Maybe this will help in fault finding your issue Mark. Only 8 of the 26 VDO master gauge connections are used and have identified each one by pin number, cable colour, purpose and source/destination as below:
Pin 1 - purple/red - keep alive +12 volts from engine fuse 3
Pin 2 - black - ground connection - to grounding eyelet shared with six others.
Pin 3 - slate/red - CAN high - shared data feed to instruments from pin 89 on ECU via engine loom
Pin 4 - blue/red - CAN low- shared data feed to instruments from pin 97 on ECU via engine loom
Pin 8 - pink - Kline - also normally used for OBD2 data - appears to only go to pin 7 on diagnostics socket
Pin 16 - pink/red - instrument illumination fed from instrument dimming potentiometer
Pin 18 - white/green - ignition switched +12 volt feed from engine fuse 10
Pin 19 - brown/white - frequency signal (0-40khz) fed from pin 86 on ECU via engine loom
I would also check engine fuse 10 (as well as aforementioned fuse 3) therefore and, by way of a shot in the dark, change both fuel pump and engine relays for Durite ones (well worth doing if you haven't already). Doubt if the issue is attributable to the sealed ECU but could of course be the Speedo. Otherwise hope this info helps.
