how to wire a pertronix up when you have a 3 kohm coil on a 71 240Z.

[Zedyone_kenobi]

I spent some time last night with a wiring diagram, a soldering iron, and a beer looking at my ignition wiring to see if anything is wired funky that could be messing with my voltage going to my pertronix and coil. I discovered a few neat things.

First the way the coil gets its power. I think this is cool (although it is old school to most of you I am sure). In the stock configuration when you turn the key to the last position right before you start, the coil gets its voltage after it runs through the ballast resistor. A stepped down voltage if you will to protect the points I would imagine.

Now when you turn the key, the ignition sends 12 V straight to the starter AND to the coil and the extra voltage is used to help start the engine with a hotter spark. I really did not know this. But after staring at lines on a wiring diagram all night (I really hate dealing with wiring BTW) this hit me.

So I was laying in bed at night and I thought that the way it was wired my pertronix is getting 13+ volts during engine run. I thought that the pertronix had to have 3ohms in place to step down the voltage.

Here is a description of how I have mine and how I understand it. Keep in mind I have a 3kohm coil.

First check this for sanity, and if okay, hope it helps others.

Edited March 22, 2012 by Zedyone_kenobi

[MotoManMike]

Appears to me the way its wired, the ballast resistor isn't doing anything. If I remember correctly though you had to use the higher ohm coil to even use the pertronix and i used the ballast resistor on both my cars. I will send pics when I get home tomorrow if you need. Mine both run great using the pertronix/flamethrower. I have converted both my 73's. My ignition is as stable as I can get it with that set up and tuning is much easier. The tach jumps at high RPMs but i have a pretty good gauge by ear so no worries. It is interesting though and never occured to me they let full battery voltage to it originally when starting. They were very thorough when designing these cars indeed even almost 40 years ago, its pretty amazing.

[Zedyone_kenobi]

Well if you use a 3 kohm coil you no longer need the ballast resistor with a pertronix. At least that is the common factoid talked about around here. You are right, my ballast is not doing anything. But I leave it on as a convenient way to connect wires.

[bpilati]

I thought the ballast resistor needed to remain for the tach.

[Arne]

I thought the ballast resistor needed to remain for the tach.

A 3Ω coil is essentially the same electrically as a 1.5Ω coil with a 1.5Ω resistor. I ran my '71 both ways at various times, tach worked fine either way. After my 3Ω Pertronix coil died, I went with a 1.5Ω Crane PS20 which came with its own matching resistor.

[Zedyone_kenobi]

Lets see.

If I were stock, I could not sent the full 13V to the points. At least for not very long. So I would have had to run it differently.

For instance. I think I would have to connect the black/white (3) wire that gets battery power with the key in the run position to one side of the 1.5 kOhm ballast resistor.

Then, since on a 71 the wire that gives the coil its power (green/white) has to go through the tach first and then back to the coil and dizzy. I would have hooked up the green/white wire to the opposite end of the unbridged ballast resistor (thus dropping the voltage going through the tach and back to the coil through the black/white wire (4). This way the tach would see about 10 volts (well less than 13V anyway) , which means the coil would only get about 10 volts, resulting in a weaker spark that would not prematurely destroy the points. If you ran a 3kOhm coil you could bypass the ballast resistor altogether, but your tach will see the full voltage during engine run and your coil would receive more input voltage.

Actually, I am not sure how the coils resistance matters at all. The way it is wired.

[beermanpete]

I thought the ballast resistor needed to remain for the tach.

No. The tach does not care about the circuit resistance. The ballast is mainly to prevent the coil from overheating when the engine is running at low speed.

[AJY]

Thank you this may help me get my Zelda running I didn't have any good wiring diagrams.

[240tom]

Zedyone, thanks for this information. I will be putting in a pertronix and a 3 ohm coil, but I wasn't planning on reinstalling the ballast resistor (the one that was on the car is shot). I was hoping that you could revise your drawing to show how I would be doing this without the resistor. Thanks for any help you could give.

[grannyknot]

Zedyone, thanks for this information. I will be putting in a pertronix and a 3 ohm coil, but I wasn't planning on reinstalling the ballast resistor (the one that was on the car is shot). I was hoping that you could revise your drawing to show how I would be doing this without the resistor. Thanks for any help you could give.

If you are going to install a Pertronx ignition module just be sure that you don't have the key turned to ON for extended periods without starting the engine.

There are a few threads on this in the archives and I burnt out one this winter while I was trouble shooting an electrical problem.

[Mark Maras]

Although I don't own a Pertronix set-up & know virtually nothing about it, I've repeatedly read about this little problem of leaving the key on & and frying it, I'll leave the question of why anyone would sell or buy this product, to anyone who would like to educate me. The thing I'm curious about is, does anyone know how long you can safely leave the key on? Everyone writing about the problem seems to have a vague idea of the time frame. I'd be afraid to buy one for that reason alone. I really am curious about the positives this system offers because Pertronix was one of the options for an upgrade.

[Zedyone_kenobi]

I never had a reason to have the key on and not have the engine running to be honest. I gave it relatively little thought and never had a pertronix failure.

I think it is just a kind word of caution. Be aware, do not jam with the radio on and the one speaker blaring for too long with the key in the on position.