Some ECM's tach signal is not strong enough for some aftermarket tach's. This method can be used to “boost” the signal from the ECM. All you do is hook one end of a 680 ohm resistor (SUPPLIED BELOW) to the tach wire, and other end of the resistor to a keyed 12V+. This may not be needed with all tachs and all ECM’s. The pre 2002 ECM’s and tach combinations seem to be hit-and-miss. So far, all 2003+ have had to have it.
Your tach’s power supply should be a keyed 12V+ source. So simply use the resistor as a “jumper” between the main 12V+ supply to the tach and the incoming tach signal wire. Make sure the leads on the resistor aren’t allowed to short to ground as they are unshielded wire and “hot” when the key is on. Make the connections then cover them and the resister in shrink tubing works well.