A3 Brake light problem

[daf95]

The Boss came home from work today and sweetly announcedthat there was a warning light on the dash of our 54 plate A3 Sportback 2.0Tdi. When I had a look it was telling me that the left stop light was out, so Itook the tail light out and had a look at the bulbs. The brake light bulbwas a bit blackened but the filament looked OK so I put a meter across itand it had good continuity. Changed it anyway . . . The body of the light hadsome water in it - I think because the seal wasn't seated properly - but notenough to reach the bulbs or corrode the tracks.

Re-assembled theunit, turned the ignition on and was greeted by the same message . . . Turnedthe side lights on and discovered that they were both working, tried theindicators and they were both working too.

Went and got The Bossto put her foot on the brake and the right hand brake light and the high-levellight in the tailgate spoiler were OK, but no left hand brake light.

I took the light unitout again and had a look at the metal tracks leading to all the bulbs, and thecommon earth track. I found a blackened spot on the 'finger' which contacts thecentre electrode on the brake light bulb, so I scraped it back to bare metaland put a dab of silicon grease on it.

As a 'control,' Itraced the earth connection in the light back to the multi-way connector, didthe same for the brake contact and connected a 12V battery across them on thebench. The brake light lit up, as did the side light and indicator bulbs when Iprobed the contacts in the connector, so I think I can be confident that thelight unit itself is fully functional.

The Boss has gone outso I don't have the use of her foot at the moment to see if I have any voltageto the connector to the light, but the warning light is still on the dash andthe side light and indicator still work . . .

Is this a commonproblem, and if so what else should I be looking at?

Any ideas please,folks?

[kite]

Bulb out warning is very sensitive.
Try new bulbs.

Sent from my GT-I9505 using Tapatalk

[daf95]

Weird thing - I've lost a lot of spaces out of the text I posted - makes it hard to read . . .

Thanks for your reply - but the brake light is still not working despite the light unit itself being in working order, so the warning is functioning correctly.

I'll have to have a look at the wiring loom connector when I can borrow a foot from someone to see if I have voltage to up to the light unit . . .

[Geraldy212]

Daf, if you have a voltmeter, look across the lamp contacts. If you get about 0.4volts, it indicates that the canbus is looking for a good bulb, and that the circuit is good. Use a new bulb and make sure that the replacement bulb is exactly the same as the old one. As Kite has suggested, the bulb-out detection is very sensitive. With a new bulb in, cycle the ignition a few times, then try again. It sometimes takes the canbus time to realise a good bulb is there, and to restore the 12v supply. Position a mirror behind the car, so you can see if the brake light is working (give the Boss a break)lol. Good luck.

[daf95]

give the Boss a break)lol.

Mmmm - very punny . . . . That's exactly what I'm trying to do (give her a brake )

Thanks for the tip - I'll try metering the voltage at the bulb as you suggest. I could also lay the back seat flat so I can see the light's innards through the boot - but I'll still need a foot on the pedal if I'm at the back end looking at the meter . . .

I do know the mirror trick - my username is derived from my HGV driving days (a DAF 95 is a tractor unit about the size of a block of flats) and if I was hitching up alone in the yard in the early hours of the morning I would reverse up near the loading bay doors to check the trailer brake lights - it's awkward seeing them at the wrong end of a 45' trailer . . .

I'll try it tomorrow and report back.

[daf95]

O-K-A-Y . . .

Went to the only parts supplier within 10 miles that was open on a Sunday and bought a couple of 12V 21W bulbs - there were no Audi-specific bulbs available, nor even blister-packed branded ones of the correct type. Only a box full of nameless loose bulbs. I figured that at 60p each I had a chance of them working, and if not they would fit the indicators on the bike . . .

Took the light unit out, changed the offending bulb and decided to go straight for the money shot without checking for the CanBus signal first. Turned the ignition on and . . . . . . . the warning light was still on

So I thought I'd try cycling the ignition on and off a few times before I asked The Boss for the use of her foot.

Guess what?

It worked . . .

Thanks!!

PS Now I've just got to try and sort out the front wheel she kerbed on Friday too . . .