I'm not sure about this but I think the EGR valve on the 300D turbo's are strictly vacuum controlled and does not detect engine RPM with an LM1815 on the EGR computer like on my 83 240D to control the EGR valve. On the 300D's that gizmo on top of the valve cover (VCV?) controls the EGR. I think you will need the mag pickup, the RPM sensor (aka tach amp) and just wire it up if you want a tach.

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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