How do I convert a positive ground triumph motorcycle to a negative ground?

Written by michelle schaefer
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How do I convert a positive ground triumph motorcycle to a negative ground?
Learn how to convert your Triumph motorcycle from positive to negative ground (vintage motorcycle image by sasha from

The original positive ground electrical system on a Triumph motorcycle can cause the centre electrodes of the engine's spark plugs to be eaten away rapidly and reduce the life of a spark plug. Other electrical components may also become subject to accelerated wear and so require frequent replacement. Such erosion may make the spark-gap too large and seriously reduce the power output of the engine. Using components from the later negative ground Triumphs makes a polarity conversion a relatively simple task.

Skill level:

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Things you need

  • Cross head screwdriver
  • Flat head screwdriver
  • Metric socket set
  • Rectifier and Zener diode from 1979 and later Triumph T140/TR7RV
  • Multimeter

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  1. 1

    Locate the battery under the motorcycle seat. Disconnect both the positive ( + ) and negative ( - ) cables at the battery with a cross head screwdriver. Lift the battery out of the motorcycle and store it in a safe location.

  2. 2

    Remove the existing rectifier and Zener diode. Mark the location of the wires on the rectifier terminals to avoid improper installation later. The Zener diode is single-wire component, either in a finned heat sink under the headlamp, or attached to the air filter box under the seat. The rectifier is a four-wire finned component under the seat near the ignition coils or fastened to the rear mud guard.

  3. 3

    Install the replacement rectifier and Zener diode in the original locations using all the original hardware and wiring. The replacement rectifier and zener diode from a 1979 or later Triumph are negative ( - ) ground components and are direct replacements for the positive ( + ) ground components removed in Step 2.

  4. 4

    Reinstall the battery by connecting the ground cable to the negative ( - ) terminal of the battery. The negative cable is the cable that connects to the motorcycle frame. Connect the positive cable to the positive ( + ) terminal of the battery. The positive cable is the cable that connects to the wiring harness through an in-line fuse.

  5. 5

    Start the motorcycle and bring the engine to normal operating temperature. Attach the black ( - ) probe of a multimeter to the Negative ( - ) post of the battery. Attach the red ( + ) probe of a multimeter to the Positive ( + ) post of the battery. Check the reading on the multimeter for a positive ( + ) 13-to-15-volt output.

Tips and warnings

  • An aftermarket solid state charging system will work in place of a rectifier and Zener diode from a 1979 Triumph T140/TR7RV.
  • Battery vapours may be flammable. Keep the battery away from open flame or ignition source.
  • A battery retains voltage even when disconnected. Avoid shorting the battery: do not contact the terminals with metal tools or parts.

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