W124 bulb fail warning light

[tpv01]

I had a look behind my dashboard and saw the bulb tell you there is a light problem was blown. I replaced it and it comes on every time i switch the lights on (main light or side light setting) after a couple of seconds.

After searching here i found that it is normally rear number plate bulbs which should be 10W so i replaced there and it still comes on.

Then I saw the next most commmon reason is corrosion on rear cluster contacts. I removed both and both looked good but gave it all a clean and it still came on.

Next i realised i had a dash illumination bulb out in the clocks so i replaced this.....suprise suprise it is STILL coming on!! what is the problem? Every single light works in the car from the glovebox light to the brake lights. The only thing i think it could be is that the car has a towbar fitted with electrical connections which must tap into the lighting circuit if connected (never tried to connect anything to the tow bar). Would this cause this fault to come up? Any other ideas??

 

[Howard]

The bulb failure warning on the w124's is so sensitive , it can sometimes tell you when a bulb is about to fail

 

[trapperjohn]

My 124 Estate has a towbar fitted and I have exactly the same trouble/condition as you. All my bulbs work and are not cheap bulbs either. Holders are all ok. I leave it alone and live with it.

 

[jonnyboy]

almost ever 124 i've had has been sensitive to none-matched brake light bulbs. had to buy matched pairs, bosch ones, to solve. an estate i had with a towbar just needed the extra relay (?) to stop the bulb failure light.

 

[BlackC55]

Make sure all the bulbs are the correct wattage.

Does it come on with the side lights? Dipped beam? Brake lights? Which ever circuit triggers the warning will be the cause.

 

[trapperjohn]

Mine comes on as soon as I go to side lights. BTW all lights work on the trailer when it is connected up.

 

[tpv01]

same as above with the side lights.

as it doesnt come on when braking does that mean its not the braklights??? same for indicators??

if iv sorted the numberplate lights and not the brakelights, dipped beam or main beam then can it only be rear lights and side lights?? or do interior light count to so all the dial bulbs need to be right??

very confusing and annoying!!

 

[230K]

Check that your rear sidelight bulbs are the correct wattage, take them out and look at wattage written on the side of them. It should match what is written on the back of the lens assembly or is it the holder?

230K

 

[jonnyboy]

check the numberplate lights are matched too. if stuck i think i have a combination relay you can have for a tenner plus p and p

 

[tpv01]

thanks all.

is this combination relay for the towbar elecs? where does it go so i can check if there is one fitted already?

this is only a tiny little problem but is it worth a tenner to stop that annoying light coming on everytime the lights are on.............probably!

 

[jonnyboy]

no its the combo relay that does bulb failure system and a host of other stuff.

am of to Royal Ascot now, pm me if you want it. where in the uk are you?

 

[Norse]

I have had this on my w210 since I bought it... Have not been able to sort it out. Nor have the dealer..........

 

[MPG]

I had the same with my W126. The secret is to replace bulbs in pairs (it's quite fussy about that) & don't forget the rear parking lamps - chances are you'll never use them but if one has blown it'll cause the failure.

 

[Howard]

Whats a rear parking lamp ?

Doesn't it just use the sidelight bulb for that ?

 

[tpv01]

you mean the white ones for reversing? Would these bring the fault up almost instantly when the lights are switched on even if the car wasnt in reverse?

jonnyboy, I may take you up on the relay offer but might try a few other things before. I am in west wales so it would probably have to be posted.

 

[Shude]

From engine start:

Touch brake pedal; bulb fail lit then brake lamp failure (2, sometimes 3 if central), this usually stays on even if you take your foot off the brake.

Select reverse gear; bulb fail lit then it's a reverse bulb (2)

Turn headlamp dial clockwise once and sidelights go on; bulb fail lit then it's one of six (possibly eight) bulbs, sidelights front (2), rear lamp (2 or 4) or licence plate lamp (2)

Turn dial clockwise to next level, headlamps; bulb fail lit then it's a headlamp bulb (2)

push indicator stalk forwards; bulb fail lit then is a main Beam (2)

Pull dial out once; bulb fail lit then front fog (2)

Pull dial out to final click; bulb fail lit then rear fog lamp (usually just the one although there is probably a second bulb inactive in the opposite housing)

Indicate either direction; failed (if not obvious at 2x the speed ) then it's probably a wing repeater (2 total)

I think that's all the bulbs although I'm prepared to be corrected! Might write this up properly and create a sticky.

 

[bolide]

Rear sidelight bulbs on early estates were 5 Watt, later cars are 10 Watt, I don't know if the other models are the same

Number plate bulbs are known for bringing up the bulb fail, as is towbar wiring without a bulb fail relay

Nick Froome

 

[tpv01]

right well if that is true it is either rear or side lights for me...but is this how the error circuit works for sure?? could it possibly do a quick spot check of every circuit and find an error or is it only when that lamp is activated, i.e. has power?

are interior bulbs definitely not included in the error circuit?

where can i find the correct wattage for all the bulbs i need to check?

 

[230K]

Hi

Thats how it works on a 1993 124.

Wattages are embossed on either the holder or the plastic part of the back of the lamp. Its all in your manual too!!!

230K

 

[bolide]

The system works like a Wheatstone Bridge - the car is divided into two halves and the bulb failure warning unit compares the current draw from one side of the car with the other. If the two sides are different/wrong the bulb fail is activated. This is why replacing bulbs in pairs tends to cure the problem

Nick Froome