How to Repair Vehicle Lights Acting Strangely

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


Triumph TR4A Front Lights
Triumph TR4A Front Lights
  • When indicating…  the flashing indicator light is dim and other lights (tail lights, fog lights, brake lights, etc) flash as well (sometimes very dimly)?
  • When braking, the brake light is dim and other lights (tail lights, fog lights, indicator, etc) come on as well?
  • If switching on a light, other lights may go off?
  • If your indicators are flashing too fast, see 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).

Bad Earth or Vehicle Earth Fault


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
  • First, turn off the lights.
  • Check the connections to the light fitting. Are they clean and serviceable?
  • If not, carefully clean, plug the unit back in and check to see if this has cured the fault
  • Still doesn’t work? Turn off the lights. Check to see if there is an earth wire (usually a wire from the light attached to the bodywork via a screw or bolt). Check the connections and clean if necessary. Connect back up and check to see if this has cured the fault?
  • Still doesn’t work? Check the light bulbs, their sockets and the light fitting condition. Turn the lights on and check to see if this has cured the fault? No? Can you swap the light bulbs?

Note: Don’t leave the wrong ‘wattage’ light bulb in the wrong socket.

  • Still doesn’t work? If the fault is with the rear lights, do you have a tow hitch? Check the tow hitch wiring and connections. Note: The tow hitch is often connected to the rear lights via plastic connectors that clamp around the existing wires and make a connection. These can become dirty and corroded, along with the tow hitch socket connections.
  • Still doesn’t work? Check to see if an earth wire goes back to the battery.
  • The solutions above should cure the majority of faults. If it doesn’t, seek the advice of an electrical engineer.

* Caution: If disconnecting the battery *

Do not short the 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…

Hints & Tips

Rear Light Connector - Moisture Protection
Rear Light Connector – Moisture Protection
  • 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.