Interesting earthing problem, might be useful to others

I was getting the van ready for its MOT and discovered the nearside rear indicator was duller than the offside.

Checked the bulb values and they were the same.

Removed the light cover, turned on the indicator and (when looking very closely) could see that the filaments of the rear light and brake light bulbs were also flashing.

Checked all the wires which were OK.

First guess was a wiring fault under the dash which was leaking current into all three wires. To check this I took the bulbs out, made up a flying earth lead and checked each circuit individually. All OK.

The only common factor that I could see was that all three bulbs used a common earth on the light assembly backplate. Checked the earth from the backplate to the chassis and got a high resistance.

Cleaned the earth and all is OK.

Here is what I think was happening. When I turned the indicator on 12V arrived at the bulb. Due to the high earth resistance the voltage saw another path which was into the backing plate, into the earth terminals on the rear and brake light bulbs, backwards through the bulb filaments, and then leaking away through the old rear light and brake light wiring somewhere in the loom.

There is nothing as strange as an earth fault on an old wiring loom. If I'm right this might be useful to others. If I'm wrong I appreciate some advice on what was really happening.

 

Earth faults are fun wait until you put the turn lighst on and the wipers go

 

The horn sounds when I press the button in the middle of the steering wheel AND when I push the indicator stalk.

 

Or ,if your say, mate or neighbour(both female) cannot get their vehicle to start or show any sign of life and you check and the earth is sooo minging ,sorted by your good self and hey ,you are a dude/hero ! a star !<yup ,verrry sexist, but i have been that hero on occasions (love a crappy earth ,me)

 

This was the only thing that my bus failed the mot on last year, the garage ran a wire from the back plate straight to the battery! I have yet to sort it properly but after reading your post I will set to and clean all the terminals and see if that sorts it!

Great post, dodgy earths do cause strange things to happen on old cars!