Installing the wiring harness on a Lambretta scooter represents a bit of an undertaking and some spare time. The process involves removing all the detachable body parts, taking off the old wiring and connecting and securing the new system to the scooter. However, once done and connected properly, the scooter electrical system should operate better given than many old systems are as old as 40 years now.
Park the Lambretta scooter in an open workspace with sufficient light and room to place removed parts. Pull off the sidepanels of the scooter to expose the battery if the scooter has one. Disconnect the battery with a crescent wrench and screwdriver. Begin dismantling the body parts starting with the headset. Use a screwdriver to loosen the headset top. Pull it up and unscrew the speedometer cable to free it.
Remove the front body parts with a socket wrench and crescent wrench and screw driver by loosening the screws that hold the legshield centre piece, the legshield, the horncast, and the front fender together on the scooter frame. Put all the spare bolts and nuts in plastic container to avoid losing them. Continue the process until the front end of the scooter tube frame is completely exposed. Do not remove any of the control cables or the lower headset parts.
Begin detaching the old wiring harness by snipping off all the ties that kept it secure to the tube frame. Disconnect by hand all the wiring ends from the headset headlight junction, the horn, and the front brake switch. Continue down the scooter removing the wiring from the engine junction box, the battery and the rear brake light. Disconnect the turn signal junctions if the scooter has these as well.
Pull the old wiring harness off the scooter once it is freed. Save it in a box if in good condition, otherwise throw it away. Purchase a new wiring harness from a Lambretta spare parts dealer and tell them the model scooter you are working on--some use 6 volt wiring and others use 12 volt. Unwrap the new wiring harness and orient it in the right direction next to your scooter.
Examine the new harness to make sure all the connectors match your scooter junctions. Replace them with the correct male, female, or bullet connects if needed. Snip off the old connector with a snipper, strip the wire insulation a ¼ of an inch and secure a correct new connector with the wire crimping tool.
Use a wiring harness guide specific for your scooter to match the wiring colours correctly to junctions. Begin attaching the new wiring harness by inserting the front wires underneath the headset and connecting them to the headlight junction. Secure the harness with a ziptie to the tube frame neck and connect the front horn wiring below. Work down the tube securing the harness with more zipties and connect the front brake switch. Continue securing to the rear of the scooter and connect the battery, rear tail light and engine junction box.
Double check all connections are in place. Reattach the headset top to the speedometer cable and headset frame. Start the scooter up and check to see all the electrical parts are working. Use an ammeter to chase down a short if a certain unit doesn't work correctly. Replace all the body parts and panelling with a socket wrench and crescent wrenches and screwdrivers once the wiring is completed.
Using the Internet can provide colour wiring templates at various scooter enthusiast websites, which makes it easier to map out which wire goes where on the scooter.
Make sure not to spill the battery acid on yourself when handling the battery. The liquid is caustic and can cause skin injuries.