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BMW Instrument cluster lights and the K-bus failure

A failure in the K-bus may cause malfunctions in the warnings lights or control modules

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.

Symptoms

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.

Fault diagnosis

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
The K-CAN can operate on just one wire without the customer noticing any issues in operation. The technical will notice a slew of K-CAN faults in the vehicle.
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-.

Possible causes

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.
Disconnect all modules successively. If you still have no communication on K-bus, test the wiring harness
Check voltages on K-bus wires when disconnecting the modules.
  • 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:
-E60 BMW 5 Series PDC module full of water.
-E70 BMW X5 damaged wiring behind the CCC/MASK. Incorrect routing of the loom.
-E70 BMW X5 water ingress into trunk.
-E70 BMW X5 wiring damages against seat rails.
-E70 BMW X5 damaged wiring on the 3rd row seats.
-E70 BMW X5 rear view camera not working. Damaged wiring around the rear fusebox.
-E87 BMW 1 Series damaged wiring rubbed through the metal dashboard frame.
-E87 BMW 1 Series damaged wiring at the IHKA module.
-E87 BMW 1 Series PDC module full of water.
-E90/92/93 BMW 3 Series damaged wiring under the rear seats.
-E90/92/93 BMW 3 Series water ingress into trunk. Damaged modules.
-E92 BMW M3 damaged wiring under centre console causing a battery drain.
-E82 BMW 1 Series damaged wiring rubbing through parking brake mechanism.
-E88 BMW 1 Series multiple warnings on dashboard, K-bus Low.
-Damaged wire from IHKA control panel 

Repairs

With vehicle connected to Autologic:
  • Press Quick Test button.
  • Press Generate button.
This should generate a vehicle report showing modules with K-bus faults.
If a number of modules show K-bus faults, check the K-bus system:
  • 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).
Check modules showing K-bus faults for water damage. Also check connectors for water corrosion.
Disconnect individual K-bus modules one at a time. If communication to other modules resumes, the disconnected module is the faulty one.