Hello All, 

The high level brake light on my Fiesta has stopped working over the past few days - I believe they contain LED's so I picked up a replacement one from my local scrapyard and plugged it in but it doesnt work.

My standard rear brake lights work fine - does anyone have any ideas? Is there a fuse to control the high level brake light?

Thanks in advance

Otis

I doubt it will be on a separate fuse. Excuse my incompetence or failure to understand what you have actually replaced, but if you have only changed the LED's, they are directional so only work plugged in one way, but by the sounds of it you have got a replacement unit?? Or at least bulbs with a proprietary connector. I've had it where I've bought replacement brake light bulbs and my head/rear lights only work when I touch the brake.

I've replaced the complete high level brake light unit - my 2 ordinary brake lights work fine. 

You need a multimeter ideally and check for 12v and ground at the high level brake light unit. If one is missing work back until it appears and between those two points you have a break. It could be a disconnected connector, damaged wire or failed earth.

sorry to sound a bit smug, but checking there was voltage getting there when the brake pedal was pressed was the first thing you should have done. Even if you did not have a voltmeter / multimeter you could have used a normal type bulb and held some wires on it.

by the way, although these brake lights are a good idea to help stop people running in to you, they are not required by law.

The MOT manual says they can not fail it for not working if it has permanently been disconnected, and it also says if there is any doubt about whether or not it has been permenantly disconnected then it should be assumed that it has.

Hi guys, 

Just a quick update. As advised I measured the voltage at the plug that goes into the brake light unit using a multimeter with the brake pedal depressed and it reads 11.2v.

I'm guessing this indicates the power to the plug is correct - I'm wondering if theres potentially problem with the plug itself, causing it not to make correct contact with the pins on the lamp unit. 

42 minutes ago, otis_b said:

Hi guys, 

Just a quick update. As advised I measured the voltage at the plug that goes into the brake light unit using a multimeter with the brake pedal depressed and it reads 11.2v.

I'm guessing this indicates the power to the plug is correct - I'm wondering if theres potentially problem with the plug itself, causing it not to make correct contact with the pins on the lamp unit. 

That voltage sounds about right yea. Check the connector pins aren't bent or snapped. Also make sure that they haven't started to rust, if they have then clean then with emery cloth.

Connect the unit to a 12v source, battery, battery chager. see if it lights up.

Isn't 11.2v a tad low?, I would expect above 12v.  To compare you could check the voltage at one of the brake lights in the main rear light units.

i know 11.2v isn't far below, and on a normal bulb it might work and just be a few % dimmer. I do not know how LEDs behave with slightly lower voltage. But the lower voltage makes me think there may be a bad connection which won't allow any real level of current (amps) through it.

As I say, compare it with voltage at another brake light on the car.

Actually whilst typing this I just went out and checked mine , it is showing 12.2v.

If you have a bad connection reducing it to 11.2v with no load (the voltmeter does not really draw any current), when you put an actual load on it, the resistance of the bad connection will cause the voltage to drop more.  You might have a partial break in the wire from the bootlid to car body where it flexes, although it is not all that likely as the number of times someone would open and close their car boot is not great (compared say to the cable on a hair dryer or hoover where you do get this problem).

If say I had a car with this problem and it was a MOT requirement for it to work (which it is not - see my post above about that) then I might run a wire from one of the working brake lights up to the centre one - yes, I know some people will say that is a bodge but it would do the job if I was desperate.

If you start the car the current should increase to over 12v.. Don't forget that leds are current sensative, Pos must go to Pos and Neg to Neg.

The LEDs won't draw much current anyway.. And should light below 11v anyway as I can't see them having anything fancy in the back of the unit 

 

As above I'd test the unit separately, and check both plugs for damage 

 

...and don't just check the 12v supply; the ground deserves verifying too.

You can run LEDs in series off a 9v battery with no problem at all so 11.2v will be ample, as above they draw very little current. Another good point made above is that LEDs are polarity sensitive so if you can, try rotating the plug through 180degrees and reconnecting. There is also the possibility that the scrappie has sold you a duff unit.


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On 5/20/2017 at 10:05 AM, karl46 said:

Connect the unit to a 12v source, battery, battery charger. see if it lights up.

It should be pointed out that some electronic chargers wont power anything unless it can see a battery presence. 
I found this out when testing a repair on one, i tried to power an electric tyre pump.

4 minutes ago, HelpIfICan said:

It should be pointed out that some electronic chargers wont power anything unless it can see a battery presence.

Very true. Just 7 years too late with that bit of information 🤣