Coil Ballast Resistor: Grassroots Motorsports forum: Grassroots Motorsports Magazine

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Aug. 13, 2008 9:56 a.m. m4ff3w Dork

The coil is dead on my 81 Fiat X1/9

I also replaced the ignition module with a new GM style:

I have a MSD Blaster II coil to go in, do I need the Ballast resistor?

The MSD literature says: "for a stock, points style ignition system, a 0.8 Ohm ballast resistor must be installed.".

On the other side, for instructions on "wiring to a stock points ignition" it says "when using with a points or stock style distributor, a 0.8 Ohm ballast resistor must be installed"

I am confused, do I need a ballast resistor with my electronic ignition?
Aug. 13, 2008 10:24 a.m. aircooled Dork

I am almost certain you do not. I believe the purpose of the ballast resistor is to keep from burning the points (which you do not have). I have a Mallory coil in my car, with the same ignition module and a Hall effect coil. With the ballast resisted power to the coil it would stall at idle with the lights on, full 12v to the coil solved that. I did contact Mallory to make certain the coil could handle it and they assured me it would.
Aug. 13, 2008 11:07 a.m. carzan New Reader

Older cars that came equipped with ballast resistors typically had 6V coils. The ballast resistor brought the 12V system voltage down to a level that the coil could handle long-term. While starting, most have a bypass circuit that allows full 12V to the coil for extra oomph while starting but the coil won't last at that voltage level and the releasing the key to the "on" position returns power to the ballast resistor circuit. If the MSD coil says use a ballast resistor for any application, I would assume it is not a 12V coil. If you turn the ignition to the "on" position and read 12V at the coil terminals, I'm thinking you need the ballast resistor. A call to MSD might be your best bet, though.
Aug. 13, 2008 12:07 p.m. Jensenman SuperDork

Remember the ECM gets its switched +12V from the ignition coil terminal. For that reason alone, I would not use the resistor.
Aug. 13, 2008 1:18 p.m. carzan New Reader

Editing my above post: Looking at the MSD Blaster II specs online, I change my vote to agree with aircooled and Jensenman that the ballast resistor probably isn't needed with the GM module.

From the website: "Most late model vehicles with electronic ignitions do not require a ballast resistor, check your ignition and manufacturers specifications to determine if a ballast is required in your application."

Still kinda vague, though. Again I say call MSD to verify. "MSD Tech Line: 915-855-7123 Call our tech line Monday through Friday, 7am to 6pm Mountain Standard Time"
Aug. 13, 2008 1:30 p.m. Roberto Bowen

no need for a ballast resistor with the GM module.

It has solid-state coil current control.

A ballast resistor is there to limit coil current and therefore keep the coil from overheating/burning out. Also to prevent points from burning from excessive current.

So if you use a GM module you do not need a second way of controlling current.