Bus systems, connecting a number of control modules, sensors and actuators, became the norm in BMW vehicles beginning with 1999 – 2000 models. The K-bus (or K-CAN), a 2-wire bus, connects the instrument cluster, climate control (IHKA), airbags (SRS), lighting system and tire pressure control. A failure in the K-bus may cause malfunctions in the warnings lights or control modules for any of those system.
The vehicle may exhibit one or more of the following problems:
- Random instrument cluster warning light failures.
- Instruments cluster inoperative.
- Climate control, airbags, lights or tire pressure sensors inoperative.
To start with, use the Autologic scan-tool to check for fault codes:
- Fault code E204 K-bus line faulty.
- Fault code E247 K-bus line Low fault.
- Fault code D904 K-bus line High fault.
- Fault code E5C4 K-bus communication fault
NOTE: The best tool for checking bus systems is an oscilloscope.
Check for any obvious issues like to water damage to control units logging K-CAN faults. Also check connectors and splices for water corrosion.
Disconnect individual K-CAN modules one at a time. If communication to other modules resumes, the disconnected module is the faulty one.
Disconnect all modules successively (except the Gateway and CAS – will need to be swapped from another vehicle). If you still have no communication on K-CAN, test the wiring harness for short to B+ or B-.
If the preceding fault codes are found, investigate the following causes and repair as necessary:
- Failure of a body module: climate control, airbags, lighting or tire pressure control.
- Water damage to a module or electrical connector.
- Damage to wiring or short in wiring.
- A short to positive raises voltage to 12 v on either the High or Low line.
- A short to ground drops voltage to 0 v on either line.
- Check the following model specific locations for wiring harness damage:
- Press Quick Test button.
- Press Generate button.
- K-bus is a 2-wire system; one wire is K-bus High and the other is K-bus Low.
- Use oscilloscope to diagnose K-bus High and Low patterns. High and Low square wave should mirror each other. A volt meter is only a quick check and not a definitive diagnostic tool.
- High = 0.4 v (this is the green wire on E60 5 Series vehicles).
- Low = 4.6 v (this is the green /orange wire on E60 5 Series vehicles).