lahrlax

Battery is dead. Car starts instantly with a booster. As soon as you hook up the booster to the battery terminal, the car horn alarm sounds. How to solve?

StayKrafty

Car alarm is going off when trying to jump start? If your alarm is tied to the cars actual horn, you could pull the horn fuse.

TomCat39

Insert key, turn to the on position (II), then connect booster cables. This tells the alarm it's an authorized re-connection of power.

Failure to do this, the alarm thinks it's thieves trying to bypass the alarm by disconnecting the battery and reconnecting it.