2010 Ibiza Brake light issue (not the switch)

[Seatiniza6j]

Hello,

im pulling my hair out over this issue.

the brake lights stay permanently on when the ignition is on, and flashes a coil light on the dash because of it.

I've changed the switch and no solution, i then found a wiring diagram that shows pin 3 and everything on that associated circuit should have 12v.

i have tested from the switch plug to the ecu, switch plug to the abs, and switch plug to the BCM.

all have continuity.

if i check VCDS, it shows at “0” under the ABS measuring block channel 001 (brake lights switch) if i apply 12v to the circuit it then shows as activated, regardless of what i do with the brake peddle.

 

[cairus]

Was an original Seat brake light switch installed or was it an accessory?

 

[Seatiniza6j]

Was an original Seat brake light switch installed or was it an accessory?

Ive tried a new aftermarket and a few used ones i got at the scrap yard, but from what im seeing i should have 12v from pin 3 of the switches plug and im only seeing 0.8v

 

[Seatiniza6j]

So ive figured out if i apply 12v to pin 4 with the brake switch connected my brakes work perfectly, now im trying to figure out why 12v isnt present on its own

 

[cairus]

I will Look in the circut plan

 

[Seatiniza6j]

It has cruise control so im going to put it back together tomorrow and go for a drive and make sure everything works, ive spent ages looking at circuits. If it all works ill just run a wire from the fuse box to the connector wire to power it

 

[RUM4MO]

Maybe this is what some other people have had happen to their similar aged Ibiza, if you find any of the threads containing this sort of fault, the trail tends to run cold, with either no follow up after getting it fixed by a garage, or "chaffed/broken/shorted wire" - with no real indications as to which wire and where it was chaffed/broken/shorted to earth, though I think that it might have been where the cabling crosses over a side rail - or even in that side rail where a "slightly too long" sticks up and chaffs the harness that runs above it - all vague stuff I'm afraid, sorry - the side rail is on the LHS engine compartment.

Edit:- I keep meaning to remove the plug at the master cylinder, on my wife's 2015 Polo, to see what effect doing that has wrt does the brake lights stay on or never come on - just to try to get one step ahead of this probable future issue!

 

[Seatiniza6j]

Maybe this is what some other people have had happen to their similar aged Ibiza, if you find any of the threads containing this sort of fault, the trail tends to run cold, with either no follow up after getting it fixed by a garage, or "chaffed/broken/shorted wire" - with no real indications as to which wire and where it was chaffed/broken/shorted to earth, though I think that it might have been where the cabling crosses over a side rail - or even in that side rail where a "slightly too long" sticks up and chaffs the harness that runs above it - all vague stuff I'm afraid, sorry - the side rail is on the LHS engine compartment.

Wiring diagrams seem to indicate it goes to the BCM, but what is the weird is its a positive wire, so if its chaffed youd think something would short/id apply voltage to the wire and it would short but it doesnt.
I bought a haynes book which suggests it goes straight to fuse 33, where some diagrams i found from the polo 6r which should be identical to my ibiza shows it going to a few systems including the cruise control.

So the unreliable diagrams make this even harder.. i will twkenyour advice and look round the rails, i will also measure resistance to the body from pin 4 to see if it has partially hit ground.

 

[Seatiniza6j]

So ive given it a drive no engine or coil lights with 12v to the brake switch plug pin4 but cruise control doesnt work, i cant find much for wiring diagrams for cruise control would be great if anyone had some