After a couple of incidents of a flat 12V battery while being away for a while, I decided to fit a remote control kill switch to the battery to disconnect it when I’m away. All worked well when I tested it with the boot/trunk open. Then went to go away, locked the car, hit the kill switch remote and the car alarm went off and kept going but at a much lower volume. There is no power from the 12V battery getting to it so does it have its own battery or run of the big battery? I switched the battery back on, the alarm volume went up and I stopped it by unlocking the car. My second attempt worked without triggering the alarm. But still curious where the alarm was getting its power from.
You haven't put a location in your profile. In a gen 2 Prius there is a fuse slot labeled S-HORN. As far as I know, the US models did not include the security-horn and there's no fuse in that slot. But models for some other places did. The s-horn has its own backup battery.
I don't recall ever seeing the S-HORN slot on a gen 3 (my memory could be imperfect though). A quick search in a US gen 3 wiring diagram does not find it. The only gen where I knew about it was gen 2, where it's present in some world markets and not in others. Now, maybe gen 3 also has one in some markets, and those markets have their own wiring diagrams on their own Toyota TIS sites.
some alarms has a low voltage detection circuit and a small battery or slow discharge capacitor to power it. Either that or you miswired the battery and accessory power wires. Hope this helps...
The siren was installed on right-hand drive cars behind the trim in the trunk. 89040-48020 Siren Assy, Theft Warning In cars for the American market, it was installed under the hood in the engine compartment.
Slots for the S-HORN Relay and S-HORN fuse are in the fuse box in the engine compartment of the Prius Gen3. For the American market, apparently, there is no wiring for these purposes and I did not find any diagrams. For the European market Gen3, the diagram assumes the S-HORN Relay and S-HORN fuse when the siren is installed without an internal battery in the engine compartment. Theft Deterrent Gen3 by MAX2 posted Feb 28, 2025 at 11:28 AM A siren with an internal battery does not assume the presence of the S-HORN Relay and S-HORN fuse in the electrical circuit at all. These parts are not needed, the power comes to the siren from the 12V battery directly and from the Main Body ECU. When the 12V is disconnected, the siren is turned on from the internal battery. In the Prius Gen2, there is no such slot with names (S-HORN). The confusion probably occurred here. https://priuschat.com/threads/twilight-zone-horn-works-no-fuse-or-relay.159130
