I think I have come up with an explanation why using jumpers to bypass the EGR computer works on the 85 model year 300D.

This is what pin 25 (output to tach) of the EGR computer looks like. It is a 3.8 v
1666 hz square wave AC signal.




This is what pins 16, 20 (transmission RPM sensor) looks like. It is a 10 v, 1666 hz sine wave AC signal. It has much higher output compared to the RPM sensor s on 83 and prior model years (around 6 to 7 mv). Due to the high voltage signal from the RPM sensor, it is strong enough to drive the tach directly.

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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