Ballast resistor at coil?

[RyanH]

Hi. Could anyone answer what is probably a simple question, but I can't seem to find an answer too. Would my 1970 911T have a ballast resistor as standard? I'm changing to Pertronix ingnitor and Flamethrower coil. Cheers...

[LjH]

coils used in an electronic ignition system need a ballast resistor that is external,built into the coil, or the electrical system has resistance wiring. 1.5 ohms works with Pertronix. Check your old coil with a meter. If it's not 1.5 ohms you either have an external resistor or resistance wiring. Also check the flamethrower coil with a meter. If it's 1.5 ohms you are good to go.
Good luck..

[RyanH]

Thanks, the coil is a 3.0 ohm Flamethrower. Would that mean if I tested it and get 3.0 ohm I do not need to add a ballast?

[wwest]

CDI systems do not require/need a ballast resistor. Kettering/inductive ignition systems require a ballast resistor in order to limit coil current in the low RPM range

Ballast resistors are high wattage, 20W and more, 0.5 - 2.0 Ohms and are mostly in the circuit between the ignition switch and the positive side of the coil, coil minus goes to the points. The ballast resistor is often bypassed via the ignition switch "start" position to compensate low battery's low cranking voltage.

Most CDI aftermarket systems will advise you to remove or bypass the ballast resistor as it will be detrimental to CDI operation, especially in the high RPM range as that is where CDI systems have the highest power load on the 12 volt buss.

Some aftermarket CDI systems have the ballast resistor bypassed as a part of the base design.

[wwest]

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[LjH]

3.0 Ohms is more than enough for a pertronix

[RyanH]

So basically, what I think I'm hearing is that with a 3ohm coil and the Pertronix ignitor I should be OK to just swap them over providing my original coil registers 1.5ohm? Electrics confuse me, the ballast resistor situation confuses me more, someone just say yes or no! Thanks to all for assisting me through 'a senior moment'

[racerpaul]

I too am confused... what coil do I need to buy when installing Pertronix on my 912E?

[wwest]

Originally Posted by racerpaul

I too am confused... what coil do I need to buy when installing Pertronix on my 912E?

It appears that the Pertronix ignitor is simply a solid state switch that replaces the mechanical points. Not many transistors can withstand the level of inductive flyback voltages a coil generates, especially with a open load circuit, sparkplug disconnected, so some sort of voltage "snubber" and/or limiter is included.

So I would suggest you rely on the manufacturer for the correct answer.

[LjH]

electronic conversions need either a coil with an external ballast resistor or a coil with internal resistance. I converted a 912 using pertronix and a Bosch "Blue coil". check the link below good luck,,

rhttp://www.pertronix.com/prod/ig/flame/coils/chart.aspx

[RyanH]

...after a lot of frustration and the car not firing I've gone back to the points. I spoke to the seller and they [kindly] put a replacement in the post free of charge but it will be a while before I attempt that again...

[wwest]

Originally Posted by RyanH

...after a lot of frustration and the car not firing I've gone back to the points. I spoke to the seller and they [kindly] put a replacement in the post free of charge but it will be a while before I attempt that again...

Many cars using the points/condensor/coil system have a high power low resistance "ballast" resistor in series between the 12 volt source and the coil + in order to limit coil current flow in the low RPM range.

Most of these bypass the ballast resistor when the starter is engaged to compensate for the resulting low battery voltage.

I can't really tell from the available schematics if your car does this or not.

If it does then the Pertronix switching transistor may blow out due to excessive current flow the first time you crank the engine.

[jjeffries]

Nothing wrong with points for a street car thus originally equipped, IMHO. Easy to install and set, they a reasonable amount of time, esp. for the low miles a collector car gets driven. I believe we get over-sold on the benefits of the points replacement devices on the market. John

[RyanH]

Originally Posted by jjeffries

Nothing wrong with points for a street car thus originally equipped, IMHO. Easy to install and set, they a reasonable amount of time, esp. for the low miles a collector car gets driven. I believe we get over-sold on the benefits of the points replacement devices on the market. John

+1... if it ain't broke don't try and fix it, unfortunately, like most owners it difficult not to fiddle!!