When the jumper cables are incorrectly connected, the polarity of the electrical system on the vehicle with the dead battery will be reversed for a few seconds. This can irreparably damage many of the sensitive electronic components that are common on todays vehicles, such as on-board computers and electronic sensors.

What happens if you put jumper cables on the wrong way?

Hooking the jumper cables up backwards can create disastrous results⁠. Possible outcomes include irreparable damage to the battery, fried electronics, and even serious personal injury. You might cause personal injury and/or damage to your car if you accidentally hook your jumper cables up wrong.

Which cable do you connect first when jump starting?

First the clamp of the red cable is connected to the positive terminal of the assisting vehicle. The other end of the red cable is connected to the positive terminal of the broken down vehicle. Then the black clamp is connected to the negative terminal of the assisting battery.