Electrical gremlins; a tale of lights, a demister and loss of power

[Headsnap]

Right, bit of an odd one for me. Sorry in advance, I'm going to bang on a bit like a dumber version of James May, in that I will likely just bore the hell out of you rather than also showing off technical talent.

Recently I've had some electrical gremlins affecting all the exterior lights.

When depressing the brake pedal, both side's indicators come on constant (front and back) rather than flashing and dipped beams come on, as well as brake lights.

With brake pedal depressed and indicators on, the indicator's flashing is quicker and they are sometimes constant. At the same time, rear fogs don't work and neither do the reverse lights.

I've taken apart and cleaned the rear light holders, they are in pretty good nick. No obvious signs of wiring damage to the wires that I can see.

This is very annoying and somewhat embarrassing, but not completely unfixable.

What has me stumped though is that when this is happening, I pressed the rear window demister to see if it has any effect and there is a sudden loss of engine power and all the exterior lights come back on, even when the switch for them isn't on. As soon as I turn it off the power is back again and the lights are off. Sometimes, including today, pressing the demister even rectifies the entire exterior lighting fault and the symptoms described above are gone.

Any light that can be shed on this (pun intended; yes, I'm hilarious. Said no one, ever) would be much appreciated. I've searched extensively on here and in the past I've found a few fixes for the lights but the demister thing is not something I've ever heard of before. So if anyone knows of or has found threads about it please slap me with a link.

Ta muchly

 

[Gaz7]

I'd be looking at all major earthing points, cleaning them all up.

 

[sockpuppet]

Does this happen in dry weather? I sounds like water getting to the electrics and shorting them out. Check whether all your carpets are dry as water can get in via cabin filter, leaky door seals and occasionally through the scuttle panel. I have also seen instances of the ECU getting wet causing problems.

 

[chriswales6]

As said above it could be water getting into the electrical system. The first place I’d be looking is the terminals on the battery, there’s a section above the battery with connections that don’t like getting damp. I’m not sure if there are also some relays under the battery?

Leaks under the passenger side of the dash are a pain but more serious on the driver’s side because there are more relays and the fuse box on that side.

There are more electric boxes and wires in the scuttle panel that could be sitting in a small swamp if the scuttle needs to be cleaned out.

Since you’re getting problems with the lights and rear demist you could also check the wiring to the tailgate. They sometimes short out under the rubber cover because of the movement while opening and closing the boot.

 

[DrewCole]

I'd put money on it being an earth leak/bad connection on the system somewhere.

It won't matter if it is wet or dry, if the connection is bad then it will fail at all times.

Check the major earth strapping points, as mentioned above, clean them all up with some wet/dry paper and make sure the contact surfaces are all clean too.

Do this before anything else and see how you get on.

 

[mty12345]

I'm pretty sure the main earth point for the rear, is on the passenger side of the boot compartment. If its not that earth point, then maybe one of the ones around the dash, you just need to find which earth point the lighting and heated window etc have in common

 

[Headsnap]

Enormous thanks to everyone for the info. Earth at the back is fine, haven't had time to check under the battery; I'm thinking this is probably going to be one of if not the only culprit. The box on top of the battery is nice and dry, save the sticky stuff that I assume is the water repellant applied during manufacture. Wire looms running either side of the boot at the top and down to the back are oddly fine; I've looked at these before too because I've had these problems in the past. I guess I've been lucky with them.

I'll post results when I've had a look at the major earth points.

Thanks again!