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.
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.
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.
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.
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.
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.
- Vintage Triumph Register: Converting Positive Ground to Negative Ground
- "Triumph 650 & 750 2-valve Unit Twins Owner's Workshop Manual"; Jeff Clew and Chris Rogers; 1990
- 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.