My temp gauge and tach were not working so I replaced SI batteries but realised that there is a small repair done on pcb trace and some of the transistors are blown.
BMW E30 SI Board info and troubleshooting - Bavarian Restoration
Alternative SI battery solution? - R3VLimited Forums
BMW E30 3-Series Service Indicator Battery Replacement(1983-1991) | Pelican Parts DIY Maintenance Article