I was just having a look at this pdf file
HERE and it seems to point to needing the BCM.. It states
GENERAL INFORMATION
The VT Series Model uses a Bus Master communication system where the BCM is the Bus Master.
The BCM periodically polls (surveys) each device on the bus and requests status data.
The devices connected to the bus are:
• Body Control Module (BCM).
• Powertrain Control Module (PCM).
• Instrument cluster (INS).
• Antilock Brake System / Electronic Traction Control (ABS/ETC).
• Supplemental Restraint System (SRS).
• External diagnostic scan tool (TECH 2).
The data provided by each device may be utilised by any device connected to the bus.
Each device has a unique response “Message
Identifier Word” (MIW) for ease of identification.
The bus master (BCM) polls each device with a serial data message which includes that devices MIW. The device
responds by putting a serial data message back onto the bus. The devices message includes its MIW and data, of
which is retrieved and utilised by any device on the bus requiring it.
Excluding the PCM, the BCM polls each device for a status update once every 300 milliseconds. The BCM polls the
PCM twice every 300 milliseconds.
When the ignition is switched from OFF to ON, the BCM will communicate with the PCM, via the main serial data
line (circuit 1221), for antitheft purposes and TECH 2 will be able to communicate with any module via the ALDL
mode. If the BCM does not receive an OK TO START message from the PCM within 0.5 seconds of the ignition
being switched on, the auxiliary serial data line (circuit 1220) is isolated via switching within the BCM.
The isolation of the auxiliary serial data line during this period eliminates any possibility of a device failure other than
the PCM or BCM, causing a problem on the bus, inhibiting antitheft communications. This period (know as short
loop time) continues until the PCM responds with an acknowledgment or a maximum of five seconds after which the
BCM will switch to the standard polling sequence.
Following successful antitheft communications, the BCM begins sequential polling of devices on the bus and normal
system operation is established.
When the ignition is switched OFF, the BCM continues to poll, allowing for TECH 2 communications and external
control of the bus prior to the ignition being switched ON.
