The way I'd do it is add a set of gold plated contacts from a miniature relay to the oem oil pressure gauge, actuated by the needle, the buzzer or light would be lockked out above 1500 rpm, which I think is the trigger point of the EGR controller.

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85 300D turbo pristine w 157k when purchased 161K now
83 300 D turbo 297K runs great. SOLD!
83 240D 4 spd manual- parted out then junked