How do I wire a ballast resistor & coil?

Written by tracy underwood
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The typical automotive ignition system prior to 1974 consisted of a coil and ballast resistor, with breaker points to interrupt the current flow when a spark was needed. The job of the ballast resistor was to inhibit current to a level that would not overheat the coil. This simple system is easy for even the novice mechanic to wire. So if you have a classic car with missing ignition components, don't hesitate to replace the coil and ballast resistor yourself.

Skill level:

Things you need

  • 18-gauge wire
  • No. 10 ring terminals
  • Crimp tool

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  1. 1

    Disconnect the negative terminal of the battery, if there is one installed on the car.

  2. 2

    Route one end of the wire from the engine compartment into the passenger compartment. Strip 1/2 inch of insulation from the end of the wire and crimp on a ring terminal. Connect it to the ignition terminal of the ignition switch. Route the other end of the wire to one terminal of the ballast resistor. Cut the wire, strip 1/2 inch of insulation and crimp on a terminal. Connect to the ballast resistor.

  3. 3

    Cut a piece of wire long enough to reach from the other terminal of the ballast resistor to the "Bat", "+" or "B+" terminal of the coil. Strip 1/2 inch of insulation from each end of this wire and crimp a connector onto each end. Connect the wire to the unused terminal of the ballast resistor and to the previously identified terminal of the coil.

  4. 4

    Locate the small gauge wire that comes out of the body of the distributor. Connect this wire to the negative terminal of the coil.

  5. 5

    Reconnect the negative terminal of the battery.

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