LED car lights - repair

If it is a diode, bypass it. If you have the polarity right it won't hurt. But why should they put a diode there? Could it be something else?

Bill

It could well be something else. It is a small black cylinder with a silver/grey end. Part of the writing on it is IN40

No electricity reaching them?

That does sound like a diode.

Is it maybe a thermal fuse? I would certainly be inclined to try by-passing it and/or probing the individual lights (perhaps using a power supply with some current limiting rather than the car battery, although admittedly this is presumably protected by a fuse)

In article , Michael Chare writes

Assuming we are talking about a leaded cylinder about 6mm long an 3mm round with a silver band at one end then it is pointing towards it being a diode. 1N40 is the beginning of a designation for 1A silicon diodes with the last 2 digits from 01 to 07 indicating the reverse voltage from

It would make sense to have a series diode for reverse polarity protection as LEDs have poor tolerance of reverse polarity. Use a multimeter on diode range to check the diode and the LEDs, 0.6V for the former 1.7-2.1V for the latter.

Can't see why brake lights would be prone to overheating.

Tim

In message , Michael Chare writes

Try a scrappie, local or on-line

Have you checked the full 12V is getting to it? Might be a bad connection losing a significant chuck of the voltage.

If it's on a hatchback boot lid, a common problem is the earth return through the hinges, etc goes high resistance after some years. Any other electrical appliances on bootlid are also impacted (such as a rear screen wiper or heater). Fitting an earth connection between the bootlid and the chassis fixes it.

Presumably red LEDs?

You should be able to test it in isolation by putting 1k in series with a PP9 battery and splashing each chain in turn to see if it lights. Try and get the polarity right as they don't like reverse biasing and may fail if connected backwards.

Possibly 1N40xx are pretty standard silicon diodes.

The wire leading to it, a fuse or a dry joint somewhere common.

It is unusual to lose them all at once. A single point failure going open circuit will only take one chain out. A single LED failing short circuit would make the remaining ones brighter until they too die.

Whatever the failure is it has to be in a common path. I'd test each diode in turn with a PP9 and 1k series resistor to limit the current.

"Properly" rather than "at all"? Define "properly"?

You have, I presume, checked to make sure there's +12v and a good earth?

Likely a diode. Can be tested easily with a DVM, as most will have a diode test facility.

Unplug the unit so one side of this diode is no longer connected. DVM black lead to the stripe end. Should read approx 0.6v. Reverse the leads - should read nothing at all.

Clearly you haven't driven behind some people who seem to live on the brake pedal.

I had the misfortune to be a passenger in a car driven by a work colleague who had no concept of smooth driving. He'd race up to the car in front, then slam the anchors on to pull back, then race up, then pull back ...

I was the car in front of him. I see him behind me quite often...

Hello Michael.

I wonder which is more ridiculously expensive - the LEDs, the brake switch or the wiring loom. If the LEDs are on the tailgate then I'd hope that the fault is simply a broken wire. Much cheaper to replace than any of the other items.

ttfn Ian.

It's tempting to say it must be something common, but don't discount faulty LEDs out-of-hand.

They do occasionally fail, it would only take one duff one on each series string to bring the whole lot down and all the LEDs are most likely from the same batch.

The obvious test to do first is to measure volts across each string, including its resistor - should be over 11V with the engine off.

Cheers,

Colin.

Even so it is unlikely that they all fail at once unless they were provoked by some external arcing or sparking.

And the brake lights meant to be on. I'd be more inclined to isolate the failed component and fault find on a bench top. A mistake involving a car battery and a dead short somewhere tends to do real damage.

As I said, they don't all have to fail. Just at least four of them. Also, I didn't say it was particularly likely - just don't discount it out-of-hand. I've had multiple low-power LEDs fail in a car before. Mind-you it was an Alfa...

You could do that if you wanted it to take many times longer than it needs to. I prefer to be careful. Probing an LED brake light with a multimeter is pretty low-risk. I'd be very impressed if you could do more than blow a fuse or make yourself slightly more fixing work on the already knackered brake light.

Cheers,

Colin.

Common diodes do have part numbers like 1N4001

Available for minimal cost from many electronic outlets and Ebay, but with p&p charges. For a one off try a Maplin high street outlet

If it's a 1N4001 any in the series with the same physical size is a substitute. Maplin seem to stock the 1N4002S