Modern Vespa

hey guys, one of my wires has melted and ruined my wiring harness, i have a parts bike but would want to know what his caused it,

Usually this is the result of insulation chaffing through where wires are in close contact with the metal body of the scooter or other hard surface. The result is a short which damages or destroys part of the wire loom. You will want to carefully check your fuses at the time you replace the harness (or the damaged portion of the harness). Once repaired, you may also want to provide some extra chaff protection in the area where the damage occurred.

Just want to add to that ^

May as well take a multimeter to it, bike on center stand (make sure the rear wheel is off the ground completely), twist the heck out of the throttle.

Voltage should go up to I think 14.4-14.6 ish... Can't remember

If you start seeing more than that, like into the 15v area, your voltage regulator is fecked, overcharging your system, making burnyburns and meltymelts.

Probably not the case, but in 17 yrs riding the only bike I've had to replace a regulator on was my Vespa so... 30 second test brings good peace of mind.

I had pretty much th same thing happen to my 2007 GTS.

I traced the cause to a junction of about 10-15 black ground wires twisted together into a cone shape, soldered and wrapped with electrical tape. The point of the cone, through vibration pierced the insulation of the main positive battery cable with predictable results.

Sloppy workmanship, if you ask me.