Fixing glowing telltale light on K models
A friend applied the official BMW service bulletin fix to solve the glowing telltale light problem on his K100RS. It worked like a champ. I plugged it into HTML in case you want to add it to your K bike home page.
Service bulletin 61 021 89 (2354), issue April 1989
Subject: Telltale light glows-K models
Battery charge telltale light glows very dimly, even though engine RPMs are at highway speeds. This condition is most noticeable at night and when heated grips are used.
Testing the charging systems proves that the charging system is working properly and no fault can be found. The purpose of the charge indicator light is to inform the owner when alternator output voltage equals battery voltage. This is done by providing battery power (gn sw) to one side of the indicator light. The light grounds (bl) through the alternator diodes until alternator output equals battery output, thereby providing a positive on both sides of the bulb, which makes it go off.
The light may glow (particularly with the heated grips activated) because some resistance has developed in the connections on the (gn sw) side of the circuit that slightly lowers the circuit voltage, so that a minor voltage difference occurs. Heated grips’ consumption exacerbates this resistance sufficiently to produce a drop of perhaps 1/2 to 1 volt, which makes the light glow. This condition does not affect the output of the generator.
Although the system will work perfectly without any change, it may be annoying to the rider. The simplest way to correct this condition is to install a diode in the blue wire. This will eliminate any backfeed if the voltage drops slightly on the system side (gn sw) of the circuit.
Installation of Diode:
Diode–part no. 61 31 1 358 268–can be installed near the alternator. Cut the blue wire approximately 2 inches from the plug-in connection at the alternator and splice in the diode. The diode and splice connections must be protected from elements with heat-shrink tubing or electrical tape. OR The diode can be spliced into the blue wire inside the electrical equipment box under the rear of the fuel tank.
Direction of Diode:
The arrow on the diode must face towards the alternator.