One for the experts on vacuum systems!

[mobilejo]

I am trying to diagnose a problem with the vacuum operated hydraulic motor mounts on my LCI e70 Xdrive40d - the diesels use mounts that have vac lines going into them, they work as follows apparently:

The damping characteristics of the hydraulic mount are set softer or harder by means of a vacuum. In this way, the vibration transmitted from the engine to the body can be influenced specifically.” (Quote from BMW training manual)
Failure of the pneumatic vacuum system will prevent the engine mounts from entering the ‘soft’ state which is activated at idle and low engine speeds.

Engine Mount Control DDE 4.0

The controllable engine mounts are switched to "hard" or "soft" corresponding to operating conditions.
Function

The pressure converter connects the engine mounts to the vacuum supply by means of hoses. When activated by the DDE control unit, the pressure converter switches vacuum through to the engine mounts so that they switch to "soft".
The engine mounts are activated at:
Road speed < 60 km/h and
Engine speeds < a value which depends on the coolant temperature as follows:
Below -40 °C: 1200 rpm
From -40 °C to -10 °C: 1200 rpm to 1050 rpm
Above -10 °C: 1050

Troubleshooting

Fault code 3520 "Engine mount control" is stored if a defect occurs in the activation of the pressure converter.
The engine mount control is switched off if one of the following faults occurs:
0500, "Driving speed signal"
0115, "Coolant temperature sensor"

Realoem diagram of relevant parts:



Now my car vibrates at idle and low revs, suggesting the mounts were not entering the soft state. To test I disconnected both vac lines ot both mounts - no change in idle at all. On a 'working system' doing this should cause idle to get "vibratey" because this would effectively switch off the soft state. No change indicates it was never in the soft state, which is what I suspected.

So I disconnect the main vac line (no. 16 in diag.) where it joins no.12 in the diag (i.e. before it is split to the 2 mounts), there is no vacuum at this line. This line (no. 16) goes in one complete length to the solenoid (no. 13).

I don't have any of the fault codes mentioned above so I can't understand why no vacuum is being delivered.

So, what do I need to test/look at next? I will try and get a new part 13. but I would have assumed a fault code if this failed? Where does this solenoid get it's vacuum from (i.e. what is on the other end of vac line 15?)

Thanks