe you should post this same thing you have in your post into the Technical/FAQ section under heading of "Gen Lamp Resistor Replacement" or something?

[montmil]

... Maybe you should post this same thing you have in your post into the Technical/FAQ section...

Done.

[Bob_Roller]

For the sake of conversation, what if you installed the resistor between the two wires and then used an LED replacement bulb in the tachometer  .

Any thoughts if this would work or not ??

Seeing as how it's a bit of a job to replace the bulb, if you have a fairing, or an LS version R65 .

An LED should rarely give an issue with failing, not that it can't happen .

[Barry]

Would an LED together with it's series dropper resistor pass enough current to energise the rotor. ?

An LED would pull something in the region of 20-30 mA  My first thought was that's not enough as the original 3 Watt charge lamp provides approx 250 mA energising current. On the other hand the 330 Ohm resistor only provides approx. 37mA energising current and that's known to work so I guess maybe the LED would work.  The reduced energising current  must have some effect in terms of raising the revs at which charging kicks in but as long as it's a temporary measure that wouldn't matter. To replace the original charge lamp and get the same low speed charging performance I assume you would need to simulate the 250 mA current draw and use a lower value 47 or 50 Ohm resistor that would dissipate 3 watts.

[nhmaf]

I would not rely on just an LED, even one with an internal/integrated resistor.   The LED current draw in not linear at low voltages, and has been pointed out, the typical common ones are rated in the  15-30mA range.   Unlike a filament bulb, the LED is unidirectional as far as current flows, until one gets high enough reverse voltage to cause reverse breakdown.    In certain, admittedly somewhat rare failure situations, a faulty voltage regulator (for example) can cause the voltage on the regulator side of the lightbulb/LED to rise above the battery/ignition switch side (the opposite of the "normal" light-the-bulb case.   In this situation, a filament bulb would still illuminate somewhat -> providing a clue to the user that there  isa fault in the charging system, but the LED will not illuminate when reverse biased.

So, if one is willing to forego the ability to visually diagnose this certain relaitvely uncommon failure mode, an LED along with a parallel bypass resistor I think is fine.   Or, one can simply go with the original filament bulb and add in the bypass resistor to ensure charging capability even if the bulb breaks.

I don't think that the alternator/regulator require 250mA source current to do their job, but I do think that the number is probably closer to the 50mA-80mA range than the 10-15mA range.   Snowbum likely has some articles on this worth looking at, too.