Finally got some time to look into getting a tach to work in my 83 240D. I took apart the EGR controller (located in passenger kick panel) and it has an LM1815 chip in it amongst othe parts, same chip as in the tach amp. I think all I need now is to find the right place to tap in to get the varying ac signal and connect it to the tach (which I will borrow from my 300D turbo).
Can someone post an external wiring diagram of the 83 240D EGR system? Can you decribe how the EGR system works and what it controls? This would be useful info in figuring out what I have to do.