Fixing non working brake light, well that was an odd one.

[Chris French]

The high level brake light on our 2009 Ford Galaxy wasn't working, replaced bulb, still not working. Didn't seem to be power to the light unit. Thought I'd leave it to the garage to faff about with.

Few weeks later it goes in for it's MOT, they investigate, no power, check for broken wire where it goes from tailgate to car body - ok.

Looking at having to track the cables through the wiring loom, but first, as it's Canbus they hook their computer to the car, tell the cars computer they have replaced the bulb, and then the brake light starts working. Very odd (and none of us have enough knowledge of how the system works to be able to explain it. Maybe it just had a brain fart?)

[Tim+]

I'm afraid this kind of chicanery is all too common nowadays. Believe it or not, this is probably how it's meant to work.

Makes a bit of a nonsense of rules in some countries that insist that you carry spare bulbs. Roadside repairs are fast becoming a thing of the past.

Tim

[Nightjar]

On 15/01/2015 22:35, Tim+ wrote: ...

Are there any that still require this? It hasn't been required in France for some years now.

[Chris French]

No, I don't believe so, my garage had not come across it before (not a ford specialist, but they have always been very good). They rang the nearby large Ford dealers and they didn't suggest it. From what one chap there said I think the garage even goggled it up and that didn't seem to turn up much - just some suggestions of replacing the wiring loom for similar faults.

It's a common car, if it was normal behaviour it would be documented.

and I've changed other bulbs on the car with no problem, why would they do it for one bulb?

Not really, since this just seems to have been a bit of an oddity (software bug? weird blip in the system, who knows)

[john james]

Maybe this one failed with the bulb shorting as the filament came off and bulbs don't usually fail that way, so you don't usually see the software noticing that sort of short and not powering that line until its told that the bulb has been replaced.

Maybe more of a documentation failure than anything else.

[Chris Bartram]

My Leon has CANBUS, and I recently had a brake/tail lamp failure (it's a single filament 21W bulb that they dim with PWM for a tail lamp). Changed it, no good, but turning off the ignition and/or lights made it work. I think once the controller detects a failed bulb, it disables it until next start.

[Chris Bartram]

I think it is deliberate, possibly to protect the CE module from a shorted output.

[Brian Gaff]

It might be that they are separate and it blew short circuit so the brain disconnected it. after all if there is a facility to tell it you changed it that has to be there for a reason. Brian

[harryagain]

Probably a wire broken inside the insulation giving an intermittant connection. As you say the likely place is where the cables is bent to and fro at the door hinge. Hard to find. You can follow the electricty by stripping back the insulation here and there. However you need a wiring diagram that shows cable colours really.

[Jethro_uk]

Did you read the OP ?

[Chris Bartram]

There isn't actually a facility to say you changed it, just the facility to clear the error from the controller.

[Davey]

My Renault Laguna recently told me that one of the rear license plate lamps had failed. I replaced it with a new one, but it didn't come on, then switched bulbs, which proved that the new bulb was good, but still only had the same location lit up, and then switched them back, and everything worked fine. At no time during this did I start the engine, just turned the light switch on and off each time. They just have minds of their own, they don't work the way we expect!

[Chris French]

In message , john james writes

Could be I guess, though still a bit surprised that it's not common enough (even though not a usual bulb failure mode) to be known about as a possibility.

I'll have to suggest it to the garage next time I'm in