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MG MGB GT V8 Factory Originals Technical - Installing Pertronix ignition

I am looking at the installation instructions and one note says: "If your ignition system has a ballast resistor, do not remove it". I am using a GM points style distributor that was with the Rover 3.5L when I bought it and a new pertronix coil.
Do I need to use a ballast resistor with this system or am I OK hooking up to the original ('76) harness?

Thanks in Advance,
Joseph Lagasse

Joe . . . I've installed a Pertronix II ignition in my buick distributor along with a Flamethrower II coil. The instructions with the unit said that I "could" use the ballast resistor, but recommended removing it, so I did. Late model cars used a resistor wire rather than a ballast resistor. It's a simple task to identify the color of the wire and cut it from the system. Haven't tried to run the engine, yet, though. It may run backwards :-)


Things might have changed with the Pertronix II, but my original unit instructions specifically warned against using any sort of ballast resistor for the Pertronics power supply - the red wire required a full 12 volts. On my Factory V-8, this entailed running a separate 12V key switched supply wire up to the coil, solely for the Pertronix red wire. I retained the original ballast circuit to protect the ignition coil and allow easy starting. I performed "surgery" on my wiring harness and placed the new wire inside the harness to retain a Factory appearance - I HATE single exposed wires snaking through an engine bay!

Paul Kile
Paul Kile

Here's how the car is currently wired for the four:

Coil (-):
WB to original EI distributor (not used)
WB to tach
WB to points distributor

Coil (+):
WLG to starter solenoid
WLG to original EI distributor (not used)
W from ignition switch

This is how the car was wired when I obtained it. No sign of a ballast resistor anywhere. Do you think that I can leave things as is, adding the Pertronix R & B wires and removing the old EI wires? By the way, what's the purpose of the WLG wire from the starter solenoid?

Joseph Lagasse

Gordon and Paul,

Thanks for the response. If I had been looking at the instructions for the Flamethrower coil installation (Duh!) it specificaly states that eight cylinder applications DO NOT require a ballast resistor. It also notes that the coil will work fine if you do have one. It even gives instructions to determine if you have a resistor in the system! I'm still curious about the wire between the coil and starter.

Thanks again,
Joseph Lagasse

If you have a ballast resistor its fitted in the feed from the ignition (W). When you crank the engine the battery voltage drops so the ignition coil has low voltage. To avoid this the wire from the starter solenoid provides raw battery voltage (no ballast resistor) while in crank.
If the voltage on the (W) feed from the ignition is the same as the battery then you don't have a ballast resistor.
NOTE:- Later Rover V8 ignitions that have an amplifier mounted on the coil have the ballast resitor internaly so you can't just check the ignition feed wire - check the voltage at the coil.

Joseph - the ballast resistor is not an identifiable component but a length of resistance wire contained within the loom. The fact you have a white/light-green wire on your coil is an indication that you have the ballast in circuit and hence probably have a 6v coil. If you bypass the ballast to supply a full 12v to the Pertronix and coil I would say that you must also replace the coil with a 12v item or you will passing too much current through the points/Pertronix. Original MG coils measured about 3 ohms for the 12v primary and about 1.5 ohms for the 6v primary. Other coils may vary, I have a 12v Lucas Sport coil that measures 2.4 ohms for the primary.
Paul Hunt

Thanks for the clarifications. I'm fitting a new Pertronix 12v, 1.5 ohm coil. All of the instructions indicate that it is OK to leave the resistor, but they do indicate that the ignitor module should be wired to bypass the resistor as Paul Kile had previously mentioned. If I can get the club engine hoist this coming week, I should have it fired up before the end of the month. Yahoo!

Joseph Lagasse

This thread was discussed between 12/06/2002 and 15/06/2002

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