I put a tach in my 83 240D using the EGR computer as the tach amp. Search for my post. There should be some info there that may help you in wiring up your tach amp and how to power it from your cluster. My 240D is now a parts car and I have sold the tach setup to a member here and he is enjoying it in his 240D.

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85 300D turbo pristine w 157k when purchased 167,870 July 2025
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked