ALTERNATOR GEN LAMP CIRCUIT MODIFICATION
©Copyright, 2014, R. Fleischer
This modification has been done by many in past decades...and the 'accepted value' of R2 in the sketch at the bottom of this article has generally been 470 ohms. The original purpose of the modification was strictly to ensure charging if the stock incandescent lamp burned out. It required somewhat higher than normal RPM to obtain initial alternator output. I will expand upon that resistor value determination. I will show that a slightly lower value is slightly better and makes an improvement in the rpm at which charging begins.
I will also show why a much lower value is more appropriate if you purposely do not have an incandescent GEN lamp; provided the low value resistor is not mounted in the instrument pod (for heat reasons).
General comments about 'modifications':
As a general rule, the 'factory knows best' is a fairly accurate statement. However, few of us own UNmodified motorcycles. Our bikes are a reflection of our personal desires. I am not in favor of many of the modifications that we all see or hear about. Some of these modifications, which I have done myself, are not very economical; or, do less than is often believed; or, are not good ideas in general; especially for the average rider/owner. I try to be honest, describe all sides of any controversy or what can happen with any modification.
What about this alternator GEN lamp modification? If done incorrectly, it will reduce reliability. You could damage the instrument 'printed' wiring connection board, etc. The GEN lamps seldom fail.
Most instrument pod printed board failures are from using wrong methods of removing a lamp, damaging the copper on the printed-circuit board. However, it is also true that the original BMW design lends itself to this type of damage; that is, the contacts for the lamps is a poor design.
I am not necessarily recommending any modifications in this article; but I have done it to most of my own BMW Airhead bikes.
I have not converted any of my personal bikes to use LED lamps in place of a stock incandescent lamp.
If what I call the low ohm resistor modification is done, you do not need the lamp at all, as performance will remain just about the same as if the lamp and instrument pod did exist. The low ohm modification is generally for those who have cafe'd a bike, or otherwise do not have the regular instrument pod or setup. The low ohm modification can be done to a stock bike, if the resistor must be located where any heat is well-dissipated.
This article discusses, at several places, the stock incandescent GEN lamp. The /5 motorcycles, only, used a 4 watt GEN lamp. All the later motorcycles used a 3 watt lamp. The 4 watt lamp allows a slightly lower RPM at which the alternator begins to produce usable output. That is because the 4 watt lamp has a lower resistance in ohms.
If you have a break in the lamp circuit, this resistor modification may not help; depending on where and how you install the resistor. Most failures in the lamp area are at the lamp's actual contacting "socket", which IS the printed circuit flexible material in the instrument pod...usually due to poor technique in removing replacing lamps.
Much more rarely, but has happened, the failure is inside the rubber plug that pushes into the rear of the instrument pod, or, a cracked solder joint at the mating male connection to that plug inside the pod (fixable by careful cleaning and soldering). If the proposed modification resistor is mounted with good workmanship under the fuel tank, there are no such problem areas, except for the quality of YOUR workmanship. For the high value resistor (330 or 470 ohms), it dissipates little heat, so it is fine to put it inside the pod. For the /5 bikes, there is no flexible foil material, so the lamp R/R is not any problem in this regard.
NOTE, again, that it is rare for a GEN lamp itself to burn out!
The actual circuit routing is a bit more complex than earlier described. One side of the GEN lamp is fed by the battery (after the ignition switch), and the other side of the lamp not only connects to the voltage regulator input (D+), but that side of the lamp also connects to the alternator positive (+) output of three SMALL diodes on the diode board. When the alternator is not spinning, or spinning slowly, let us say below ~1200 RPM, the alternator output is extremely small, if any at all. The GEN lamp will be lighted, as the battery current flows through it and then through the regulator and rotor, to engine case ground (battery negative). Once the alternator stator output increases enough, the three small diodes rectify the stator output (rectify means to change A.C. to D.C.) and the voltage on both sides of the lamp is now approximately the same, and of the same polarity (+) ....and the lamp has so little voltage drop across it, that it appears to be, and is, not lit.
The lamp will not and can not supply enough current to come anywhere's close to fully energizing the rotor when large outputs from the alternator are required..., that function is done by those small diodes, supplying the several amperes needed. As RPM rises, the small diodes pass far more current into the voltage regulator, which supplies the current needed to more fully magnetize the rotor, which causes output of the alternator to greatly increase. As the output voltage from the diode board diodes begins to approach the desired amount, the regulator begins to reduce the current flowing into the rotor from the small diodes.
The voltage (well, current flow) being regulated is that of the small diodes output, and and not directly the big diodes output, which goes to the battery and rest of the bike's electrical system. It is the LAMP that separates the main output of the big diodes from the small diodes output, as far as 'sampling' the voltage is concerned. There are some more complex reasons why the small diodes are there, and why this seemingly strange sampling method was done. I will not get into these reasons, they will only further complicate things here.
