Vespa Dies When Braking

She runs and idles well (maybe a bit rich), but she dies when I apply the brakes. I did a quick search and found nothing. I know somebody is going to say that I should use the search function.

I am aware of the brake light safety feature on these bikes. The bulb is correct and intact. I know I'm probably going to have to tear into the electronics and compare it to the wiring diagram, I was just wondering if anybody could point me in a the right general direction.


Bike is a 1982 Vespa Grande without turn signals.


Re: Vespa Dies When Braking

Double Trouble Dan /

One of the brake light wires is grounded out. Remove one from the brake handle and see if it dies. If not try the other. If that one causes the problem , use a continuity tester test it to ground. Follow that color with a wiring diagram until you find the shorted wire. Usually it will be either in the brake light socket, where the wire comes out from flywheel or in the headlight. let me know what it does .then we can go from there. Here are the blinker wiring diagrams. i can get you the others if you need.

Re: Vespa Dies When Braking

Thank you very much for the tip. I'll look into it tomorrow. Any chance you have a diagram for the basic grande without blinkers? I have one somewhere, but it is probably buried. I haven't touched anything moped related in a couple years and I need to dig through all my parts and manuals.

Re: Vespa Dies When Braking

Never mind, I found it in the Wiki

Re: Vespa Dies When Braking

brad, listen carefully:

the brake switches and brake bulb are providing the ground / return for the ignition stator coil.

pulling the brake levers opens the circuit, leaving the brake bulb as the only ignition ground, lighting the bulb.

if applying the brakes kills the ped, it means you're removing the switch ground, and the alternative brake light circuit is open.

peds with the bulb bypass resistor are rare.

the singular exception is clutch issues.

the OP needs to learn how to diagnose a brake bulb circuit, power and ground.

if the taillight works, it's prob. not the ground.

2 connections to a 9v battery, done.

Re: Vespa Dies When Braking

Double Trouble Dan /

Glen , I am more used to my sachs Eagles , which have the resistor.

I get your drift. Can u explain the 9 volt battery, how u use it? i use a 7 volt transformer or my old 6 volt battery charger for some diagnosis..

Re: Vespa Dies When Braking

i never just look at a bulb, touch the side and bottom contact to a 9v battery, proof positive it's not the bulb.

then you backtrack, find the wire feed to the bulb, isolate and touch that and motor ground to the battery, prove out the wiring (colors).

you can just jumper the plugs on the back of the housing if it's accessible, proof the socket is correct.

this will not work with headlights, but the rest, it's foolproof without buying expensive tools.

this is 100% more accurate then continuity testing.

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