Thanks chaps,
After another 2hrs of methodical checking, here's how it stands:
Overdrive circuit is on the same fuse as the rear window heater (?). Pull fuse - nothing works. Fuse in - O/D electrics OK. Problem with gauges and OD indicator lamp still persist.
Wiring checked on gear shift - inc the 'allow start' on P and N - all OK. Earthing is OK.
Rear floor up - All wiring to the fuel sender unit Ok. (Well impressed with how secure they made all the wiring in this area).
Fiddled around with the voltage regulator transitor on the back of the dash circuit board (nice of volvo to put it in a socket, not direct soldered). Connected - some response on the fuel gauge - not right down, but the OD lamp lit the whole time. Pull the transistor - OD lamp goes out. fuel gauge right down.
So ... either the voltage reg. transistor socket or possibly the dropping resistor that feeds a control voltage to the trani failed ? Will check this next go.
Learned a lot but out of time for now. Its drivable - Overdrive works and the lock off switch ( side of the top of the gear shift) works. Just v inconvenient wit no fuel or temp gauge and the OD lamp on all the time.
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