I read through the pertronix instructions before I installed the unit, but iii checked the voltage while it was running, and I guess that throws off the volt meter. I checked it in the KOEO position, and I was getting 7 volts at the coil, so there is a resistor. Would you happen to know where it's mounted? I looked all over the engine bay, and was unable to locate it. Also there is only one vacuum hose connected to the distributor, and I disconnect, and plug it when I'm checking the timing.
I'm not really up on 73's, so parts and other things could be far different than the 71 I'm familiar with, but I do have quite a bit of experience with Pertronix modules now, having gone through this routine myself.
First, you did NOT say which Pertronix Ignitor module you bought. That is very important for us to know. ALL modules should be run on a full 12V at the coil, engine running (or key on?). However, the basic Ignitor I , can run with a resisted voltage according to Pertronix. Likewise the Ignitor II, but NOT recommended. The Ignitor III MUST be run with a full 12 V UNRESISTED power supply. From my personal and costly experience, I would NOT suggest using the Ignitor III. I have written about this issue many times. It is the
construction of the module for the Ford application that was a major problem,
not the electronics. Unless Pertronix have redesigned the stupid little plastic bearing between the two plates, you WILL have timing issues because the top plate will move around, altering the timing. I'm not going into the full details here, but in a nut-shell Pertronix replaced the Ignitor III with a II and a free matching coil, problem solved. ALL Pertronix modules should be used with the MATCHING coil.
On a 71, the resistor wire is located were the harness plug on the s/column meet the main harness connector. Is it the same on a 73, I don't know for sure. It is a large PINK wire in the main harness connector. As mentioned I believe that wire powers the tach, but as I don't have one, I don't know for sure. In the instructions for the Ignitor III module, it requires a#10 wire directly to the coil. In my case, I pulled the pink wire connector pin out, unsoldered the wire and inserted and soldered the "red" #10 wire. It was not an easy job I'll add. No going back now!! However, it is possible to add a Chrysler style ballast resistor on the firewall if you absolutely need it for a return to points or maybe that tach, just guessing on the tach! You will need to make sure that it has the correct ohms. (there are many types)