Plug Wire Mystery

[Max Power]

I have a 393 windsor which I recently built, and I decided to get new plug wires to match the motor (old ones orange, new ones blue). I was running 7.5mm Autolites, and I bought Moroso Spiral core 9mm wires.

My car won't run with the Morosos. I have a Petronix coil running off the factory ignition connection (resisted), a Pertronix Ignitor I and a stock cap with Champion plugs. The car runs great with the Autolites.

The resistence on the Morosos is less than the Autolites as measured with an ohmeter. When I replace one wire at a time it will run without a miss that I can notice, but when I tried to start it after replacing 4 wires it would not start.

The Moroso guys are baffled as to why it wont run, as am I. I will try and run 12v to the coil this weekend, but I have never had a car refuse to run just with a plug wire change. Thoughts?

 

[69 Capecod]

coluld one of the wires be broken. to the point where its not getting enought juice to flow threw? did you try to four dirfrent wires I would try four then try four dirfrent wires. Or is the coil to week to send the correct spark threw the wire?

 

[DarkBuddha]

That is wierd... My first thought is that it has something to do with the reduced resistance of the plug wires, but it simply shouldn't cause this kind of problem unless the lowered resistence has increased amperage enough to allow the coil spark to fuse the plug electrodes, which seems unlikely since the car still runs fine with the Autolite wires, right?

But, from looking at Pertronix site for Support tips, I think I might've found something... Specifically, the Pertronix system has a maximum amount of resistance allowed from the coil, so I wonder if the reduced resistence of the new plug wires has caused a problem. Here is the text from Pertronix site:

"What type of coil can I use with the Ignitor™? How do I check my coils resistance? (12V negative ground only)

To determine if your systems coil is compatible with the Ignitor, some measurements should be taken prior to installation of the Ignitor. Caution… While performing this test, never leave the ignition switch on for more than 30 seconds at a time.
Set your voltmeter to a 15 or 20-volt scale. Attach an 18 or 20 AWG jumper wire from the negative coil terminal to an engine ground. Attach positive (red) lead of your voltmeter to the positive side of the coil, and the negative (black) lead to an engine ground. Turn the ignition switch to the run position. Now read the voltage at the positive coil terminal. Turn the ignition switch off. If the voltage measured is approximately 12 volts, no resistance wire is present. A typical resistance wire will provide 9 - 6 volts.
The next step is to determine the resistance in the primary ignition. Label the wires attached to the coil terminals and note their appropriate location. Make sure that the ignition switch is off and disconnect all wires from the coil. Adjust your meter to the lowest ? ohm scale. If you are using an analog style meter make sure to zero the needle. Measure from the negative terminal to the positive terminal. Write your measurement down.
Now the maximum system amperage can be determined, divide your voltage measurement by your coil resistance measurement. This will give you the system current or amperage.
Four cylinder engines should not exceed 4 amps. Six and eight cylinder engines should not exceed 8.5 amps. If the total amperage in your system is higher than the amount recommended for your application, you should install a ballast resistor.
Example Voltage 12
Resistance 1.5
12 / 1.5 = 8
Total amperage 8"

Helpful? Good luck!

 

[Max Power]

Helpful in a way, but I am using a Pertronix coil with the Pertronix Ignitor. You have to assume that the coil and ignitor are meant to work together.

None of the Moroso wires are broken. They are brand new.

 

[stangjunkie]

which wires did you replace??
could it be that if you replaced 4 plug wires and it would start however one by one it would.... HAve you changed the coil wire?? maybe it can feed seven 7.5mm wires and one 9mm but cant feed more than that... if you havnt yet change the coil wire first sounds like it may not be feeding enough

 

[mustangdave]

Could the spiral core design be causing electrical interference with the Petronix? I didn't think the Petronix needed a resistor?

 

[Max Power]

I tried it last fall with all 8 and it wouldn't start. I switched back and it ran fine.

This spring, I thought I would try it again in graduated steps. First, just the coil wire swap. It ran fine. Then added one cylinder and it ran fine. When I stepped up to 4 cylinders, it wouldn't start.

Moroso guys were baffled and the Pertronix guy says the only thing he can think of is to try running 12v to the coil and see what happens. I thought I tried that last fall, but I will try it again this weekend.

 

[mustangdave]

I have a sneaking suspicion that will solve your problem. I use an aftermarket coil and I had to run a wire from the solenoid to the coil for 12 volts to make it work.

 

[2nd Mustang]

http://www.nightrider.com/biketech/truth.htm

I remember reading this once before. Don't know if this will help, but it is interesting.