hey all, this weekend ive been working on an issue with my wifes mk 3 1.0l tsi where the high level brake light will briefly flash then go off/really really dim when braking (the other lights are fine).

what ive tried so far :

obd - no fault codes

Fuse - Big old 40 amp intact and tested fine with multimeter. in addition the other lights attached to this fuse work ok.

9v battery - tested light unit its self, lights up fine.

checked 10 pin connector in luggage compartment, using voltage probe can illuminate bulb from connector.

continuity check of 12v and ground leads, fine.

using digital multi meter get a brief 2v when brake applied then nothing much (0.6v ish which seems to be almostconstant)

things of note - a lot of insulation cracked in gaitor between boot and boot lid.

boot lock stopped working and auto electrics place repaired the broken earth to fix it.

i just cannot work out what the fault could be and have no idea where to look next. could the j519 be faulty? i know which pin to check but getting to it is gonna be hell from what i can see, or am i missing something stupidly obvious? (i can take criticism!!)

please if anyone has ANY suggestions im open to them as im rapidly running out of hair!! many thank yous in advance!

19 hours ago, qwackers98 said:

things of note - a lot of insulation cracked in gaitor between boot and boot lid.

Did you try wiggle test for this area, the 2v and 0.6v might be a wiring/connection issue - but I'm no electrician or diagnostics expert. Checking with say a 21w bulb might give, er, a clearer view of full fat electric getting through the wires and/or connector. Did you disconnect the connectors and look at the connectors, plug and socket, for corrosion and bent pin(s).

That me out of ideas.

I don't know about 2018 cars without looking it up but for Mot purposes certainly for earlier cars the third brake light isn't required as long as you inform the tester that it has been disconnected.

Hello, welcome to the forum.

I'd strongly suspect damaged or fractured wiring between body and rear hatch - you mention cracked insulation in gaiter.

It is commonly reported for the internal copper wire core to work-harden and fracture inside the insulation.

If you have had an earth repair I would start there. It may be a loose connection or the wire has broken again plus 40amp is a huge fuse for brake lights are you sure its the right one. They are normally 10-15amp?

@qwackers98 Please share the VIN with me so that I can look up best wiring info for you.

I'll start a private message conversation with you for this purpose.

At first glance, the fuse for the brake light switch is no. 13 in under-bonnet fuse box, 7.5A rated.

So the wiggle test was wiggled and didnt show anything different, the repaired ground was on the boot lock (and it wasnt a great job either as the shrink tubing wasnt shrunk).

From what i can see the high level brake light is on a different fuse than the normal brake lights (which work perfectly). The fuse also does the parking lights(checked and working) and some other bits. I automatically ruled out the brake switch as i assumed if the normal left and right brake lights work then the switch must be functioning, but i could be wrong.

Thank you all so much for your comments so far, i truly appreciate it. I see a few people have had a similar issue in the past but noone ever says what caused it. i will revisit the wires and repair them all anyway soon as i can.

@Breezy_Pete soon as i can get it ill send it across. Thanks.

I'll check later, but I believe it's usual for all brake lights to be parallel connected off the brake light switch output.

Yeah, well that last reply of mine is some well out of date crap! 😆

Circuit for high level brake light in your car looks like this:

I think you said that if you stick 12V up pin 7 of the 10-way it works well, so I have to speculate that either the BCM isn't giving the right output on pin 9 of T73c, or there's something wrong with the wire between there and the 10-way connector on the left in luggage compartment.

So stupid question, is there an easy way to check connector t73c? Which is part of the J519 as from what i can tell its up vehind the fusebox in the dash on the drivers side. All id need to do is stick 12 down the pin and see what we get at the light if it dont work then its the wiring i guess. If it does then 🤷🏼‍♂️