there may be one, but i have never seen an Autometer tach that the instructions said that you couldn't use it if it had aftermarket coil - they all say not to connect the tach to the coil if you have aftermarket ignition and to connect only to the tach terminal - this means if you have MSD 6AL, MSD SCI, etc. ignition, you connect the tach to the tach terminal on the ignition control box and not to the coil - if you don't have aftermarket ignition, just external coil, you connect the green wire to the negative terminal of the coil - Autometer does say not to connect the green wire to the O.E.M. tach signal wire (usually blue) that runs from the distributor to the gauge cluster - Quote: "the signal to the cluster tach is not the same signal as what our tach needs to see (12v square wave)."