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Technical An Odd Problem...

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Technical An Odd Problem...

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ryng123

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Hi there.

First time posting but I've been around the site for a couple of years. It's been the source of a lot of great info so thanks to everyone who has contributed!

Just this morning, I noticed that my Seicento Abarth's rear windscreen heater switch also activates the middle brake light. Now, this seems very odd to me and it's something that I've never seen on another car. I was hoping that someone could clarify whether of not this is supposed to happen and if not, what the cause may be.

Thanks in advance for any help and I look forward to spending much more time on the forums now that I've finally got around to registering.

Cheers :)
 
My guess would be an earth fault from tailgate to car.
The brake light and rear window will each, and separately, be fed with 12v when switched on. Both will complete the circuit via a common earth wire from the tailgate to the car. This earth has either broken, or has a poor connection to the car. Break is most likely at the point where the wires pass across between the tailgate and body, in the rubber tube.

When the screen is switched on, 12v is fed to one side of the screen wiring, then looks for an earth to complete the circuit. Not finding one, the common earth connection joins together the screen, third brake light and probably the rear wiper as well. So the electric current uses the brake light to find a path to earth, through the light, illuminating it, then probably via its feed wire to the other two brake lights and earths through them. They may also light, bright, dim, or not at all.
You may also find the brake light on when the wiper operates.

Find and follow the earth wire, repair the break, or replace the wire, or make good teh connection. That is likely to be near one of the rear lamps.
 
Right: now back home, I've found that the brake light has stopped coming on with the heater.

Could be that something has been loose and driving has shaken it back into place again. I did find one plug below the dash that wasn't quite in right, but it's hard to tell if it has anything to do with the lights or heater without getting the multimeter out.

Cheers (y)
 
portland_bill said:
My guess would be an earth fault from tailgate to car.
The brake light and rear window will each, and separately, be fed with 12v when switched on. Both will complete the circuit via a common earth wire from the tailgate to the car. This earth has either broken, or has a poor connection to the car. Break is most likely at the point where the wires pass across between the tailgate and body, in the rubber tube.

When the screen is switched on, 12v is fed to one side of the screen wiring, then looks for an earth to complete the circuit. Not finding one, the common earth connection joins together the screen, third brake light and probably the rear wiper as well. So the electric current uses the brake light to find a path to earth, through the light, illuminating it, then probably via its feed wire to the other two brake lights and earths through them. They may also light, bright, dim, or not at all.
You may also find the brake light on when the wiper operates.

Find and follow the earth wire, repair the break, or replace the wire, or make good teh connection. That is likely to be near one of the rear lamps.
Click to expand...

Thanks for that!

Even though I'm an electrical engineer, I'm not all too familiar with auto electrics so wasn't really sure what to look for. Also, the Seicento is my first car so experience ( or lack thereof ) probably plays a factor too!

I'll be sure to check out the earthing at the back of the car over the weekend when it's a bit less dark :)
 
Last edited:
ryng123 said:
Right: now back home, I've found that the brake light has stopped coming on with the heater.

Could be that something has been loose and driving has shaken it back into place again. I did find one plug below the dash that wasn't quite in right, but it's hard to tell if it has anything to do with the lights or heater without getting the multimeter out.

Cheers (y)
Click to expand...

Its working again because you've opened and shut the tailgate, or the loose connection appreciated some of the potholes you found on the way home. A job for the weekend, trace the wires and make a good connection.

Car electrics used to be very simple. Some of it still is. Each item gets a feed, and the earth return is via the car body. Most likely cause of any electrical problem is a bad earth. Always seek these out and sort them as a first diagnosis point.

Good luck, and don't forget to swear a lot while doing it.
 
portland_bill said:
Its working again because you've opened and shut the tailgate, or the loose connection appreciated some of the potholes you found on the way home. A job for the weekend, trace the wires and make a good connection.

Car electrics used to be very simple. Some of it still is. Each item gets a feed, and the earth return is via the car body. Most likely cause of any electrical problem is a bad earth. Always seek these out and sort them as a first diagnosis point.

Good luck, and don't forget to swear a lot while doing it.
Click to expand...

Thanks again for the advice.

I'll take it on board for now and the future, and I'll be sure to post an update if I find the problem (y)
 
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