Bit more involved than that.
I have just bought from Germany a Frankia T640 FF2 and upon getting it home find a few problems, first was that the Oyster system didn't work properly, Ten Haaft sorted that but then realised the dish didn't park on engine start and looking deeper the fridge worked all the time if 12 volt selected engine on or not as well of course to all other outputs associated with engine running operation.
Some more in depth investigation required, located the Electroblock EBL 220-2 and the 25amp
fuse which should only be fitted if a compressor or AES fridge is fitted was in place, hence continuous supply to the non AES fridge.
The black sense wire for dish was attached to the correct wire on the EBL but didn't energise, then looking deeper I found the wire from pin 4 of BL 12 was simply tucked behind the controller connected to nothing, attached to the end of it was a plug of the style used on the EBL, on checking, no signal is / was present on the wire.
I opened up the engine
battery area and found the D+ wire there connected to nothing, and nothing to connect it to, this wire didn't appear in the area of the Electrobloc , when searching in the double floor area I found the other end, still with its original Lumberg pin crimped on.
I can only guess that the previous owners had removed some equipment without re-instating the original system.
I've replaces all connections and confirmed that the Electroblock works correctly as it should but now need the D+ signal to operate it.
I've checked the bottom of B post and do have a small wiring block there, unfortunately it is not populated with a D+ signal.
As a note the wire (yellow and marked by Frankia as D+ ) in the engine
battery box has an 8 mm ring terminal crimped on and it appears to have never been fitted to anything.
I did think about using a D+ simulator but not sure how that would work as the
solar system would be supplying the habitation and engine
battery with a constant charge which I'd guess would cause it to think the alternator was working.