Introduction

The GEN lamp supplies a very small initial current to the alternator rotor to enable the alternator output & charging process to begin as soon as there is modest rpm.  If the lamp burns out or corrosion or other problem at the lamp, the alternator usually will not provide any output. This article describes, in simple fashion, a popular modification which eliminates lack of charging from lamp burnout (which is actually fairly rare).

General comments about ‘modifications’:

As a general rule, ‘The factory knows best’ is a fairly accurate statement. However, few of us have UNmodified motorcycles. Our bikes are a reflection of our personal statements and desires; and are thus often modified for many reasons. I am not in favor of some of the modifications that we all see. Some modifications, which I may have done myself now and then, are not very economical…or…do less than is often believed. Some modifications can REDUCE reliability.

If alternator GEN LAMP modification is done INcorrectly it will reduce reliability.  You could damage the instrument ‘printed’ flexible wiring connection board…or produce a potential short circuit under the gas tank…etc. …AND, these lamps seldom fail. Unfortunately, most folks never inspect lamps, especially the harder to get to instrument pod ones, for sagging filaments which is a sign of impending failure.  I do NOT recommend checking pod lamps for sagging filaments.  This is particularly so on the older types of instrument pods that contain a printed circuit board, which may no longer be available (substitutes are available from KatDash).  The original flexible printed circuit board is somewhat fragile; so I recommend replacing lamps only when they have failed.     BUT:  when a GEN lamp fails, it nearly always fails at an inopportune time.

I am not recommending everyone do this GEN lamp circuit modification….BUT, I have done it to my own Airheads that use the stock type of charging system, no matter what year, and no matter what mix of stock alternator components from any year.   This modification also bypasses problems from microscopic or other cracks that tend to arise on the instrument pod flexible connection film.  With cracks, the lamp does not work and there is typically NO charging.  The flexible connection film is easy to damage during bulb changes.

There is a schematic diagram and a LOT of information on the alternator system at:
https://bmwmotorcycletech.info/trbleshootALT.htm
You may want to click on that link and read that article now. Consider printing the schematic located in that article.

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If the GEN lamp fails, rare as that may be, you MAY not get any charging, as the GEN lamp energizes the rotor from the battery (through the voltage regulator), such that the alternator, once spinning fast enough, will begin actual output and charging. Do NOT depend on hearsay that 5000 rpm or so will produce charging, due to ‘some residual magnetism in the rotor’.  THAT may work, or may not.  The residual magnetism, if present, is usually VERY small.

The modification described below allows the lamp to fail and you still get charging…and you can replace the lamp elsewhere’s, and NOT by the side of the road during a downpour.   Failure of lamps can also be caused by board/film problems, excessive vibration, large road bumps ….or even normal smooth road riding.

The rubber mounts for the instrument pod were changed long ago by BMW to a different durometer (a different softness of the rubber) that better dampens the vibrations.  That helped mostly with longevity of the instruments themselves, and not the film material or GEN lamp.

How the lamp circuit works, simplified:

One side of the GEN lamp is fed by the battery (from a point after the ignition switch, assumed here to be turned ON).  The other side of the lamp connects to the alternator output from the positive output SMALL diodes on the diode board. That lamp/diodes point of connection is ALSO “D+” of the voltage regulator. The lamp supplies only a very limited small current to the voltage regulator,.  The current passes through the voltage regulator to the Df brush terminal, and the tiny current (alternator not yet spinning) flows through the two rotor brushes to chassis ground (battery negative).

When the alternator is not spinning, or spinning slowly, the alternator output is essentially zero. The lamp will then be lighted brightly, as the battery current flows through it and then through the regulator and then the rotor, to engine case ground (battery negative). Once the alternator output increases enough, the three small diodes rectify the stator output and then the voltage on BOTH SIDES of the lamp is now approximately the same, and same + polarity (approximately +12 volts), and the lamp has so little voltage drop across it, if any, that it appears not lighted.  That is how things are supposed to work.

NOTE!….A DIM lamp at cruising rpm, sometimes only seen at night, usually means some sort of corrosion at some connector, or a bad rotor or poor brush contacting. A bright GEN lamp at cruising rpm usually means a bad rotor, bad regulator, bad diode, worn out brushes, etc.  A bright GEN lamp usually means no charging at all.

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The modification consists of installing a common cheap standard electronic part called a resistor. The resistor to is connected across the GEN lamp electrically, but not necessarily physically/mechanically at the lamp itself.  The resistor will produce a small amount of heat (ONLY during the time the lamp is normally lit, such as key on, engine not running, or, at idle rpm), but this is seldom longer than a minute or so; and in any event, the heat amount is small…especially with a 470Ω value.  That omega symbol means OHMS.

There are TWO ways to do this modification that I approve of:

#1. Neatly clean the area and solder the resistor across the bulb socket wiring itself. The resistor may be secured from vibration by cementing it with a tiny dab of silicone RTV or similar. Use short lead lengths. For the /6 and later type of instrument pod, use a very sharp Xacto knife, very carefully, to remove the protective coating layer in two small spots of the printed flexible wiring.  Have the copper bright and shiny.  Use a small soldering iron, very quickly, with rosin core electronics-type solder.

#2. Alternatively, you can 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 very neatly, with no chance of bare wires, nor vibration breaking them. Insulate the resistor and wiring with shrink tubing, and use proper plastic wire-ties, as required.

 

Resistor discussion:

Most types of resistor styles 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 (those typically are ceramic-cased). The resistor value CAN be the popular value used for many years for this modification, 470 ohms, but my tests indicated that 330 ohms is better, and will allow earlier charging, at lower rpm’s (this effect is SMALL).

**All BMW alternators from all years on our Airheads can use those values**

Resistors NORMALLY come in certain ‘standard’ values, and 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 at idle rpm. Too much heat MIGHT (not WILL) injure the instrument pod parts…if that is where you mount it, over the very long term; 470 or 330 ohms is OK. Remember that the resistor is only energized as much as a modest level when the alternator is not spinning and the ignition is on (pre-starting)… OR; when the alternator is at a low (idle to perhaps 1500 rpm) speed. Under the very worst conditions I can think of, the 330 ohm resistor value would produce about 0.6 watt of heat. If you want to be extra sure of not damaging the printed material when mounting the resistor at the lamp ‘socket’, then use a 470 ohm resistor, which will produce only about 0.4 watt under worst conditions. The printed flexible material in the /6 and later pods has to be VERY carefully scraped to get through the protective layer down to the copper material….don’t scrape away any copper. I use a very sharp Xacto knife, avoiding the pointy tip as it can be too sharply scratchy…& I use the blade on quite an angle.

The resistor should be rated at 1 watt or more, so it will essentially last forever. If you try to use a resistor size of more than 5 watts you will have problems fitting it into your instrument area…or…installing it neatly under your gas tank.  Ceramic bodied wire-wound resistors (the actually resistor is encased in white ceramic material) are available in a 2 or 3 watt size that work nicely.

NOTE that the Author’s website has a lot more information on this modification:

https://bmwmotorcycletech.info/genlampresistor.htm
HTTP://BMWMOTORCYCLETECH.INFO/INDEX.HTML

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Snowbum #1843

updated 02/12/2021 (minor clarifications)