[Schulisco]

The 1980 SC was the first 911 with electronic lambda control. The box is under the passengers seat. The fuse #4 supplies also this box. When it's broken no power supply is there and lambda control is fully out of service. Esp. the cycle valve doesn't operate any more, which manages the fuel distributor pressure. Do you have an oxygen sensor ("Lambdasonde") still installed?

Refer to Andrews (user AndrewCologne here) fabulous website according those engines:
https://nineelevenheaven.wordpress.com/der-911-us-sc-3-0-motor-mit-g-kat/

Schematics you find here:
https://classickabelboomcompany.com/wiring-diagram-porsche/
Unfortunately not for the 1980 model, but you can use them of the '81-'83 models. There only slight differences, I suppose your car doesn't have the acceleration box (an additional electronic box which enrichens the mixture for 2secs on accelerating, but no necessary for older cars)

Thomas