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I am planning to do some car fixes and want to know what precautions I must follow in order to make it safely. Specifically, I'd like to know is for what kind of work, the battery should be unplugged.

I assume that everything related to lights must be done with a disconnected battery. But what about tasks like door replacement? There is obviously some electricity involved as well, for the window motor for example. Do I have to disconnect battery in that case?

  • On the contrary, lights are difficult to work on (you always end up testing) with the battery disconnected, and tend to have their own switches/fuses. So pulling the relevant fuse (and checking) is often the best way if there are bare wires around. – Chris H Aug 30 '16 at 11:01
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These are some general guidelines when working with electricity on vehicles:

  • Always remove any jewelry, to include rings, necklaces, watches, etc. If one of these items should come in contact between a hot and ground, it will instantly go hot (if there's enough amperage flow) and will burn into your skin requiring surgery to remove. These are a "no matter what" situation. There are also some physical reasons for removing these items as well.
  • Disconnect the battery whenever working on anything which is voltage sensitive, such as the Engine Control Unit (ECU - vehicle's computer). This especially applies when working on the airbag system (SRS), though other precautions need to be taken there as well.
  • Disconnect the battery whenever working on parts which need high amperage to operate, like the starter and alternator. These are items which, if the main power were grounded, would cause large issues for you and your vehicle. (Think thermonuclear meltdown.)
  • Generally no need to disconnect the battery when changing out anything which involves low current or anything which is switched. That means it has no live wires going to it when switched off. This includes doors, light bulbs, etc.
  • For some equipment, you must remember to reconnect them before reapplying power. You may not risk endangering yourself or damaging equipment, but you may put the equipment in a state that you are not prepared to fix. At that point, a technician with the proper tools might be needed to resolve the problem. The SRS (airbag) system is one case where this might happen. (It's not that the bag will deploy. It is that your SRS will not be in a fully functional state.) If you are removing a door, for example, you have to keep this in mind. Read and follow a good repair manual. – Jim Aug 30 '16 at 8:29
  • @Jim Thanks for the tip. Can you give an example/link of a good manual? – Eugene S Aug 30 '16 at 8:40
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A car battery isn´t capable of generating enough voltage to shock you (but the ignition coil can.) It is however capable of generating enormous current and can easily melt things and start fires.

You need to be careful with anywhere that is not fused. That means the battery itself and the starter motor. You should also be careful with electronics.

Circuits like lights and doors are protected by fuses so no real harm should result. If you are working on a part like a door (which contains electric windows, wing mirrors, etc.) and are worried that you could damage a cable and cause a fuse to blow, simply remove the fuse first.

You may find that the wiring loom for the door has a plug on it, though, so you can unplug the cables for the door instead. But the reason for this is more for simplicity of manufacture than repair. (If you are replacing an entire door, try to find one from the with the correct plug on it as manufacturers can change the style from time to time.)

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When welding. Both wires are advised, and there are other concerns as well. But they'll teach you this at welding school.

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