Modern Vespa

I'm attempting to find and fix a possible yet-to-be-determined wiring issue with the starting circuit on my 2015 GTS 300. For complete details, see the post: "Started as a part spark plug post; now down the rabbit hole we go."

Thankfully, Jimc was able to provide the 2015 wiring diagram at: Unfortunately, when I view the wiring diagram on my computer/iPad, it is in black-and-white. I am a complete novice and unfamiliar with reading the wiring diagram and testing wiring itself, so I have some questions...

Does anyone have a color wiring diagram? Or a color copy of just the starting wiring?

Should I be testing voltage at specific points in the starting circuit? For example, should I test the voltage at each point in and out of the starter relay? How about voltage at the starter itself? If so, what should the voltages be at specific points?

Or, am I just looking for continuity in the starting circuit at different points? I feel like I have continuity, at times, but the voltage coming across isn't enough to power the starter.

Where are the ground points on the 2015 GTS300? The grounds mentioned in the manual don't seem to correspond with the locations on the scooter itself.

Finally, if I do need to replace wiring, what should I use for wiring and connectors? Is there a specific brand someone can recommend? Is there a kit that has different gauge wiring and connectors? I already need to replace at least one connector, having banged it up pretty severely in my attempts to "fix" the scooter.

Thanks again.

For the wiring diagram: It is in color. There is also a color key at the bottom right-hand corner. The colors are marked on the diagram lines, using the abbreviations in the color key, though they are very small and hard to read.

If I got tired of reading the diagram on my computer I would maybe send the file to a FedEx or Kinko's Office store and have a large scale print made, in color if possible though the color keys would work if they can be blown up big enough to see. Note that all three pages are the same, but in different languages. Only the first page in Italian and English would need to be printed -- unless you prefer Spanish or French...

Also -- ground points: any bare metal or bolt on the frame of the scooter will work. First check that there's continuity from the negative battery terminal to the frame. Voltage and continuity are a little more difficult topics until you get the basics *. I'll try to find a better explanation for you.

* Ohm's Law and Kirchhoff's circuit rules

See this:

How to use a MULTIMETER to diagnose electrical problems.

This could be helpful, though it's somewhat dated. It describes an analog multimeter while most meters used today are digital (DMM).

Don't use the Amps and milliamps ranges on the meter. Measurements of current (Amps) are rarely helpful and can easily wreck the meter if not used carefully.

Hope this helps. Good luck!

Robot at Scooterwest has a video on Diagnosing Electrical Problems on Vespa/Piaggio Scooters:

https://vespamotorsport.com/videos/

Thanks, JKJ-FZ6. I can use the wiring diagram to identify the wire gauges and colors of the wires, but can't trace the starting circuit. For example, if I trace the BiNe-0.5 (black and white 0.5 gauge?) wire from the relay, it appears to go to the stop lamp and also into a stop push button (at two points?) and then reforms as a black 0.5 gauge wire that goes to the horn and into a fuse (and other places?) and continues on. What points on the diagram are part of the starting circuit? As you suggested, I'll have a professional print the entire diagram on a giant piece of paper (which will hopefully be in color)... When I do so, does anyone know if the starting circuit will be highlighted in an individual color? Thanks again.

You've just described it. From fuse 4, the circuit goes through either brake switch into the coil, with the ground for the coil provided by the ECU when it's happy the kill switch etc is OK, and the start push-button (from fuse 2) is pressed.

So check you have power both sides of fuse 4 (the horn working will be a clue), and that the brake light comes on when a lever is pulled in. Check you have power on both sides of fuse 2 (the immobiliser switching off is the clue here). If so, and you can't hear the relay click or engine turn over when you press the start button then double check the kill switch is working correctly.

Schuman, I'll send you a drawing where I've highlighted the starting circuits in colors.

Normally the wiring in your scooter, from the battery to the starter and back to the battery has very low resistance, less than one ohm. The voltage at the starter motor will be nearly the same as the battery voltage when the starter relay is closed and the motor is trying to start the scooter.

But when the electrical resistance goes up in any part of the circuit, the voltage at the motor is reduced (there's a "voltage drop" at the higher resistance). High resistance could occur in a loose connection, a corroded terminal, a broken or pinched wire, or other reason. Then most of the power from the battery is going into heat at the point of higher resistance and much less power gets to the starter motor.

For example, if the starter motor draws 5 amps of current when it's running, and there are two ohms of resistance in the wiring, then (5 amps x 2 ohms)=10 volts (*) is being dropped by the resistance and only 3.5 volts is present at the starter with a 13.5 volt battery (**). (This is an idealized example; in the real world there are additional factors like internal resistance in the battery and inrush and stalling current through the motor.)


*Ohms Law: Voltage (volts) = current (amps) x resistance (ohms)

**Kirchhoff's circuit rule: The sum of voltage increases (from a battery) and drops (from resistance) around a closed loop is zero.