Common failure modes:
When one or both brushes is worn enough (they tend to not wear evenly), the snail spring that supplies pressure onto the brush may begin to contact the plastic brush holder, and that can greatly reduce pressure on the brush. This is a common complaint and shows up as the brush nears the end of its life... as noticeable GEN lamp lighting (usually dimly) ...but often with increase in brightness as rpm rises. The reason for this is the slight wobble (called run-out) of the rotor...which moves the brushes in and out ever so slightly, and the snail spring is also slightly bottoming and cannot fully follow the brush movement. You can think of all this simply as the brushes are not making consistent or high enough pressure in contacting the rotor slip rings. It almost always happens on one brush first.
Brushes are 16.5 mm long when new, measured from square end to middle of concave end, and are worthless (but, read on...) once the spring contacts the holder. Brush life depends on riding conditions....dust, dirt....and how much output the alternator is called upon to produce, over time/miles. Generally, 60-80,000 miles is a typical brush life. I have seen the outer (forward-most) brush wear down to unusable, in 20,000 miles, under very dusty conditions on the Airheads that have the better ventilated front engine cover.
If one or both brushes have worn to the partial contacting point, and you are on a tour, you can put a tiny piece of thick paper between snail spring end and the brush outer end; this will keep the snail spring from contacting the brush holder, and you can ride on, and likely for a very considerable distance, even 5000 or many more miles would not be unheard of with a thick piece of paper. REPLACING the brushes is a bit of a bother, much easier to do it at home than on a tour. I do recommend inspecting the brushes maybe once a year for length (just use a dental mirror and light, and see if the snail spring has lots of movement left), the added piece of paper will work well, and is not difficult to do, although it is a bit fiddly if the inner brush is the problem one.
The basic added resistor modification:
This consists of installing one common and inexpensive standard electronic part called a resistor. I will, later here, also describe using an LED in place of the stock lamp. I ran actual tests for the optimum value for the resistor modification for the GEN (alternator) lamp circuit....and this is applicable to both a LED conversion and the stock GEN lamp.
Any incandescent lamp has a fairly low resistance when cold (not lit) and that resistance increases a fair amount when the lamp lights up. Whilst a possibly nerdy point, this was taken into account in this article.
The pod-mounted resistor, which substitutes for the lamp, if imperfectly, if the lamp or lamp connection fails, is connected across the GEN lamp electrically, but not necessarily mechanically at the lamp itself (or its socket), although that is a nice place for it if you use the specified higher resistor values in this article.
For those that are interested in the energizing current value into the rotor, with the engine off, ignition switch on, GEN lamp lit, it is on most models ~ 0.2 ampere. The added pod-mounted resistor of the modification itself passes little current, typically around .040 ampere or so, and you can see that the vast majority is passed by the lamp. If the lamp burns out, the much smaller resistor-allowed current will initially adequately energize the rotor, although alternator output becomes first available at an elevated rpm. The resistor modification does NOT make the system perform EXACTLY as when the lamp is intact and lamp properly operating in the circuit. Maximum output, and output at any reasonable riding rpm is not affected; only the lower rpm area is affected. The reason is that the resistor's purpose is to allow the alternator to reliably produce electricity, should the lamp fail, and once the alternator is producing usable electricity, it self-energizes. The maximum output of the alternator is not diminished (and the rpm at which maximum is reached is not changed either).
There are TWO methods for the modification, that I approve of:
Neatly clean the area and solder the resistor across the GEN lamp socket wiring itself. The resistor can be made more secure from vibration by cementing it with a small amount of silicone RTV or similar, although not absolutely needed,...that is because I suggest quite short resistor lead lengths as you do not need leads vibrating nor shorting to anything. The resistor will produce a small amount of heat during the RPM/time the bulb is normally lit, but this is seldom over a minute or so in duration at a stop sign, and in any event, the heat amount is small ...especially with a 470 ohm value. Those installing the resistor inside the instrument pod and across the printed circuit material might well use the 470 ohm value, considering the heat, but I have successfully used a lower resistance value (330 ohms) there. I clean the area by starting with a sharp Xacto knife on a very flat angle, scraping away the coating over the copper, very carefully. I may use a small piece of slightly abrasive sandpaper afterwards, perhaps 320 grit. I have also used both pencil and typewriter erasers after the initial knife or sandpaper. For the /5, there is no flexible printed material, and almost any way you want to install the lamp NEATLY in the headlight bucket is OK. For soldering to the flexible material in the pods, you really want the exposed copper to be clean and shiny prior to soldering. Soldering should be done with a smallish soldering iron, but with a reasonably decent sized tip, with rosin-core solder (60/40 common type is best), and quickly ...you do NOT want to damage the area.
Install the resistor at the terminal of the ignition coil that connects to the battery circuit. This is the terminal that has the green/blue wire. There is often an unused male spade connector available there. Connect the other side of the resistor to either of the blue wires coming out of the voltage regulator plug [this is D+]. You need to do this neatly, with no chance of bare wires, nor vibration breaking them. Insulate the resistor and wiring with shrink tubing, and use proper all-plastic wire-ties, as required. For SOME, this may be a preferred point of attachment, to avoid damaging the flexible circuit at the lamp (if you have a later bike with the instrument pod, not a /5). Mounting the resistor as in this method #2 creates less chance for ham-fisted folks to damage that thin flexible printed circuit board. The drawback of Method #2 is that there are probably more chances for messy workmanship, and vibration problems. This method #2 is the way to go if you are going to use the low ohm resistor, say 50 ohms or so, as described later in this article, as that resistor can get much hotter than the higher ohms resistor (470 or 330 ohms) that I recommend if soldering the resistor onto the pod flexible material.
