W126 bulb failure warning lamp

[tog410]

I've just bought a 1988 300SE, and can't make the little orange warning light that tells me an exterior bulb has failed go out. It is constantly lit when the lights are switched on, but all the lights appear to work. The handbook says it will also come on if bulbs of the incorrect type are used, so is there something I'm missing there? As far as I can see they are all of the correct wattage.

Any tips?

 

[TomJones]

Check bulbs on rear licence plate

 

[tog410]

I finally noticed one of these had blown yesterday, and changed expecting all my worries to be over, but it made no difference! All head, tail, side, fog and number plate lights are now working as far as I can tell.

I've just noticed another thread that mentions tail lights in a W124 should be 10W not 5W, so I shall check those again in case the 126 is the same.

 

[m7m]

When the light is on, current passes through a resistor creating the voltage that the sensor measures. If the voltage is below a certain level, then the sensor thinks the light is not working. Hence, the warning light turns on. To me, this could mean one of two things:
1. using the wrong bulb (wrong wattage) causing a small current to flow through the circuit.
2. the sensor is faulty.

 

[219]

Quite a common problem is that the tail lights are 10W , not the 5W fitted to most other cars - if you have two 5W bulbs fitted , it is the same as one 10W one being out !

I had an intermittent problem with the monitoring circuit on my 500SEL when I first got it . Problem went away and never returned after I resoldered all the the electronic modules to cure intermittent high idling speed .

The solder in a lot of the modules under the bonnet will have deteriorated with age - forming 'dry joints' - it is a very common problem and costs no more than a few hours of your time as long as you are proficient with a soldering iron ( otherwise find someone who is and buy them a case of beer/bottle of Malt ) .

The modules are VERY expensive to replace but easy to fix .

Check the tail lamp bulbs first , ensure all contacts are clean and free of corrosion .

Oh , and replace ALL the fuses with new ones , blown or not : they oxidise at the ends too , creating high resistance joints which then burn - often when you pull them out you will see a little circle burnt away where they have been contacting the fuse box ! They cost buttons to replace so well worth doing .

 

[ianrandom]

Ditto all the above. Mine was always like it til recently. I put the correct bulbs in the rear. But also the contacts in the rear lamps need tlc, cleaning and bending a bit. It had all got a bit damp back there as the seals for the rear lamps were flaky.

 

[MPG]

I had exactly the same - even though all of bulbs were working the failure light was still on. One tip is to replace the bulbs in matched pairs. Although my tail lights were both working one bulb was burning a little brighter than the other. Putting a new pair in made the problem go away - two years later still no issues.

Oh - and just to echo Pontoneers comment - change the fuses - it costs nothing but can stop a myriad of strange electrical gremlins from raising their ugly heads. Just do a google search for "continental fuses" and you'll find them.

 

[tog410]

Well thanks for all the tips. I've now sourced my problem – despite me saying that all the bulbs were the correct wattage it turned out that one tail light was a 5W after all. I'd checked one and it had been 10W and since both were exactly the same size and style I may not have actually removed it to see what it said on it, just assumed it was the same. Moral of the story - never assume anything!

The other tips are also useful, especially re the fuses. I shall go through and replace them and clean all the other bulb contacts too.

Otherwise the car is going beautifully - I've had it just over three weeks now and the first thousand miles have been great