Octavia MkII SE 1.6 TDI Estate - Electrics Fault Rear Lights

[UKPoohbear]

For some time, the "bulb blown" warning light has been lighting up on the dash. I check and all lights are working. Ignore and forget the warning light and a few days or weeks later it goes out. The warning light coming on/going out seemed to be associated with the rear doors or tailgate being opened/closed but I can't be definite about that.

 

A couple of months back, the warning light came on - I checked and found a rear light wasn't working. Removed the cluster and the bulb looked OK, so I checked with a meter. Bulb wasn't blown. Replace bulb and it worked - except, there's another rear light not working. Checked bulb - bulb is not blown. Went through this 3x - finally, all lights working - except dash warning light is still on. In desperation, tried slamming rear doors/tailgate but warning light stays on.

 

Very frustrating and annoying as I don't know when a bulb really does fail!

 

Anybody any ideas?

 

I don't have any circuit diagrams but Is there some sort of distribution point for the rear lights which could be where the fault is?

 

Thanks,

[pikpilot]

It is quite common to find the bulb is OK following a failed bulb warning and the bulb not lit when on the car.

The bulb check is made soon after starting and if the bulb current is low, the warning light comes on AND the supply is removed to that bulb.

 

So, a poor connection at the bulb holder caused by water corrosion or fretting away of the solder pip on bulbs that have them, can result in a poor contact at the bulb leading to low current draw and the warning light coming on. If you check with a meter on the bulb you will see there is no supply as this has now been removed as a safeguard. You then take out the bulb and test it with a multimeter or another supply and it works!

Often, putting the bulb back in again you find it now works for a while because you have disturbed the corrosion.

 

Facelift cars with DRL bulbs are particularly susceptible as the DRL are mounted on the plastic bumper, which bounces around causing the solder tip of the bulb to wear until contact is lost due to vibration. Note also that DRL bulbs are different and have a non standard thicker filament to withstand the vibration so check carefully when buying replacements.

 

So, in your case look for contact corrosion at the bulb holders. Look too for water in the boot on the left hand (passenger) side as there are connectors in there.

[silver1011]

The blown bulb detection system is quite sensitive, it will detect failing, rather than already failed bulbs too.

 

The vibrations can cause the filament to briefly or temporarily detach, triggering the warning, but reattaching prior to you getting round the back of the car to check.

 

For the sake of £20's worth of bulbs I'd be looking to renew them all.

 

Note that the brake / tail and fog / tail on the estate models that use twin-filament bulbs are 4W not 5W. 

[Stevie D]

If you know somebody with vcds it'll tell you which lamp is causing the error or narrow it diwn significantly - mine was the high level brake unit, was still working but obviously not well enough! 

[UKPoohbear]

Thanks all. I hadn't appreciated that the blown bulb detection system is quite so sensitive!

 

There's no water around the rear light clusters but I'll have a look for corroded/worn holder connectors/bulb pips.

[UKPoohbear]

OK - I thought I had the problem solved but no 😞

 

Went all around both rear light clusters, no significant corrosion obvious, gave the  solder pip on all the bulbs a bit of a rub with some emery, bent up all the contact tags, rotated the bulbs  back and forth a number of times when re-inserting.

Started the car, waited a minute or so, got out went round the car and checked all lights working, got wife to come out and confirm brake and reversing lights were working - which they all were. Got back in the car and went to drive off and the bulb failed warning light comes on !!*$@***

 

 

[UKPoohbear]

Sorry - wasn't quite finished ....

 

Checked all the lights carefully. Finally worked out that it is one of the rear tail/fog lights which sort of doesn't work. It's a single filament bulb and works as a combined fog/tail light. I guess when the rear fog light is turned on a higher voltage is applied to increase the brightness?

 

Switched bulbs around and the same rear fog light doesn't come on so not the bulb.

 

I guess there's a bad connection somewhere in the fog light circuit - one works, the other doesn't. Any suggestions as to where to start looking? My first assumption would be that as one rear fog light operates and the other doesn't, it's somewhere in the rear near the clusters but I guess that would be too easy?

 

Thanks,

[langers2k]

Stock behaviour is that only the drivers side rear fog light works.

 

You're also correct about only the 21W filament being used in both the fog/tail and brake/tail lights. You'll notice there is a wire missing that would have powered the lower powered (4W?) filament on those bulbs. Also completely normal for this age of Octavia :)

[UKPoohbear]

I'm pretty sure that both rear fog lights used to work but can't be 100% certain .....

 

I've also noticed that the failed bulb detection test isn't run until the light are turned on. It's only when I turn on the lights that the failed bulb warning light comes up.

 

It also seems that the supply is not removed from failing/failed bulbs. In attempting to sort out what was going on, I completely removed the fog/tail light bulb on both the passenger and driver side, which I would have expected to be the same as a blown bulb. I then checked the sockets with a volt meter. The driver side showed a low/high voltage with no bulb as the rear fog light was turned off/on. While the passenger side stayed at a low voltage with no bulb present.

 

I'm begining to think the fault is with the supply to the lights rather than anywhere in the rear light clusters?