Have just bought an Audi 80 TDI 1994

First - no Haynes workshop manual for this model - anyone know of an alternative?

Second, oil light comes on as normal and goes off when engine is started.
At 2200 rpm, the oil light comes back on and beeps three times.
Above 2200 rpm, the light stays on, but beeping stops.

Both oil sensor switches on oil filter housing work.
When sensors are disconnected, the oil light does not come on when starting engine, but still comes on and beeps at 2200 rpm.

Q. Is there a third sensor somewhere? If so, where is it? Is this likely to be an electrical fault?

I have put in a new Audi oil filter, changed the oil and flushed the engine. But this has made no difference to the oil light.

I am still driving the car and it appears to be fine -any ideas?!!