From The E28 Wiki

OBC I Went Dead

By Wiseguy, Mon Mar 06, 2006

Original thread: My OBC1 Went dead (Pre-85)

Problem: My OBC1 Went dead (Pre-85)

Solution: Check the fuse that's INSIDE the OBC Computer, located underneath the steering column. Inside that silver box is a small fuse (I always used SFE4 fuses from Radio Shack) in an inline holder. I bet it's blown.