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brake lights, bad engineering and terrible diagnosis


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Took the mk2.5 focus in for an MOT, part way through bloke say you've got no brake lights.  I thought this a little odd as just the week before I knew all 3 where working 100%.  Rang owner and asked if 20 mins beforehand was the cruse working when she had driven home from work, it was.  (whilst I get the second brake light switch is to cancel cruise if you brake, it ought to be part of the brake light circuit having assumed the three stop light bulbs must ALL be independent).  Looked at the two pedal switches, both connected up and mechanically operating.

Now having been out of motor tech world for 30 years and (unfortunately) owning a Merc, where its standard on every one ever made for them to regularly have brake light switch failure, I was already wondering if this was the problem.  This was exacerbated when I asked the MOT bloke what he thought was wrong, he instantly said "brake light switch as you've lost all three".  When I said is it common, he said "all day long go to ECP and get one".  So then I ask, will they have one in stock?  "Yes" he replied.

So with 30 mins before closing, I nip a mile up the road, noticing cruise control works and cancels as it should, give Reg number and get a brake light switch.  Go to fit, its the wrong one, elec connection won't fit either multiplug on the car.   Take it back get the other switch, which I now know will at least fit one of them.   Wire (not fit) the new switch on the lower of the two multiplugs, no brake lights, drive up the road cruise still working !!!  So its go nothing to do with the switches, great wasted an hour!

Get to parents house round the corner try to find fuse.  The Manual is at a different house ! No idea which is which, so start to test all, inaccessible, too dark, struggle like hell.  20 mins later having checked 50% of them including all 20 amp ones, I give up as can't see what I'm doing.  Parents wake up and I get on their PC and a LED torch.  On the internet and eventually find a fuse list, 15 amp fuse 132.  Can't read the number …. magnifying glass and 5 mins hunting 132 is OK, so another 30 mins wasted! 

So then decide to start checking the basics, unscrew left rear light cluster.... bulb well popped.  Get a pair of stop tail light bulbs and fit a new one on the left. No brake lights !!!!   its not looking good.  Pull the right cluster off the car, bulb blown, fit the second bulb ALL three brake lights now working.

All of which means, I'm a total knob and Ford should be shot for wiring all three brake lights in series.  How can this be legal in this day and age? Surely the middle light should at least be wired to the cruise switch on a totally separate circuit.  If I was in gov NO car would be allowed to be wired like this.



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You are completely wrong.

The brake lights can never be connected in series. Three 12 Volt light bulb in series require a 36 Volt power source. The Focus MK2/MK2.5 only has a 12 Volt power source. Just like any other car the brake lights are connected in parallel.

The problem you experienced is that the filament of a broken bulb can cause a short circuit between power/ground (and in case of a dual filament bulb also between both filaments). The Focus MK2/MK2.5 is a pretty advanced vehicle with a fully computer controlled fuse box (GEM module). When the GEM module detects a short circuit it disables the power to the brake lights to prevent further damage. 

The cause of the problem could easily be found by performing proper diagnostics using a suitible Ford specific diagnostic system. The DTC code stored in the GEM module would have pointed you to the right direction immediately.

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advanced.... if they were shorting and with a massive gap between the posts the bulbs don't appear to be, how come the one that was definitely OK still didn't work. 

That's not advanced that's blo0dy dangerous 

obviously as have been moved about whilst taken out and sat in the boot for 3 miles the bulbs may not be in the condition they were…. but checking continuity just now both bulbs are open circuit on both brake light filaments

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