Hi Everyone,

 

I have a 2007 Octavia Scout.  The auxiliary brake light (at the top of the back window) stopped working: it did not light up when I braked, and I got an MoT advisory for it.  But I noticed that two or three LEDs in the unit were permanently on, very dimly, and were still on with engine off, ignition off, car empty and locked.  They were very dim, so you could only see the LEDs were lit when it was dark.

 

So I got a new light unit and fitted it today.  It seems to work fine.  But half an hour ago a neighbour knocked on the door and said "you know your high brake light is on?"  Sure enough the entire row of LEDs was dimly lit (it is dark now, so they were clearly visible).  When I put my foot on the brake pedal they all came on brightly, as they should.

 

Fiddling about, I noticed that the light goes off completely when the tailgate is unlatched and opened.  It comes on again (dimly) when the tailgate is closed.  So presumably there is something up with a switch or sensor at the lock.

 

For the time being I have left the tailgate ajar to save draining the battery.

 

Can anyone suggest what I could try to sort this out?

 

Thanks in advance.

LEDs are in fact capable of being illumined dimly with only a small current being drawn through a high resistance. 

When opening the rear hatch the current is cut off  by the microswitch in the lock mechanism.

So, the question is where is the high resistance that connect to the 12V source?

 

The usual reason is a water leak bridging two cables. Corrosion product can do the same.

Look in the left hand side of the boot behind the trim as this is where most of the wiring goes. Look also at the boot lock as there is wiring there to the microswitch. In fact, follow the wiring right back to controller.

Finally, a failing transistor/FET in the controller can do the same.

 

It's a treasure hunt but by using a modern multimeter and common sense you will find it.

2 hours ago, pikpilot said:

LEDs are in fact capable of being illumined dimly with only a small current being drawn through a high resistance. 

When opening the rear hatch the current is cut off  by the microswitch in the lock mechanism.

So, the question is where is the high resistance that connect to the 12V source?

 

The usual reason is a water leak bridging two cables. Corrosion product can do the same.

Look in the left hand side of the boot behind the trim as this is where most of the wiring goes. Look also at the boot lock as there is wiring there to the microswitch. In fact, follow the wiring right back to controller.

Finally, a failing transistor/FET in the controller can do the same.

 

It's a treasure hunt but by using a modern multimeter and common sense you will find it.

Thanks for taking the time to reply, pikpilot.

 

I'll chase the cables back to the boot.  Yesterday I had a look at the ones inside the tailgate trim, going down to the lock, and I couldn't see or feel any damp or anything else amiss.  I'll have a look behind the left boot panel next.  On my last Octavia the rear window washer supply pipe had popped off a junction there, and had filled the spare wheel well with washer fluid, so there is definitely potential for a damp problem there!  This particular car hasn't had this problem in my ownership, though.

 

I have a multimeter.  Whether I have any common sense remains to be seen.  I certainly don't know much about how to use the meter, other than for basic continuity testing.

 

Last question: where's the controller, and what does it look like?

 

Cheers,

 

scandalxk

Following @pikpilot's advice I removed the panelling on the left side of the boot and found the bottom of the compartment behind the panel had two or three mm of water in it.  Some had spilled over the welded seam into the spare wheel well.  The wiring loom, clipped to the bottom of the compartment, was soaking wet, and so was the carpet in the boot.  This happened on my previous Octavia but has not happened previously on this car.

 

I dried everything up as well as I could, then tried to work out where the water was coming from.  On the previous car it was because the rear window washer pipe had popped off a connector, so it was simply pumping the washer water into the compartment behind the panels.  The clue was that no water was coming out of the washer nozzle, but instead could be seen collecting under the spare wheel.  This time the washer has been working fine, so I had no reason to believe water was getting out somewhere else.

 

The rear window washer pipe junction (the grey connector visible at the bottom left of the photo) doesn't seem to leak when I operate the pump.  I suppose it might if the nozzle was blocked, but I haven't tested that.  Perhaps the nozzle froze up when we had a frost a few weeks ago?  This connector looks a lot more substantial than the one I remember from the old car: that was simply push-on; this one appears to have a bayonet or click fitting.

 

The pipe coming down from the nozzle was dry, with no sign of water dribbling down.  Same at the other end of the compartment, where the pipe arrives from the front of the car.  There also appeared to be no smell of windscreen washer fluid, but maybe that had all evaporated leaving only the water.

 

Is there anywhere else the water could be coming from?  What is the purpose of that big rubber vent on the outside wall of the compartment?  Could water get in there?

 

(The brake light is now working fine, not staying on dimly when it shouldn't, presumably because I have dealt with the short by removing the water bath...)

 

scandalxk
 

Edited 12 November, 20223 yr by scandalxk

A few months ago I was helping a friend to find to a water leak in the boot of his Golf. It turned out this a common problem on that car due to failure of air vent shown in your picture. I have not heard of this as a source of water in the boot on the Octavia (which is based on the Golf).

The dust on your vent suggests it is not leaking BUT on my friend's car the vent needed resealing to the body.

 

Worth having a look.

23 minutes ago, pikpilot said:

A few months ago I was helping a friend to find to a water leak in the boot of his Golf. It turned out this a common problem on that car due to failure of air vent shown in your picture. I have not heard of this as a source of water in the boot on the Octavia (which is based on the Golf).

The dust on your vent suggests it is not leaking BUT on my friend's car the vent needed resealing to the body.

 

Worth having a look.

Oh, that's interesting...the vent is slightly loose in its fitting, and doesn't appear to be sealed at all, but there are stains on the panelling which suggests water could be leaking down from it.  Or possibly not.  I'll have a closer look tomorrow.  Thanks.

Edited 12 November, 20223 yr by scandalxk

OK, fingers crossed it is fixed now.  I'm pretty sure the water wasn't coming from the washer tube, as it did on my earlier Octavia.  The pipe junction is more substantial than the earlier one and there was no sign of leakage.  However, the big rubber vent mentioned above was loose in its fitting, and when I looked more carefully today I saw that there are two clips on each side of the unit which were not engaged.  You can see the nearer two in the photo above.  I clipped them all in place and immediately noticed residual water squeezing out of the foam rubber seal on the back of the vent... 

 

There are several videos on YouTube about the Golf problem @pikpilot mentioned, and it seems as though on the Golf the water originates in the roof gutters and gets to the vent via the back lights.  I don't think that can happen on the Octavia.  However, I remember a few weeks ago hitting a puddle in the Lake District a bit faster than I intended, and it turned out to be a bit deeper than I thought.  On the Octavia the boot vent opens into a void between the inner and outer bodywork just behind the wheelarch, and without taking the exhaust box off to have a look, it appears likely that water could get jetted into there in those circumstances. 

 

So, boot and spare wheel well dry, brake light working correctly, all the internal panelling back in place (finally).  I'll report back if it turns out not to have worked.  Thanks for your help, @pikpilot.

Glad you got it fixed.