Specifications
50
Testing Testing Bus Systems
The LM reduces a number of components that were used in the past, such as:
• Numerous relays
• Flasher unit
• Crash control unit (relay)
• Cold check relay
• Dimmer switch
NOTE:
i Note the following regarding the LM module:
• The LM is informed by ZKE when a crash has occurred or the alarm was tripped for headlight/
flasher activation.
• When reverse is selected, the LM is signaled to turn on the back-up lights.
• The LM can compensate for a faulty brake light bulb by using the tail light filament. It increases
the light intensity by using pulse width modulation to apply more current. This may make it
more difficult to diagnose a faulty light bulb, compare both rear lights, or check the one that is
less bright.
D Bus (Diagnostic Bus)
The scan tool initiates communication on the D Bus (diagnostic bus). The D Bus is only active
when the scan tool is connected to the diagnostic socket and there is two-way communication
between the scan tool and a control module.
The D Bus is the oldest bus system used in BMW vehicles. It was introduced in 1987 as TXD,
which provided communication between the DME and a scan tool. The D Bus may still be referred
to as TXD in BMW training literature.
The D Bus communicates in series, one control unit at a time, with all control modules in the
vehicle that are capable of communicating with a scan tool, some of which may be on other buses
and communicate through the Instrument Cluster module or IKE gateway.
NOTE:
i The D Bus communication speed to the scan tool may be considerably slower than the internal
bus speed or the communication speed between buses.
The scan tool currently communicates via the D Bus to the following modules and buses:
• Powertrain (engine, transmission, EML, ABS, everything on the CAN bus)
• Airbag (MRS)—K Bus
• Climate Control (IHKA)—K Bus
On vehicles produced up to model year 2001, use the 20-pin underhood diagnostic connector.
The two links are:
• RXD pin 15
Later control modules (from 1997) no longer required the separate RXD to establish
communication, so pin 15 may be removed.
• TXD pin 20