How to Repair Vehicle Lights Acting Strangely

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?


The Symptoms:

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?
  • The bulbs are good, but the light just doesn’t work?

If your indicators are flashing too fast, see the article – My Turn Signal (Indicators) Flash Fast on One Side?

The Causes:

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.

Damp can also cause a multitude of faults…


* 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 *

Fault Finding

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):

  1. Are the bulbs are OK.
  2. Check the supply at its source and along the circuit.
  3. Is there a fault code (it may need to be cleared).
  4. Check for contact resistance and open circuits (which may be intermittent), slight tarnishing, weak contact pressure, corrosion, contamination, etc.


  • Disturbing components may temporarily clear any faults.
  • Broken wires can be intermittent due to temperature fluctuations and movement. Carefully wiggling, wriggling, jiggling, or just plain moving the wires about, can sometimes highlight issues?

Bad Earth or Vehicle Earth Fault:

The Solution

The solution, is to find the bad connection on the ‘return path’ of the electricity. 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?


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)

  • 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.

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.

Actual Fault Examples:

(Strange things happen – With vehicle electronics)

Here’s some examples…

  • My wife’s Mini – A hand brake Fault?

A fault showed intermittently on the dash board, as a handbrake warning light and another weird warning symbol. The handbrake symbol would vary in colour (red and sometimes orange). Wandering around the vehicle, I noticed a rear side light had gone out.

I fixed the rear sidelight, (a bad connection to the lamp) and the dashboard errors disappeared! But it showed a handbrake warning? It’s always worth looking around the vehicle for unrelated faults. They could actually be related in some way. Electronics can do strange things…

  • An electronically operated boot (trunk) lid opening randomly?

On occasions, when you opened the driver’s door, the boot (trunk) lid would pop open? The rear number plate light housing (which contained a micro switch to open the boot manually) had water in it. But, what that’s got to do with the driver’s door..?

It turns out, the curtesy light (which came on when you opened the door), went to the same electronic control unit as the boot (trunk) lid switch. Light comes on > damp switch > boot opens…

Strange things definitely happen with vehicle electronics!