I have a 2000 Ex with an Si gauge cluster. The tach works fine with the OBDII ecu. I converted to OBDI P28 ecu with convertion harness and the tach is dead.
I tested the OBDI ecu in another Civic and the tach works. I tested a second OBDI ecu and harness and have the same problem, no tach. Does anyone know what I can try next?

 

try a different cluster!

 

If you still have the stock cluster try that out.