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2008 S40 T5 AWD
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Discussion Starter · #1 ·
As the title states, i recently bought a 2012 XC60 t6 for my wife to daily drive. Recently we got a brake light bulb error on the dash so I went to swap bulbs only to find that the bulb is not the issue. I had previously installed LED bulbs in the brake lights for brightness/longevity but the left rear stopped working.

things i have done so far:
  • swapped bulb with right brake light which was working, did not resolve issue
  • checked bub for polarity due to being LED, did not work in either orientation
  • replaced left brake light bulb with standard filament bulb, also swapped right side to standard bulb as well to rule out low current draw possibility sometimes associated with LED bulbs, did not resolve issue
  • checked fuse #22 (5a) in fuse box B under glovebox, found to be in working order
  • tested wiring on both sides with a multimeter, right reads slightly above 12v, left is reading slightly less than 6v at the connector

I have looked over the wiring and did not identify any physical damage to wiring or connectors.

any suggested course of action from here other than taking to the dealership? Is there a relay or some other common issue to check?

would like to resolve this soon and am willing to do the work myself with suggested items to check. I’ve had many VWs, Audis, and other cars with crap wiring over the years but never seen an issue like this before.

i have also done some online searching but havent identified anything that seems to be relevant to this model specifically.
 

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have you checked the earth (ground) connections, always worth checking, it sounds like 6V is your issue as I have found with LED's if you drop below a certain voltage, they will not come on, it could still be a faulty bulb as you have checked it in both orientations and it did not light, did you swap the failed bulb into the known good socket and test it?
the low voltage is a concern, that could be a bad connection somewhere, try unplugging and replugging any connectors (and check for corrosion) that may cure the issue.
how does the brake lighting system work, is it one feed that loops from brake light to brake light, or two seperate circuits, if the former, then the issue is after the first break light, if the latter, somewhere in the circuit for the failed light.
 

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2008 S40 T5 AWD
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2 Posts
Discussion Starter · #3 ·
To answer some of those questions, it is not easily identified where the ground connections actually attach to the body but i did examine the ground wires for damage and their secureness within the plugs at the terminal junctions. Ground wires on both left and right brake light connectors appear to be a bluish grey color, and the positive wire on both sides is purple with brown stripe, i will mention that there are (2) positive wires (purple with brown stripe) entering the same pin on the connector for the left brake light while there is only one positive wire on the right tail light. Yes, i did reorient the LED bulbs as mentioned previously as well as seapping the bulb from the right brake light into the left, this did not resolve the issue. I also swapped in new incandescent bulbs to both left and right, did not resolve the issue.

I did play around with a few things though and got mixed/confusing results. I ran a jumper wire from the positive of the right (working) brake light to the positive of the left brake light. After doing this, the brake lights both worked but only with the engine running and not wile the engine was off. And the brake lights would only work for about a minute before both would shit off on their own. Turning off the engine and restarting would make both sides work again but only for about a minute. I repeated this cycle several times with the same result. Checked the brake light fuse with the car off and was found to be in proper order.

I ran out of time and daylight so i will pick it up again tomorrow but for now i’m planning to switch the jumper wire to connect to the “other” positive wire for the left brake light after some further diagnosis with the multimeter and i’m assuming one of the positive wires is switched power and the other is constant with input from the brake light switch/relay.
 

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so basically the left brake light gets the supply and loops it to the right, one thought on the 6V thing, if one of the lamps is getting a supply from a different lamp via the ground the two bulbs in series will drop 6V each (this is indicative of a bad ground) try jumping a ground across from one lamp cluster to the other, this should eliminate this issue, as if it then works, it will be an issue with a ground. (i would expect this to exhibit on all bulbs though)
 
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