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GEN Lamp Failures, Resistor Modification,
For BMW Airhead Motorcycles


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 modest rpm is had.  If the lamp burns out, the alternator may not provide any output. This article describes the system and a popular modification which eliminates any 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 many of the modifications that we all see. Some of these modifications, which I have done myself, are not very 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 pod ones, for sagging filaments which is a sign of impending failure.  Note that 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.     When the GEN lamp fails, it probably

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