Faulty Car / Vehicle Lights ?


* If you are unsure about working on vehicle electrics, seek the advise of an electrical engineer *

Bad Earth or Vehicle Earth Fault

Triumph TR4A Front Lights



This type of fault is usually down to a bad electrical connection somewhere. This fault is generally known as a 'Bad Earth'. On vehicles where the bodywork is made of metal, the return path for the supply (back to the battery) is via the metal bodywork. On cars made of non conductive materials, such as fibreglass or plastic, the return path for the supply is via a cable. Due to the bad connection(s), the electricity is trying to find its way home another way. It does this by passing through another light fitting. The lights tend to be dim, because the electricity passes through more than one light bulb (a bigger load).


The solution is to find the bad connection on the 'return path' of the electricity (see diagram). The 'return path' or ‘fault path’ can be in a number of places.

Rear Light Connector Rear Light Connector - Moisture Protection

Caution: If disconnecting the battery. Do not short the connections or yourself between the live supply and the earth (or body work). It will give you a shock! (12v is not enough voltage to kill you. Batteries larger than 12v may be a different story.). The hydrogen gas given off by the battery could explode (via a spark) sending acid and battery bits everywhere.

Hints & Tips

Electrical connections can be protected by applying a thin coating of electrical lubricant, waxoyl etc. An electrical cleaning fluid or lubricant can improve connections by preventing oxidation and assist in cleaning where the contacts cannot be reached.

Beware when applying oils, greases and wax, they may damage cable insulation, rubber and plastic. Read the product instructions.

Do not get substances, dirty fingers on the light bulb glass, it will reduce their life span.