Yes, as Rob said, I studied the wiring diagrams for some time
and concluded that the wire coloured 'SN' is shown on the wiring diagram (page 14-14 of Haynes) as connected to pin 23 of the ignition ECU, then not connected to anything (terminates at a connection block). This pin is also not shown on Fig. 13.80 in the supplement (all other pins are identified). I was hopeful that it may be for the digital boost gauge.
It may (on the other hand) be for an electronic diagnostic connector. I think there is a three-pin sealed socket floating around near the ECU on the Uno Turbo. So it would be a good idea to find this socket if possible and check the wiring colours to see if that's where the mysterious SN wire crops up (I haven't even worked out what colour SN is
)
The approach I suggested to Rob was to unplug the instrument cluster plugs and look for the copper traces on the circuit that the plugs fit onto. In theory, there must be a trace for the boost gauge that does not have a corresponding wire in the standard loom? So this is where you would connect the signal. The pins are easily removed from spare Uno wiring using a small screwdriver.
-Alex
