© Copyright, 2012, R. Fleischer

genlampresistor.htm
article #19

Purpose of this modification:    After modification, the stock Bosch BMW Airhead motorcycle alternator will then produce electricity even if the GEN lamp burns out (however rarely that happens) ... or; if the GEN lamp printed circuit material (it is common for it to crack at the lamps area) does not connect to the lamp properly, etc.

NOTE!...This modification has been done by many in past decades...and the 'accepted value' has generally been 470 ohms. I will expand upon the 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.

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, in my mind, very economical...or...do less than is often believed, or, are not good ideas in general.  I try to be honest, describe all sides of any controversy or what can happen with any modification.

So...what about this alternator GEN LAMP modification?...well...if done INcorrectly, it will reduce reliability...you could damage the instrument 'printed' wiring connection board...or produce a potential short circuit under the gas tank...etc. ...AND, *these lamps seldom fail*. Unfortunately, most folks never inspect them for sagging filaments, a sign of impending failure.  Most folks will never take a cover off the instrument pod until one or more lamps fail, or there is some other maintenance reason; of course, ham-fisted folks can damage the printed circuit board in the pod, CAUSING lamp problems. When the lamps do fail, they fail at rather inopportune times. So...while I am not necessarily recommending this modification....I have done it to my own BMW airhead bikes.

Theory:  If the GEN lamp fails, you will USUALLY not have any charging. At key turn-on to the RUN position, the battery positive (+) connects to the ignition switch, then from that switch electricity goes through the GEN lamp, and then through the voltage regulator, to the Df rotor brush.  A small current flows, due to the internal resistance of the lamp, and some effective resistance in the voltage regulator, and that small current travels through the Df rotor brush and associated slip ring, into and through the rotor windings, back out the D- slip ring and brush, and then to chassis ground, making a complete circuit.  The alternator, once the rotor is spinning fast enough, will begin producing useful output, but the rotor must be FIRST slightly magnetized by the above circuit, until such rpm is reached that the alternator self-energizes.   It is true that increasing rpm to about 5000 rpm or so possibly can allow charging due to some residual magnetism in the rotor.  You can NOT depend on this!   While you COULD do some temporary jumper wiring or other things to get the alternator operating in the case of a burned out lamp, it is far messier than what is proposed below; which is permanent.  The modification described below allows the lamp to fail and you still get charging...and you can replace the lamp elsewhere's...at your leisure....rather than by the side of the road during a downpour.    The charging with the modification, and a burned-out lamp, will not begin at as low an rpm as with a good lamp, but you do NOT have to spin the engine to 5000 rpm either!

NOTE that if you should have a wiring failure in the lamp circuit, this resistor modification may not help; depending on where and how you install the resistor.  Most wiring failures in the lamp area are at the lamp's actual contacting of the printed circuit flexible material in the instrument pod....usually due to poor technique in removing replacing lamps.

Here is some additional explanation, the actual circuit routing being a bit more complex than described above. One side of the GEN lamp is fed by the battery after the ignition switch, and the other side of the lamp connects to the alternator output of the positive output of three SMALL diodes on the diode board. That point is also "D+" on the voltage regulator. When the alternator is not spinning, or spinning slowly, let us say under 1200 rpm or so, the alternator output is essentially zero. The lamp will then 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 the SAME polarity (+) and the lamp has so little voltage drop across it, that it appears UNlighted.   The lamp will NOT supply nearly enough current to fully energize the rotor when large outputs from the alternator are required..., that function is one that the small diodes do, supplying the several amperes needed.

 A dim lamp at a relatively modest to high rpm usually means some sort of corrosion at some connector(s), or a bad rotor or brushes. A bad battery can also cause the alternator to produce a very large output in such a way that the lamp might glow dimly.   A bright lamp at riding rpm USUALLY means an open or shorted rotor or bad regulator, or a bad diode board, and likely there is no charging (no alternator output) at all.   

When one or both brushes is worn enough, the snail spring that supplies pressure onto the brush may begin to contact the plastic brush holder, and reduce pressure on the brush.  This quite often shows up as the brush nears the end of life, as noticeable GEN lamp lighting (dimly) with slight increase in brightness, as rpm rises.  The reason for this is the SLIGHT wobble (called runout) 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.  Brushes are 16.5 mm long when new, from square end to middle of concave end, and are worthless 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.    Generally, 60-80,000 miles.  I have seen the outer (forward-most) brush wear down to unusable, in 20,000 miles, under very dusty conditions. 

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 move the snail spring from contacting the brush holder to where it is again pressing on the brush, and give reasonable spring pressure to the brush.

The modification consists of installing one common and quite INexpensive standard electronic part called a resistor.   

I ran tests for the optimum value for this resistor modification for the GEN (alternator, really) lamp circuit.

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 nerdy point, perhaps, it WAS taken into account. 

The resistor, which substitutes for the lamp 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.

