MyE28.com

Having wrestled with my (E28M5) tach and instrument cluster I found that the tach will not work without a working code plug. They are available from Lithuania via eBay. I would contact Joseph at ProgRama in south florida. They actually have new SI boards too and can simply test your cluster for function and provide a diagnosis. If you are in europe then definitely go to Ivo. 

Back to mine, I have a high amp draw somewhere on fuse #5 and there are many possibilities for this!!  Good Luck, 

Bill

Can you show a picture of your connections to the tachometer and the car body? Also the connections of the multimeter with the voltage and frequency readings.
There is another check you can do but seeing what you are doing would help.