| This sure sounds like a voltage breakdown in some of the secondary wiring (spark wires) of the system from the higher voltage achieved by the Pertronix ignition upgrade. If you are getting spark at all, the Pertronix is good.
Scott has been a real sport to jump in to help with this problem. IMHO, when the first part of the thread says "Everything worked great for about a month", that lets Scott completely off the hook for being responsible for this problem. Just the heading of this thread points fingers at Scott and that's not fair. If you took an electronic part back to Pep Boys, told them it worked for a month, and wanted them to fix some new problem, they'd snicker and show you the door, or sell you another part.
Sometimes a rethink of the problem will help, and an understanding of the system would really help you. To jump the Pertronix power source around to different places in 'hope' of fixing the problem is scary. The installation says to use a solid 28 vdc source, and that means to avoid the ballast entirely.
Think about the voltage at the output of the ballast. The reason the voltage varies from 28 vdc (or whatever generator output voltage is) is because the Pertronix switches in the coil's primary winding in series with the ballast, drawing current, and current flowing through a resistor (the ballast) causes a voltage drop across it. Every time a spark is generated, the voltage at the ballast drops. The reason the voltage "looks" like 17-18 volts is because it's a series of spikes and the voltmeter does it's best to average those spikes. The 'apparent' average voltage will vary with the meter used.
Hooking the Pertronix to either end of the ballast is asking for trouble. Think about it, when the Pertronix fires (conducts) the voltage at the ballast drops considerably. If the Pertronix is getting power there, then it's source voltage drops. Not exactly easy for the Pertronix to operate under those spiky voltage source conditions.
You should pick up 24vdc from the main source, like the B+ terminal on the voltage regulator. Install a relay to provide key-switching of that voltage source. (The relay should turn on when the key is activated.) FYI, there are key switched wires to several of the fuses along the front of the firewall.
If you think your ballast is bad, you can check your ballast with an Ohmmeter. It's just a resistor.
Sorry this got long.
Bob |