AWD Problem
 

Need some help please. I have 57 plate XC 90 D5 with 69,000. Symptons, when doing a turn on full lock both left and right both forward and reverse the rear wheels are juddering as if they are going to lock up. If I disconnect the DEM all is fine. I have checked the oil pump on the Haldex Unit and it is working fine. The oil is clean. I sent the DEM of for testing and it came back stating it had no fault codes. I have plugged into VIDA/DICE and have the following fault codes
BCM-0094 comms prob with DEM.
BCM-0115 comms prob with SAS, signal missing.
BCM-0148 comms prob wrong configuration ID
HCM-2987 Lost comms with BCM.
CEM-1A64 Comms with AOC control module signal missing.
I am thinking perhaps there is a fault with the BCM and I need to send it off to be looked at, I am hoping one of the experts on this helpful forum will be able to give me some advice.

Apply systematic diagnostic methodology:
Check battery voltage first, don't do anything unless battery is fully charged. Connect a charger if you will be working on the car with ignition on but engine off, otherwise you run the risk of a flat battery which will play havoc with diagnosis.

Clear all codes, monitor what returns, in what order, and how quickly

Check what devices on the car cannot communicate

Look for commonality in the CAN bus connections

Check the physical CAN bus wiring to the affected devices is OK
Measure with Ohmmeter and Voltmeter, Monitor with scope

But my gut feel says you have CEM problems: remove, open and inspect for any water damage

Agree with above in that you should resolve all fault codes first and then see if you have a problem.

Another quick test is take it to an MOT test station, put both the front and rear wheels on the brake test rollers with the engine running and the gearselector in D (but not throttle). If the car jumps out of the rollers (or shudders in them) you have a locked up AOC coupling.

Exactly the same symptoms on my 06 185 XC90 when I got it, felt like a tractor with the difflock engaged, front wheels skipping on full lock.

Before you get too drawn into the Haldex coupling, check the front geometry.
I checked the tracking with my old gauges, wheels were parallel.
Got a mate to put his laser gear on it, wheels were parallel but there was play in the rack drumsticks. Changed them, was like brand new.

I wouldn't have suspected this as a problem, as it drove fine in a straight line.

AWD Problems
 

Thanks to all that have answered to my call for help. Tomorrow I am going to have Battery checked,its on charge tonight. I will also delete fault codes and as adviced, see in what order and how soon they re-occur. I will talk to my local MOT station and arrange to have it put onto brake test rollers and see what happens. I have recently had tracking on all four wheels checked and done quite a few miles since and had no problem. I will give up-date at weekend as to how things are progressing or not!!