What is a dirty Tach signal?

A dirty signal means that there is too much “noise” (interference) mixed with the Tach signal on the line making the actual Tach signal hard to lock on to. This is a condition often seen in high-performance engines that have some type of spark “boost” device like a MSD Ignition or other similar system installed. A dirty Tach signal can also be caused by poor routing of the Yellow tach wire.