Earthing problem?

[Tyrone]

When I have my lights on and indicate left, both the indicator and the near side tail light flash. I assumed a bad earth (which is currently via steel bolt holding on the wheel arch), tried cleaning it up, no joy, so I took it off and extended the earth to the rollover bar connection to the chassis. It still does it

 

With the indicator on by itself no problem, lights on by themselves no problem, with foglights on the nearside tail light and fog are dimmer than the offside.

 

The is no corrosion or water inside the light cluster.

 

Anyone got any advice or ideas.

[duffym]

Bri , have you tried running a wire direct to the battery neg terminal, just as a temp measure to prove whether or not it is an earth fault?

[jono]

I had a similar problem when I rebuilt the back end over the winter. I traced it to some corrosion in the bulb carrier within the light unit.

 

I'm no electrican but once I had removed the bulb and cleaned the socket up the problem disappeared.

[Petrolhead]

I looked at this at the Church Mouse. Brians rear light are mint, no corrosion but the earch is knackered as I ran a wire from the light to the spare wheen carrier and all was fine.

 

He just needs to run a fresh earch to a good point on the body and jobs a goodun

[Chris W]

Tyrone

 

This is definitely an earthing problem and is caused by 2 functions (in this case the left rear indicator and left rear sidelight)sharing a common earth. ie: the earth side of the 2 lamps are connected together and then they run to earth. If the earth connection is corroded, or in some other way a high resistance (eg: loose), then the current through the indicator lamp flows throught this "resistance" and then to earth. In doing so it creates a voltage drop across this "corrosion resistor" (just as if it were a real resistor) and effectively raises and lowers the voltage on the earth side of the tail light, as the indicator flashes on and off, which therefore causes the tail light to "flash".

 

You should find that the tail light doesn't actually go hard off when it "flashes" but probably goes from full on to a softer glow at the flash rate of the indicators.

 

For those interested in the maths, the indicators are 21W lamps (about 7 ohms) and so carry nearly 2 amps each. If the corrosion adds just 2 ohms to the earth resistance then the total resistance in the indicator circuit will now be about 9 ohms which means the current reduces to about 1.3 amps. Now, 1.3 amps through 2 ohms will cause a voltage drop of close to 3 volts.

 

This may not sound much but it's a 25% voltage drop. However, the brightness of a bulb is dependent on the power it is dissipating and since power is proportional to voltage squared, then at 75% of the normal voltage, the power will have dropped to 0.75 squared which is about half power or, in other words, half brighntness. Hence the softer "glow" of the tail light as the earth side of its connection moves up and down by maybe 3 volts or even more, on every flash cycle, instead of staying at zero (ie: earth).

 

The fog and rear light cause the same effect but since they are both on, and are not flashing, you get the softer glow (ie: a dimmer light) all the time they are both on.

 

Check the earthing arrangement and clean it up/tighten it or run a fresh wire from the lights concerned to a good chassis earth and the effect will disappear.

 

Chris

 

2003 1.8K SV 140hp see it here

 

 

 

Edited by - Chris W on 10 Jun 2006 01:17:38