There are TWO methods to do this that I approve of:

Method #1. Neatly clean the area and solder the resistor across the GEN lamp socket wiring itself. The resistor should be secure from vibration by cementing it with a very tiny bit of silicone RTV or similar. Very short lead lengths, you do not need leads vibrating nor shorting to anything. The resistor will produce a small amount of heat during the time the bulb is normally lit, but this is seldom over a minute 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 there; read onwards.  I clean the area by starting with a 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, if need be, avoiding copper loss, perhaps 320 grit.  I have also used pencil and typewriter erasers.  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 fine by me.

Method #2. Install the resistor from 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).   I have never seen overheating from the use of the lower values at the pod.  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 modification can be done to a stock /5, which is somewhat different, no PC of course, but the same situation applies to the GEN lamp. ...and can be done at the voltage regulator in the same sort of manner....or in the bucket.

Resistor discussion: Nearly any type of resistor may be used. It can be the old-fashioned type called 'carbon composition', or any modern type of 'film' or 'deposited carbon' type....or even a 'wire wound' type. The resistor value CAN be 470 ohms, but my tests indicate that 330 ohms is better. 

All BMW alternators from all years on our Airheads can use the same value resistor, even though three different rotor resistance's were used over the years, because the resistor value is very roughly a hundred times higher than any rotor resistance. 

Resistors NORMALLY come in certain 'standard' values, you could use 270; 300; 330; 360; 390; 470...but I prefer the 330 value, as a good compromise between charging characteristics and heat produced. Too much heat MIGHT (not WILL) injure the instrument pod parts...if that is where you mount it, over the long term.  Never seen it happen though...remember that the resistor is only energized at ANY heat producing level at the same time the lamp would normally be brightly lit...that is.....when the alternator is not spinning and the ignition is on (pre-starting)...OR; when the alternator is at a low rpm, idle or somewhat above. Under the very worst conditions I can think of, a 330 ohm resistor would produce about 0.6 watt. If you want to be ultra sure of not damaging the printed material, and you will be mounting the resistor at the printed circuit socket, then use a 470 ohm resistor, which will produce only about 0.4 watt under the worst conditions. 

The resistor really should be rated at 1 watt or more. However, most modifications have used the smaller physical size of a 1/2 watt resistor, and I have NEVER heard of any failing.  If you try to use a resistor size of 5 or more watts you may have problems fitting it into your instrument area or neatly installing it under your gas tank.  There is no need for such a large physical size resistor.

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 about 0.2 ampere. The resistor itself passes little of that, 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 available at a somewhat 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 produce something, at some reasonably low rpm, should the lamp fail.   NOTE again, that the maximum output of the alternator is NOT diminished, and the rpm at which maximum is reached is NOT changed much either.

I once had an inquiry from someone who is building a Cafe Racer Airhead, and who is eliminating the instrument pod as such, and the discussion we had was how the lamp resistor substitution might be with no lamp....and I got into it a bit deeper for him.   After think it over, I decided this might be somewhat useful information EVEN IF the lamp is still used on your bike:

The GEN lamp is rated at 12 volts and 3 watts.    Ohms law says that the lamp resistance is 48 ohms when energized by 12 volts.  NOTE AGAIN that is the resistance when the lamp is fully lighted-up.  That is very considerably less resistance than the value of lamp bypass resistance used earlier in this article.   Keep in mind that once the alternator produces enough electricity, the lamp is, for practical purposes, not involved...it is extinguished automatically, as charging begins.

It is a property of incandescent lamps that the LIGHTED resistance is MUCH HIGHER than the UNLIGHTED resistance.  The unlighted resistance of the stock lamp is about 5 ohms. These are REAL nerdy things to mention...as the effect is very tiny on performance for practical purposes. So, the lamp is around 48 ohms lighted fully, and around 5 ohms UNlit.

If you wanted almost exactly the same charging as stock when the lamp only was in the circuit, beginning at a typical 1500-2000 rpm and wanted the same charge curve at the lower rpm area, then, if NOT having a lamp, you would want to install a standard 47 or 50  ohm resistor, rated at 5 or 10 watts.  That resistor SHOULD NOT be installed in the pod...because if you are at idle rpm, the resistor will dissipate 3 watts of heat, and heat up to pretty hot within a minute.  If you had no instrument pod, you could install such a resistor elsewhere's. It would be OK to have the resistor in the pod and it would not create problems from heat if you never idled the bike very long.  Yes, you could do this across a stock system that kept the lamp.

The resistor could be mounted/soldered onto brass eyelets you put into a thin piece of fiberglass type printed circuit material, and mount the tiny board someplace convenient....perhaps along the frame backbone.   You could leave the lamp circuit in the pod alone....or decrease the lamp wattage size, your choice, if keeping the lamp.

For the Cafe Racer conversion, with no pod; a clean and neat mounting of the resistor someplace convenient and yet protected, is all that is necessary.

Revisions:
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

© Copyright, 2012, R. Fleischer

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