I got out my EVTM book and traced out the tach wire to the VMM. It comes from an ignition suppression resistor that has its other end connected to the coil. Another ignition suppression resistor is connected to the same point on the coil and goes to pin 4 of the EEC. If the input impedence of the tachometer were too low it could load up the tach signal in effect shorting it out causing a problem. The best way to check the input impedence of the tachometer is to measure its DC resistance to the ground connection. Although, it's not the same thing it will give you a good indication. If the resistance measures more than several 100K ohms, the higher the better, you're probably Ok. If it's less and in the kilo-ohm or lower range it could be a problem. Now that we've thoroughly confused you Larry, try it and see if it works Ok. It's easier than running a wire thru the firewall.