Lambretta LD Wiring

The Cassell Book of the Lambretta Page 93 (Fig 57)

The earlier Lambretta models have a very simple lighting circuit comprising a headlamp, horn, rear lamp and lighting control switch.  All these units operate directly from the flywheel magneto and are operative only when the engine is running.

Later models introduced a rectifier and a battery which enabled parking lights to be fitted, with pilot light in the headlamp and bulb in the speedometer for illumination after dark.

The output from the lighting coil is about 27 watts at 3,000 revs per minute, which approximates to a road speed of 30 miles an hour and this just balances the consumption of the head and tail lamp bulbs which are 24 watts and 3 watts respectively.  The nominal voltage is just 6 volts which is reached under these conditions.

From this it will be seen that if more powerful bulbs were fitted in the lamps (that is, bulbs having a higher wattage rating), then the brilliance would be reduced owing to the voltage dropping below that which is needed.

Many owners fit a 5 watt bulb to the rear lamp of the Lambretta and wonder why the light is less brilliant than the standard 3 watt bulb.  Greater brilliance can be obtained by fitting a bulb of lower wattage which will permit the voltage to rise, but this cannot be taken to extremes or the bulbs will burn out quiet rapidly when the engine speed is enough to create extra current.

Rear lamp bulbs have a habit of burning out when the headlamp is switched over from main to dipped beam.  The reason is that the generated current is momentarily forced through the rear bulb.  This happen usually when the piece of copper inside the dip-switch, which should move with the lever in order to bridge the circuit at this moment, becomes bent and is not operating correctly.

If after the wires at the terminal block have been removed for some reason and the replaced, a very dim light is given by the lamp bulbs, it may be that connexions at the terminal block have been reversed during assembly ( See fig 57 above – Terminal Block – Rectifier and Lighting Switch wires put on the wrong way around).

By looking at the wiring diagrams, you will see that the blue coloured wire from inside the flywheel magneto is the main feed and this joins a brown wire at the terminal block.  The two green wires connect together and join a wire from the ignition coil.  MK III machines have a 6 volt battery which is charged by a rectifier supplied with current through a red covered wire (On a Marelli Magneto, if correct wired colours are used.  Green for Filso, if correct wired colours are used – See Magneto diagrams for more information in this area – Click Here for Magneto Diagrams)   (See Fig 58 above) and if this is confused with the brown wire connexions, then the output of current is very small and the lights very dim.

Note: You will see from the internal coils, the lighting coil is (normally) bigger than than the ignition coil and therefore produces more power.

Page 99 Cassell book of the Lambretta

New wire should be of good quality and of 40/36 s.w.g.

• SWG = British Standard Wire Gauge –
• 40/36 = High Quality 40/36 SWG PVC Insulated Flexible Multi Strand Tinned Copper Wire –
• Current Rating: 10 Amps Max
• Conductors’ Gauge: 40/36 means 40 Strands of 36AWG equivalent to 21AWG