To describe some of the possible faults, I’m going to use vehicle lights as an example…
If a vehicle has a bad electrical connection (dirty, tarnished, or corroded connections), it can cause all sorts of strange behaviours, such as lights dimming, other lights coming on, flashing, etc. Or, no lights working at all?
If your indicators are flashing too fast, see the article – My Turn Signal (Indicators) Flash Fast on One Side?
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 (higher resistance).
Or, in the case of where the light’s just don’t work, it can be a suspect connection, or a broken wire. Worst case (with a modern car post 2000). A control unit is defective, or has a glitch.
* Caution: If Disconnecting the Battery *
Do not short the battery connections, or yourself between the live supply and the earth (or bodywork). It will give you a shock! 12v is not enough voltage to kill you, (but batteries larger than 12v may be a different story). In rare cases the battery could explode (via a spark, igniting hydrogen gas given off by the battery) sending acid and battery bits everywhere…
* If you are unsure about working on vehicle electrics, seek the advice of an electrical engineer *
For a ‘Bad Earth‘, see the section below…
On modern cars (post 2000) fault finding has a few more things to take into consideration… The vehicles electronics often try to be helpful, with fault codes, faulty bulb indications, etc. If there is a change in the circuit parameters, they can switch off supplies if a fault is detected (protection from damaging sensitive electronics, melting wires, fire etc).
To get over this, try and trace to see how far the actual supply gets. I wouldn’t be surprised if a control unit is switching off the supply because it thinks there is a fault? I have a BMW Mini that has been throwing an intermittent wobbly on the rear left hand indicator for a while. Nothing wrong with the lamp, swapped fitting round etc. Works for a while, then comes back with a fault!
Because of the ‘modern electronics’, you may need to check all the connections in the circuit to make sure there is no added resistance (corrosion, bad contacts, etc). This type of fault can be a pain, as when you connect and disconnect the circuit, it creates a good ‘new’ connection (no fault found), but over time, days, weeks, months the fault reappears. If you can’t find anything wrong, It can sometimes come down to slight tarnishing of any contacts, weak contact pressure (the connector), corrosion, contamination, etc. If there is a slight change in resistance values or voltages, outside of the control units parameters, the vehicle CPU can switch the circuit off!
As an example… (as a summary):
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.
Don’t leave the wrong ‘wattage’ light bulb in the wrong socket. As this can lead to lights with the wrong brightness (one bright tail light and one dim, dim brake lights, etc, etc)