Author Topic: Simple modification to BMU for LED running and brake lights  (Read 534 times)

Offline rbm

  • ^ Quintessential Motobricker
  • Posts: 1941
Simple modification to BMU for LED running and brake lights
« on: January 27, 2019, 09:05:29 PM »
The Bulb Monitoring Unit (BMU) on the K-bike will light up the Red triangle indicator on the dash if you swap the OEM incandescent lamps for LEDs.  This is annoying.  The recommended solution for this situation is to tear the BMU apart and jumper the leads on the plug to bypass the electronics.  I reverse engineered the BMU recently and noticed a relatively easy modification that can be made to the BMU circuit card to stop the dash indicator.

Fair Warning:  I haven't tried this modification so please do your own research before trying out this suggestion.  This is specukation on my part until I am able to test this modification in my workshop.

Refer to the picture below.  It is a photo of the inside of the BMU, annotated with part numbers to match a schematic I drew of the BMU module (see my link below).


The modification is to desolder one lead of R1 and pull it free of the board. This resistor is pictured in the upper left corner. The best lead to extract from the board is the one nearest the corner. 

To accomplish this with the least struggle, first heat up the joint with a soldering iron and straighten out the bent wire.  Then, using a jeweler's screwdriver, apply leverage to the resistor on top of the board, prying up on the lead as you heat the joint with a soldering iron from below.  When the solder melts, the lead of the resistor will pull free.  Just leave the resistor hanging in the air. 

This mod will disconnect the indicator driving circuit and stop the indicator from illuminating. The nice thing about this mod compated to other suggestions is that this modification is reversible, to be able to bring the board back to factory original condition.

The BMU schematic I drew is posted in this thread.  http://www.motobrick.com/index.php/topic,10566.0.html

This Motobrick post gives the BMU connector pinout information:  http://www.motobrick.com/index.php?topic=906.0

This Motobrick post talks about another way to bypass the BMU functionality:  http://www.motobrick.com/index.php/topic,331.0.html
  • Regards, Robert
Toronto, Ontario

1987 K75 - Build Blog @http://k75retro.blogspot.ca/

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