Another souce of a signal to control the oil pressure alarm is the EGR computer. Some models have it and some don't, which will add to the confusion. I know the 83 240D has an EGR computer because I have tapped into its tach circuitry to drive a 300D turbo tach. An EGR computer typically looks at RPM and use the signal to control the EGR solenoid.

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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