Last week I replaced a brake bulb on daughter's Aygo.
Possibly coincidentally, yesterday she noticed that every time she brakes, her dashboard lights up and her headlights come on.
On close inspection,
when she manually switched her lights on, her three rear brake lights illuminate.
when she presses the brake, the her head and taillights come on.
There is obviously a potential of an earthing issue elsewhere but in the first instance I am going to remove the bulb and see if that makes a difference. I am just trying to get my head round the logic of all this. If the bulb has two terminals, and its holder, how can one filament light up for side lights and the other for brake lights?
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>>how can one filament light up for side lights and the other for brake lights?
Because the third contact is the body of the bulb. So on the bottom you have one contact for the brake lights and one for the side lights, but both earth through the body of the bulb.
If you intended to replace just a brake light, rather than a combined brake/rear light, then perhaps you have used the wrong bulb?
Last edited by: No FM2R on Wed 6 Mar 19 at 11:19
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You have used the wrong bulb, its easily and often done. There are two types of dual filament two pin/body earth bulbs, they look very similar, you have used the wrong one, causes the issues you have described
Last edited by: Zero on Wed 6 Mar 19 at 11:34
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>>You have used the wrong bulb, its easily and often done. There are two types of dual filament two pin/body earth bulbs, they look very similar, you have used the wrong one, causes the issues you have described
hopefully I have ,made a mistake with the "other bulb from the twin pack" then - have a plastic tub full of bulbs !
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Check the bulb you have taken out with the known good side. The giveaway is the that locking pins sticking out the body are in a differnt place
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Well that was straightforward - took light out, replaced the bulb from the other bulb in the twin pack and its working fine!
Will see if it was just a bad bulb or whether this does something similar after a few days!
And while I was there I stripped the boot carpet out to get it dried - common problem with these cars is leaking into the boot.
Cheers for the info on the bulbs - every day is a a school day!
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I've also known people somehow manage to bend one (or both) terminals in the base of the bulb holder, resulting in them touching one another, which gives the same result as you were getting.
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Halfords website is useful for looking up which bulb is required - all you need is the registration number.
I was amazed at how many slightly different variations of brake/tail light bulbs there were - 21/5W, 21/4W, extra robust, quick light-up, and all with different part numbers and prices.
Gone are the days when a box of 5Ws, 21Ws and 21/5Ws would fit everything.
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Toyota Aygo - how does a twin filament bulb work? - No FM2R
