You have no "Tach amp". It's built into the EDS computer. You have 3 common possibilities:
1: OVP fuse or relay is bad. If you have an ABS or Airbag light on, this is probably your issue.
2: Tach head is dead. Usually replacing the capacitors and cleaning the grounding points on the circuit board will bring it back. If your idle speed is normal and the A/C works, this is likely your issue.
3: Blown fuse for the EGR/EDS system. If you have the aluminum bullet fuses in your fuse box, replace them with copper or brass ones, the original aluminum ones cause way too many headaches.

__________________
Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)