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Strange Brake Light Problem?

484 Views 5 Replies 3 Participants Last post by  ChriberV8, 
C
Hello! New to the forum, and new Touareg owner. (2003 V8).

I have encountered a very strange brake light problem.
My right rear brake light stopped working (legislation station pointed this out + got 6 weeks to fix before not allowed to drive).

So, I bought new bulbs.
None worked. (still normal back light and indicator worked. Also brake light right rear and 3rd brake light worked fine).

I got the multimeter:
Measured connector, then the backplate connected to the connector (where bulbs are mounted). Power on normal rear back light lead.
And when braking power on the break light lead (!?).

I switched to a spare bulb backplate.
Same result (!).

And for Both backplates, as soon as I put the bulb in the backplate, the rear back light is litten, But, the brake light does Not work when braking!
I tried to get the backplate leads push up much more forcible to the bulb leads, but no difference.

When I measure the brake light lead, without braking, I get a strange result. 4-5V on the Multimeter.
Now, it would be possible to have a diagnos bulb test this way, but is this not a bit far fetched?
It would be as easy to make the diagnose run when brake pedal is depressed, and when 12V is applied to the bulb?

Another thing is that, the indicator chime/tick has disappeared, and at the same time I get no more warnings in display for broken bulbs.

Anyone having any idea of what this could be?

  • ECU locked up? (does any ECU run the rear brake lights?)
  • Diagnose ECU not working that interferes with the function?
  • The 4-5V shall not be on the lead for the brake light when no brake is depressed, indicating short circuit on the wire harness?

I would be very happy if anyone has any similar experience, and clues to what could be wrong!

And,
What a community! Impressive! :)(y)

/Christian
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C
jeffnhiscars said:
I'm not following exactly what's happening so I can't say if you might have a short or just a bad connection. I doubt it's a control module and my gut says check the contacts where the bulb goes in. The spring tabs may be bent or it's not grounding properly for example. Use nickel plated bulb bases not brass. Osram is your friend
Click to expand...
Thank You for Your reply.

Yes, I check the spring leads. I also tried raising them (loading with mor force to the bulb). Checked the bulb position to the leads.
They Are worn. But I expected to see bulb at least blinking when wiggle it if so. But Nope. It is strange since I measure 12V at the spring leads if depressing brake. (and 4-5V if Not depressed!).
I will try brush the lead surface at the weekend. Also to disconnect battery for a while if something is stuck/locked.
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Treg-V-eight said:
What bulbs did you get and use? Only the P21 4W ones will work.
If you have correct ones, then check the where the plastic w/metal leads mount to the light assymbly as I believe there are some small screws or connections that hold this together that may have come loose or broke and need some attention. I had to run a screw in mine to hold the plate in place for good contact as I had tried to replace the bulb multiple times to find out it was the connection.
Click to expand...
Hello Jeff,
Thank You for Your reply.

Yes. That is a good point.
It seems the grounding is good since the normal back light (in the same bulb) is working fine, i.e. grounding leads at the plastic seem to work.
(and the 12V for the ordinary red back light). But Not the brake light, as soon as I put it in.
I tested with a 2nd spare plastic back plate, and it had the same results.
(it would be my normal luck though, if both are broken/bad in the same weird way :D).

I will check this further in the weekend. Thanks!
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ChriberV8 said:
Hello! New to the forum, and new Touareg owner. (2003 V8).

I have encountered a very strange brake light problem.
My right rear brake light stopped working (legislation station pointed this out + got 6 weeks to fix before not allowed to drive).

So, I bought new bulbs.
None worked. (still normal back light and indicator worked. Also brake light right rear and 3rd brake light worked fine).

I got the multimeter:
Measured connector, then the backplate connected to the connector (where bulbs are mounted). Power on normal rear back light lead.
And when braking power on the break light lead (!?).

I switched to a spare bulb backplate.
Same result (!).

And for Both backplates, as soon as I put the bulb in the backplate, the rear back light is litten, But, the brake light does Not work when braking!
I tried to get the backplate leads push up much more forcible to the bulb leads, but no difference.

When I measure the brake light lead, without braking, I get a strange result. 4-5V on the Multimeter.
Now, it would be possible to have a diagnos bulb test this way, but is this not a bit far fetched?
It would be as easy to make the diagnose run when brake pedal is depressed, and when 12V is applied to the bulb?

Another thing is that, the indicator chime/tick has disappeared, and at the same time I get no more warnings in display for broken bulbs.

Anyone having any idea of what this could be?

  • ECU locked up? (does any ECU run the rear brake lights?)
  • Diagnose ECU not working that interferes with the function?
  • The 4-5V shall not be on the lead for the brake light when no brake is depressed, indicating short circuit on the wire harness?

I would be very happy if anyone has any similar experience, and clues to what could be wrong!

And,
What a community! Impressive! :)(y)

/Christian
Click to expand...

Small update 5th of February, (Snow outside, and no garage):

I took the plastic backplates, with the spring tabs, and had a more detailed look at them.
Both the original and the replacement backplates, with new bulbs in.
Put power on the connector leads, and both worked fine (unfortunately).
So problem is prior to the rear lamp module.

Finding a schematic, also indicating the Fuse is actually feeding the whole right rear light side, thru J393.
And since both the rear light and indicator works when mounted in the car I conclude it is Not the fuse.
Rectangle Slope Font Line Parallel
(SB15)
Fuse - SB15
Next step will be to find the J393, and the pins T12c/12 and T12b/11.
It will require some tear down work :(

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