Mk1 Cabrio Oil Pressure Warning

Recently bought a mk1 Cabrio 1.8 8v Carb
It has a low oil pressure warning buzzer and light when warm and revved over 2500rpm exactly.
This warning stays on until you either turn ignition off or rev to just past 3000 rpm
Ive replaced sensors… changed oil… replaced oil pump… 
What could it be? The only thing i can think of is wiring and the pressure warning pcb

check you have the right switches first, the one in the filter housing should be a 1.8 bar and side of head 0.25-0.3bar. VW ones are white for 1.8 bar and blue/brown for 0.3, but aftermarket could be anything.

usually the filter housing one should have a black/blue wire and head yellow, but sometimes these colours are switched on the later models