How to Check Your Car Electrical System

Batteries are dangerous. They may only be 12 volt but it is direct current so if your body or tools make a connection between live and neutral terminals (which includes the bodywork) the current holds you instead of jolting you away like the alternating current in your house would. Remove watches and jewellery when working near the battery. The electrolyte is a very powerful acid capable of serious corrosive skin burns and should be washed off immediately. If it gets into the eyes, flush it out with lots of water and seek immediate medical help. If it is swallowed, do not induce vomiting but seek urgent medical attention. Do not inspect batteries with a naked flame because they give off hydrogen which ignites explosively when mixed with air.


Batteries are often ‘maintenance free’, but that doesn’t mean they can be ignored. Flat batteries are the most common reason for breakdown organizations to be called out, though this is often down to driver errors, like leaving the lights on.

Even with a maintenance-free battery you should still check the connections are tight and the bracket holding it in place is sound. Also look for the furry residue left by leaking electrolyte which suggests the casing is damaged and the battery should be replaced.

With batteries that are not maintenance free you must check the level of electrolyte. Some have translucent sides with minimum and maximum level markings to make this easy but if it hasn’t, you remove the filler plug or plugs and shine a thin beamed torch in. The electrolyte should cover the metal battery plates inside by about a centimetre and if it doesn’t it should be topped up with distilled or deionised water, but do not over fill it. This is easier if you use a container with a thin plastic hose attached.


Don’t forget to check the lights for damage and failed bulbs. You must turn on the ignition to get all the lights to work. If a headlight works on beam but not dip, or vice versa, it usually means the bulb has failed, but if it works on neither it is likely to be a wiring problem because it is unusual for both filaments to go at the same time. In some cars the rear lights also have double filaments with the second coming on as a brake light, but most cars now have separate brake lights.

If you haven’t got a helper, you may be able to check brake ligh: by the reflections in a vehicle behind or against your garage door. Remember, if one brakelight bulb has failed the chance-are the other will go soon, leaving drivers behind with no warning you are stopping.

Indicators flash quicker if a bulb on that side has failed, though don’t rely on this because a failed side repeater indicator, and some high tech electronic systems, may not produce this effect.

Check indicators individually, not by using the hazard warning light switch, which is usually a different circuit. If there is a wiring fault on the indicators it might not be revealed by the hazard lights.

Many modern cars have light emitting diodes (LED) instead of bulbs in side and brake lights and indicators. Most high level brake lights (the one in the centre at the back) are LED. These do not have a filament to burn out so they should last the life of the car, but this also means that if they don’t work, you probably have a wiring problem that needs checking.

Related posts:

  1. How to Troubleshoot Ford Super duty Brake Light Problems
  2. How to Maintain Your Car Battery
  3. How to Replace Tail Lights
  4. How to Replace the Headlight Bulb
  5. How To Check Your Car’s Oil Level

Filed Under: Guides / DIY


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