Jump start 2008 -- horn sounding????

Discussion in 'Gen 2 Prius Care, Maintenance and Troubleshooting' started by oldman1956, Jul 13, 2026 at 12:00 PM.

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  1. oldman1956

    oldman1956 New Member

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    My 2008 has a dead 12 volt battery. Went online an found the location for placing the positive clip of my recharger (inside the fuse box). Hooked up my recharger there and to the engine block. Plugged in the recharger and the horn started blaring on and off. Haven't heard it in a while but sounded to me like the same sound for when one pushes the panic button on the fob. Tried the car door and it had locked! Unplugged the rechrger and unlocked the door with the key....left the door slightly open. Plugged it back in. Horn again. got inside and dash was lit up. Hit start....horn stopped....well quieted quite a bit; eventually stopped. Now the amp measure on the recharger goes to 6 and stays for a few seconds and then drops to zero. It waits a few seconds at zero and then goes back to 6. What is happening and am I killing my car? Any suggestions?
     
    #1 oldman1956, Jul 13, 2026 at 12:00 PM
  2. Tombukt2

    Tombukt2 Senior Member

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    Sounds like you're setting off the alarm that's the horn that's located just above crank. Pulley on frame rail
     
    #2 Tombukt2, Jul 13, 2026 at 5:54 PM
  3. Dennis Brown

    Dennis Brown New Member

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    Since you can have power to the car via the charger at the fuse box junction connector, use the inside door unlock button and unlock all doors/trunk. Then go around and open the trunk. Access the battery (remove the various floor items) and then move the charger to that location, disconnect the battery ground (10 mm socket), and then charge the battery there. Besides being more direct, this will prevent any car components to function since they now have no power (the ground is disconnected.) The charger will deliver its full power only to the battery now.
     
    #3 Dennis Brown, Jul 13, 2026 at 7:29 PM
  4. BiomedO1

    BiomedO1 Senior Member

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    Possibly; how old is the battery currently in it? Have you tested it to see if it's still viable?
    I would connect that charger directly to the battery and observe what it does. If you have an old school battery charger, does it have reverse polarity protection? Is it a "smart" battery charger with a reconditioning function?

    I've hooked up battery chargers on cars that has set-off the alarm, but it usually shuts-up when you disarm the alarm or remotely unlock the door.
     
    #4 BiomedO1, Jul 13, 2026 at 7:34 PM
    Last edited: Jul 13, 2026 at 7:46 PM
  5. ChapmanF

    ChapmanF Senior Member

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    Yeah, that can happen. When the battery power dropped below the threshold for keeping ECUs awake, the car was all closed and locked and blinking the alarm-armed light. Now you've opened the hood and hooked up jump power, so it wakes up and says "hey, who jimmied the hood open?! is this a heist?".

    Disarming with the fob unlock button oughtta work.
     
    #5 ChapmanF, Jul 13, 2026 at 8:55 PM
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