On certain P2 and all P1 and P3 models , the modules on the infotainment network communicate by a fibre optic network.
On P2 platform only XC90 have optical networks.
But unlike modern day networks this is an old fashioned daisy chain, which means the data passes through every module in sequence.
If ANY one module is faulty or without power the WHOLE network goes down.
The network overview in Vida is especially helpful for diagnosing the fault.
The network overview in Vida shows modules with 3 colours.
- Grey (not expected to be present)
- Green (Happy)
- Red (not online)
So in the above image from a P3 we can see ALL of the expected infotainment modules are red.
Which lets us know that the problem is with the IHU (integrated head unit) , IAM (Integrated Audio Module) or ICM (Infotainment control module depending) on platform / year .
P1 / P2 – ICM
P3 – Pre sensus – ICM
P3 – Sensus – ICM
P3 – Sensus Connect – IHU
How do we know? Well, the “Head” of the network is the IHU / ICM (depending on model year) and is the only module on the canbus . It is the module that reports in the status of all the other infotainment modules. This Is the MOST to CAN bridge.
To word that differently, you can unplug a working IHU or ICM from the optical network and it will still stay green. Nothing will work, but it will be green hahaha.
So if your ICM / IHU is red thats where you need to start your investigations.
It gets a little trickier as the problems move around and this is where a MOST bypass adapter can be helpful, more on that later.
Say your car has a DAB radio tuner, and its fuse blows. The entire optical network goes down every module (except the HEAD) will be red, so how do you know which one is faulty.
Well Vida has some useful routines to help understand
If you were to go to Diagnostics, Vehicle communication , (select head module) , Advanced you can see two routines of use.
- “Communication on MOST” – This routine checks for MOST LOCK , which really means is the fiber optic network functional, is there a complete circuit.
- “Open Circuit on most” – This routine instructions you to remove the master fuse form the infotainment then put the fuse in then it waits to see which modules are talking or not talking and it will tel lyou which one it is. Handy but only works when the optical network is complete, and when you have a fully dead module, its never complete.
So what do you do then?
Well, then you need one of these….
A MOST bypass (a few quid from amazon) allows you to bypass a suspect module and complete the network.
Amps fail more than any other module, so unplug the MOST connector form the AMP, plug the network into the bypass and then see if the rest of the modules go green. If they dont , then you haven’t identified the correct module, so move to the next one and try again.
Once you have identified your fault (i.e. fuse or wiring or whatever) , and your confident the module itself is faulty you can replace it with a compatible unit.
Ensure ITP security is disabled and swap in a compatible module and life should return to the infotainment.
Read here about ITP security and how to disable it.
There are some tell tale signs you can get in the car to help make a guess at where to start looking.
Infotainment wont switch on atall, likely candidate is the HEAD of the network.
Infotainment comes on the goes off after a few seconds, likely candidate is ITP security is being enforced as someone has already tried to fix the issue with an ebay module. (i see this a lot with “I just bought this car but the radio turns off after a few seconds”)
Infotainment comes, stays on but no infotainment controls work, likely candidate is that there is a break in the MOST network.
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