BMW/Mini – K_BUS system errors

Autologic Diagnostics’ Technical Help Desk looks at a common diagnostic complaint on BMWs and Minis.

Fault

Symptom: Loss of communication from a number of modules
Brand: BMW and Mini
Models: All
Cause: Individual control module has taken down the K_BUS system or could also be a wiring fault

Fix

1. Are K_BUS faults stored in a number of control modules on your Autologic device?
2. The K_BUS faults stored will be shown as a 12V square wave signal. When the car sleeps this sits at 12V and when active this switches to a square wave signal, 1V below battery voltage.
3. Note down the modules with the K_BUS faults after a quick test.
4. Check for water ingress into the modules that have stored fault codes.
5. Next find the buzz bar. This is located behind the glove box, with white/red stripes and yellow hoops.
6. Remove all cables from the buzz bar and find the wire that is generating the square wave signal. When checking cables you will need to use an oscilloscope.
7. Relocate the cable generating the signal onto the buzz bar and systematically check all of the other wires to see if they’re going to earth when the ignition is off.
8. If any are going to ground, mark it and tie it back before reconnecting all remaining cables to the buzz bar.
9. Carry out a quick test to see if any control units are not communicating.