Car check-ups expose an individual to several dangers, one of them is shock. An electric shock can be triggered by a wrench when it touches a piece of metal, disconnecting the main negative. By touching a positive electrical part, the wrench acts as an arc welder, so it's a good idea to disconnect the main negative wire first. To ensure that no damage is caused to your batteries, you need to make sure that they recognize both negative and positive signs. The negative terminal is marked with a black wire and a – sign, while the positive terminal is marked with a red wire and a + sign. For safety, before disconnecting the terminals, make sure you understand the instructions