Modern Vespa

Hello, fellas! I have a 2009 GTS 300 and it won't start.
I had this issues when the engine stalled when I was running over potholes and bumps for a while, especially when I was with a passenger until one day it just died.

When I insert the key the red light doesn't come on. I tried both the master key and the regular one and neither works.

I don't have a Vespa dealership where I live and the guys at my local motorcycle shop told me the problem is with the immobilizer system and there's nothing they can do.

I started digging through the forums and found some people with the same problem so I ordered a new immbilizer antenna. I replaced it today and nothing happened. The red light won't go on when I insert the key and the scoot won't start at all.

There are also some issues with the fuel pump but I will handle them later. I'd like to solve the immobilizer problem first. Did anyone have any similar issues? Where should I look next?

The red immo light is supposed to NOT come on when you insert the key and turn on the ignition.

PS, the potholes might have caused the underseat storage to hurt the fuel injector and crack it.

But it should blink when you insert the master key and it doesn't. Also, if it works properly it should start blinking for 48 hours once the key is out and it doesn't do that as well.
Also, if there was a problem with the injector it would have at least revved the starter but it's completely dead.

Big guy here, but my injector cracked riding down the freeway, no major bumps prior to failure. Stopped dead, no restart, trailered home and later discovered the crack. It'll take you all of 5 minutes to inspect it.

Does anything work? Brake lights? Seat opener?

Yes, everything works except for the immobilizer.

Rereading your prior comment....guess this isn't at all helpful. Sorry.

Your reasoning is completely correct.

I agree the problem comes from the immobiliser system.
And it is not the key and not the antenna.
It looks like the immobiliser is simply not working.

Can you explain in some more detail when and how the isssue came up? Was it while driving the engine suddenly died?

Can you also give us a bit background. It is a 2009. How long do you have it, how much do you use it?

I do not have detail on how the immobiliser system exactly works, but maybe someone else here has. My question: where does it get its energy from?

Did you check the fuses?

It's all in the ECU in the throttle body, except for the antenna that provides the key code signal.

This Vespa is from 2009, but I know for a fact that it was sitting in a wealthy man's garage for 10+ years without any use. His mechanic took care of it and took it for short rides every once in a while, so when the guy who I bought it from got it the scoot had about 150 km on the odometer.

This guy then drove about 1500 km and sold it to me in August of 2021 and 4 years later it has about 25k km on it.
The second owner said he had replaced the fuel pump (turns out this was one of those 2009 Vespas with a faulty fuel pump that got recalled).

I service it early. About a year ago I started noticing that it stalls when I hit bumps or holes but it wasn't happening very often so it wasn't bothering me until it did. About a month ago I was riding with my wife when after another bump the 'fuel system error' light turned on. I took it to the shop immediately and they said that everything is OK and there are no issues with it even though the error light was on the whole time.
They said that I'd need to go to an authorized service center to turn off that error light and we don't have any in my area so I just let it be.

I took it back from the shop and went for a short out of town ride with my wife and on our way back it started stalling on every bump and finally died after hitting another one. (The roads in Eastern Europe aren't the best ones)

And actually, after writing this I think the problem might be in the ECU unit. I will check it tomorrow.

Also, the reason why I thought it was the immobilizer antenna was because I took the scoot back to the shop again and told them they did a bad job and they should fix it.
Their electrician took a look at it and said it was the immobilzer acting up so I just took his word for it which in retrospect wasn't the best idea lol.

The main connection to the ECU might have jolted loose? I've not heard of that before though.

Also, the harness might have been chafed over those 25k km enough to open-circuit a wire - again unlikely.

The antenna gets a switched 12V feed, the ECU also, but the ECU also has a permanent 12V feed.

Circuit diagram here: https://manuals.wotmeworry.org.uk/Vespa/GTS300%20Super/

Note this has layers, so you can highlight the different functions.

the immo will not stop you while riding. It does not stop the engine while running. the immo chip in the key does not even have to be present after the first few seconds of turning on the ignition, the ecu will only check it in those first couple of seconds.

you say "everything works except for the immobilizer". What is 'everything'? Have you as Peter suggested checked the fuses (with a multimeter not your eyeballs)? Do the brake lights work when squeezing the levers (check both levers individually)? Do you hear the fuel pump prime when turning the key on?

By 'everything' I meant headlight, break lights, dash lights, turn lighs, and horn.

So I started checking the fuses today one by one with a multimeter. I took out the first one in the main fuse box in the glovebox and it was fine but after I inserted it back my headlight stopped working.
I took out the rest and checked them invidividually and they're ok BUT then I checked the fuse box contacts and found out that all of them in the main fuse box are shorted together somehow? I can pick any two random contacts of the fuse box to check with a multimeter for continuity and it always rings so there's that.

But after all that I found a blown fuse in the underseat fuse box.
I will replace it tomorrow but looks like I will have to look for shorts in the wiring as well.

Did you look at the circuit diagram?

With the fuses removed, one side of fuses 4,5,8 and 9 will be shorted - the orange wire from the ignition switch, one side of fuses 6 and 7 will be shorted with permanent 12V on the red/black wires. The other side of the fuses may well show some continuity, e.g. through a light bulb to ground, then back up though a relay coil or other device. Not a short mind, but enough to sound off a continuity buzzer or bleeper.

Looking for continuity across those circuits is not a good diagnostic tool for anything really.

The headlight should only work with the engine running, the headlight relay coil gets its ground from the ECU when the ECU is happen to let the engine start and run.

This is weird because for all the years that I owned this scoot the headlight turned on once I set the key to ON position. I clearly remember that because I drive from work at night and when I kill the engine with the kill switch button the light always stayed on.

Anyway, I replaced the blown fuse and the scoot started. The 'check fuel system' light is still on and when I took the scoot for a short ride the fuse burned again after about 5 minutes of riding.
I guess I will have to continue checking everything with a multimeter, this time for voltage.

Maybe subtle but ...

The thing to understand is that the headlight doesn't come on until the engine is started BUT it will remain on when using the kill switch (or if the engine stalled).