Originally Posted by Graceland

Another thing to check - make sure you are still getting full battery voltage up in the high rev range - remember, on boost, the fuel pump will draw much more current and the voltage the ECU/IAW sees will drop significantly if the wiring is old and past its best. The IAW needs above 12 volts to make the rev counter work efficiently, any less and the rev counter will fail to work.

How do I know this - I found out accidentally when a coolant hose split, thus soaking the alternator, reducing the output to 9 volts and the rev counter stopped working utill the alty was producing the correct voltage once again.