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It sits above the oil pan contact surface with the engine case, above the frame tube, behind the right engine cover. It has a rubber dust boot over it. Mine is 82 A4, but I think it's true to other models. Your problem might be the sensor, or crossed wires as it was my case when I was figuring out where they go. I'd go on a limb and suggest the sensor is fine. It's easy to cross those single wires under the battery. You can't go wrong with connecting the sensor - it comes out of the harness. The problem would be close to the all electronic cluster behind the battery. I don't think there is any danger playing with those wires. It's probably just this - a black/yellow wires crossed.

Thanks Stan, I'll check that out.
As you say, mine has all sorts of dodgy wiring and connections, I'm surprised the 'only' faults I've got is this oil switch and the slightly odd hazard flashes.

Richard.

So now that we've had our "poke".......
Blue (BLUE/white) wire ('79/'80) from the oil PSI switch runs in the same sleeve as the 3 alt. wires. The bullet plug connects on the right, behind the side cover. Power comes from the Main circuit BROWN (30A) so don't bother looking for an individual Oil-light fuse. There isn't one. Oil PSI OPENS the switch. Remove the wire from the OIL psi switch and touch it to the frame. Got a light - Bad switch! No light - bad connection elsewhere or burned-out bulb. Check the bulb. If the bulb is OK then use a voltmeter to confirm power to the bulb-socket center terminal. (Careful !..... Put a piece of Shrink-tubing over the meter-probe.) If there's no power then check the 4-pin plug in the H/L housing. Specifically the BROWN and BLUE/white. Got power - swap one of the other idiot- light bulbs to confirm the circuit. If you still don't have a light then it points to the oil PSI switch. Before you remove it have a 1/8" MPT plug ready. You'll need to plug the port or you'll have oil all over the frame and floor ! Check the switch with a meter to confirm continuity (contacts closed) You may need to replace it.

Thanks Scotch, perfect diag's info.

Oil switch solved! Yipeee!

Found the cunningly concealed beastie,
Boot off,
Terminal earthed to block.. nothing.. damn, suspect faulty switch.
Buzzed the switch through, no continuity.
Switch taken off, finger over the hole (on sidedstand the flow is very slow) and buddy (who restoring a 1950s TVR) found an imperial bolt with similar thread, which I finger tightened into the hole as temp bung (dodgy I know but no where near thread-damaging tight),
On the bench, oil switch still no continuity,
Checked for gunk in the oil gallary, none,
Thought about peeling open the crimp to investigate (and most probably scrap it! but hey at 9pm..) then had an epiphany and decided to whack the boss down hard on wooden benchtop a few times and..
Bingo! continuity! Ha! gotta admit this is only a 'shops are shut so no spares' strategy.

Have refitted, tested good on ignition, test engine started (on 3 or 4 good cylinders, hey ho..) and extinguishes, relights when engine stopped, so functionaly sound.

Time will tell if the 'bodge' has longevity...

but thanks for the guidance guys, one of my quickest fixes ever!

Let me be the Devil's advocate. Would you really want to rely on a switch that tells you if you're about to have a catastrophic engine failure on a 2 wheeled vehicle if the fix for it is to whack it on a bench top? I wouldn't have gone that far. You determined that the switch was faulty when you ground the wire to the frame, so I would have stopped there and bought a new switch. Switch can't cost more than $20.00.