No you don't need the old wire, unless you plan on using the ignition module, I replaced mine entirely. On the hot spark the negative to the coil does your firing, Positive volts should be present at coil with key on at all times. Where are you measuring voltage, you will not get a reading by probing pos and negative on coil itself. If car is not running with key on you should get 12 volts positive on both coil terminals, as you are reading through the transformer. You need to probe around at the resistors and find the wire that is 12 v. with key on. Connect that to pos on coil, connect hot spark as directed. Note if your car is like my 280 was the original ignition module uses the positve side of the coil to fire spark, which is oposite of what you will have now. Make sure you are not still using the old wire to coil+ as it will not work.