This modification can be done to a stock /5, which is somewhat different, no pod, no flexible pod circuit board material, but the same situation applies to the GEN lamp. ...and the modification can be done at the voltage regulator in the same sort of manner....or in the bucket.
You may want to mount the resistor, of whatever value, differently than my two methods, above. That is up to you. I can possibly advise you on your proposed modification, if you inquire on the Airheads List.
Low ohm resistor information. Use of LED lamp instead of the incandescent lamp:
The GEN lamp is rated at 12 volts and 3 watts (except /5 uses a 4 watt lamp). Ohms law says the 3 watt lamp resistance is 48 ohms when supplied by 12 volts. This is the lamps resistance when the lamp is fully lighted-up. That is very considerably less resistance than the value of lamp bypass resistance (330 ohms, etc.) used earlier in this article. Once the alternator produces enough electricity, the lamp is, for practical purposes, not involved ...it is extinguished automatically as charging begins. That also means that any added resistor is not producing heat, when the alternator is producing electricity. The unlighted resistance of the stock lamp is just a few ohms. Example: the 3 watt rated lamp is around 48 ohms lighted fully, and around 5 ohms not lighted. The /5 lamp, 4 watt rated, is about 36 ohms fully lighted. A lamp is nearly ideal for the purpose of both indication and providing initial rotor magnetization.
If you wanted nearly the same charging versus RPM as when the lamp was operational and in the circuit, say, beginning at a typical 1500-2000 rpm... and wanted the same or similar charging curve at the lower rpm area,... then, if NOT having a lamp, you would want to install a standard 47 or 50 ohm resistor (36 ohms for the /5), rated at a standard 5, 7, or 10 watts. That resistor SHOULD NOT be installed in the pod...because if you are at idle rpm, where the lamp is normally fully brightly lit, the resistor will dissipate 3 watts (4 watts on /5) of heat, and heat up to pretty hot within a minute or two.
If you had no instrument pod, or just wanted to, you could install such a low value resistor elsewhere's. For the /5 you can mount the resistor to the inside bucket metal. That can also be done to the later models, but it is a bit more work, as the proper connections are not already in the bucket.
I HAVE installed such a resistor inside a pod, quite successfully, but the resistor was mounted on a metal plate (for heat dissipation) I made-up, the plate being mounted to the pod housing. Two folks who I chatted with long ago had mounted the low ohms resistor on the back side of the instrument pod or the underside, with wires leading to the inside. Still, these are not ideal places for the resistor. The best place would not only allow for the heat under all circumstances, but would eliminate the pod umbilical cord connections (that rubber plug at the front of the pd), ETC., from the occasional pug problems.
The resistor could be mounted/soldered onto brass eyelets you put into a small piece of fiberglass type printed circuit material, and mount the tiny board someplace convenient....perhaps along the frame backbone. You could even use a screw-mountable metal-cased power resistor and mount it on the copper side (for heat transfer) of a piece of printed circuit board. You do not need a lamp at all with the low resistance modification, but having some sort of lamp will still offer an indicator. For the Cafe Racer conversion, with no pod; a clean and neat mounting of the low ohm (say, 50Ω) resistor someplace convenient and yet protected, is all that is necessary; with consideration to the heat produced.
Nerdy point: While there ARE special resistors available that have characteristics similar to lamps, and some other electronic devices also have such characteristics, these items are NOT part of any modification YOU should be doing; they are not easily found; usage is more complicated, difficult for most folks to understand, so I am NOT getting further into these devices.
9. If you are using the low value resistor (perhaps 50 ohms, and 5, or 7 or 10 watt rated) for R2, you will not likely be mounting it in the instrument pod.
04/11/2003: add .htm title; edit for clarity, add information on snail spring bottoming, etc.
02/06/2004: clarifications on wording, nothing at all substantial
11/22/2009: more clarifications.
01/10/2011: Add 'A bit of Nerdy Information"
06/03/2011: Clean up a bit
09/29/2012: Add QR code; add language button; update Google ad-sense code; edit article for clarity and brevity. Later, the troublesome language button script coding was removed.
12/05/2013: Add more details and re-write article for clarity.
02/02/2014: Add some emphasis, clarify details between /5 and later and how charging circuitry works, more on the LED installation, etc.
03/05/2016: update meta-codes, minor other improvements, such as fonts and left justification.
07/02/2016: Update metacodes, scripts, H.L., etc. Add Lamp sketch and re-write article to incorporate sketch and details, try to clarify for whatever value of resistors will be used, etc.
04/08/2017: Fix minor typos.
©Copyright, 2014, R. Fleischer
Last check/edit: Monday, July 22, 2019