Technical Service Bulletin Secondary Resistance

5k Resistor Usage, Twin Plug systems, Non resistor Rotors

Porsche vehicles that are using ignition rotors that have less than 5000 ohms resistance in the rotor will need to use a 5000-ohm resistor fitted directly to the coil wire. Failure to use this resistor will result in secondary current leakage through the ignition transformer, (coil), to ground. This is extremely important in vehicles that are using the Bosch-style CDI system. As energy can be transmitted directly back into the CDI box at secondary voltages, (45,000 volts and above), resulting in an immediate CDI failure. Ignition transformers (coil) will also be damaged by secondary voltages moving through the system too quickly. Causing where the Transformers’s insulation can no longer stop the energy from leaking out.

Common distributors that will require a 5K coil resistors are:

  • All twin plug ignition distributors used in the 1965-89 vehicles including the Marelli S121 models and clones. Bosch twin plug distributors and their clones, JB racing distributors, and Patrick Motorsports distributors. All of these distributors will require a 5K resistor.
  • Marelli S112AX/BX using aftermarket rotors
  • Bosch 911SC and 930 models from 1978-83, (SC) and 1978-94 930 models, this is because the original rotor has been discontinued. The replacement rotor, (930.602.902.01), only has a 1K resistor installed. See 6 pin bulletin for more information on this.
  • Any distributor that is fitted with a rotor that has less than 5000 ohms

If you are unsure about any rotor then using your ohmmeter check the resistance of the rotor by measuring from the center of the rotor to the tip of the rotor. If the resistance is less than 5000 ohms either replace the rotor or install the secondary resistor into the coil wire.

If your rotor does not have the correct 5000-Ohms resistance you need to add it.

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