VT instrument cluster config

[pman92]

Hey guys,
Had a VT in at work today. Customer had put a new instrument cluster in it with trip computer or whatever. Tacho didn't work and SRS light was on.
The SRS DTC was something like '53 - configuration mismatch with cluster'. And the cluster had to be reconfigured to 3 loop SRS from the 4 loop SRS it was previously set on.

Looking at a wiring diagram I found the the SRS module talks to the BCM and cluster. Am I right in saying that the BCM part is only because the BCM is the bus master and will be constantly polling the SRS for its presence on the network? And the SRS only communicates with the cluster to say if the SRS light should be on or off?
If so why does the number of airbags/pretensioners need to be known by the cluster?

[vs ss]

I can not help as to why the cluster needs to know ,but I say it is on the bus so that When the airbags deploy a message gets sent to the pcm to shut the engine down.

[Gareth]

One of the reasons for SRS to BCM comms is to unlock the doors in the event of deployment, pretty sure the SRS cant shut the engine down.

Yoda might be able to add to this?

[vs ss]

It is in the code for the pcm to turn the fuel pump off.
Hopefully someone else can confirm.

[yoda69]

Yes will shut the fuel pump down if airbags deployed and unlock doors.
Not sure the exact reason for the different cluster programming but know it does need to be done.
Think with telematics fitted vehicles it even calls the call centre automatically if airbags are deployed.

[antus]

The dash needs to know how many airbags are installed in the car, if theres a mismatch you'll get a dtc as it thinks a missing airbag has failed or it sounds like it detects the mismatch. The factory software can update if you can get it, or probably a tech2.

[pman92]

Thanks guys.
It's updated all good with aftermarket scan tool but just wondering why the cluster really cares.
I would of thought the SRS module would be the only one needing to know. I noticed it lists resistance and capacitance readings in the live data for all 4 circuits (drivers and pas air bag and pretensioners), and also lists them as enabled or disabled.
In this case they were all enabled except pass air bag.

Is it possible the cluster requests that resistance/capacitance data and then verifys it? Sort of a fail safe incase the SRS module doesn't detect it for whatever reason.

[jacomms]

So it is from VT - VY (not so sure about VE)
The Cluster needs to know what info to report on as it gets RX data from the SRS as well as any other module sending data to the cluster.
So you need to tell the cluster what you are using and it will ignore any null data as such.
Tech2 will allow you to set all dash data, enable police mods on the cluster trim speedo & tacho cal etc.
hope this helps

Justin