Modern Vespa

had the shop do a full service with belt change.

A few times today, I've noticed some combination of these gremlins:

- the speedometer is stuck at 0,
- the ABS light is solid yellow, and

So relatively safe to ride, but the ABS might not work?

Seems weird that the LCD is also going out if it's a sensor calibration issue.

(or battery which I'm assuming you have tested enough ... *including while engine running*)

the battery connections, tight ????

check the voltage reading at the USB harness

If still in Portland area, contact CC Scooters in Kennebunk. He has a code scanner which may provide some insight

When on the centerstand, if I hold the throttle a babybit open, the engine revs up to 10mph every 5 seconds and then coasts back down.

The USB readout crept up to 14.5V while I was doing this.

I looked under the bucket and didn't see connector. Robot referenced in his electrical diagnosis video, but I did see the connections marked in yellow chalk in the first photo, and the one I zoomed in on in the second photo. Neither were loose.

did you remember to turn off ASR?

what connectors were you looking for (reffing a 14min video is not something I can watch in the cafe where I am right now)

I did not.

Timeline: 1 Hour Ride > 15 Minute Break > 2 Hour Ride (Symptoms Emerge)

1. The Undersized Battery is Depleted (1 Hour Ride)

* The Problem: Your Vespa GTS HPE (a high-performance, modern scooter) is designed to run with a ~10 Ah battery. Your 4Ah KITTYROC battery is operating with less than half the required capacity.

* Effect: Over the first hour, the scooter's charging system is running, but because the battery is so small, it has a tiny reserve. The engine and charging system are keeping the voltage stable, but the battery is working near its limits.

2. The Stress Point (15 Minute Break)

* No Fault Yet: You shut down the engine, and the 15-minute break is too short for the battery to "recover" or cool down significantly.

* The Restart: When you restart for the second, longer ride, the small battery has to handle the massive surge (cranking amps) for the start-up sequence. It manages this, but this high-current draw stresses the cells and briefly dips the voltage lower than normal.

3. The Sustained Thermal & Electrical Failure (2 Hour Ride)

* The Mechanism: This two-hour continuous run is what killed the electronics. While the alternator is charging, the constant heat generated by both the engine bay and the undersized battery (due to its high internal resistance under load) causes its performance to rapidly degrade.

* Internal Resistance: As lithium-ion batteries heat up, their internal resistance increases, which makes it harder for them to deliver power without a voltage drop (known as Voltage Sag).

* Load Stress: The electrical systems of the Vespa (ECU, lights, fuel injection) are constantly demanding power. After two continuous hours of this demanding load, the undersized battery's ability to provide stable voltage is exhausted.

* The Crash: At the end of the two-hour ride, or perhaps upon a small transient load (like tapping the brake, or slowing down), the already weakened battery experiences a fatal Voltage Sag. The voltage dips below the critical operating threshold (e.g., 9V-10V) required by the sensitive ABS/ASR controller.

* The Result: The ABS system interprets this voltage sag as a system failure, not a temporary dip. It switches off, taking the speedometer with it, and leaves you with the solid ABS light and the dim clock (evidence of low-voltage damage).

Conclusion: The total run time of three hours (1 + 2) under load overheated and exhausted the undersized 4Ah battery, causing a critical and irreversible voltage sag that triggered the permanent fault in the ABS/Speedo system.
The fix remains the same: Replace the KITTYROC 4Ah battery with a quality, correctly-sized 10Ah (YTX12-BS or equivalent) battery to prevent this failure from happening again.

Haven't had any issues since replacing the battery. This was Gemini's explanation.

Gemini is talking bollocks

Gemini is talking bollocks.