blown bulb indicator?

[Legoman]

anyone else find the blown bulb indicator is a bit tempramental? , mines telling me i have a blown bulb , but i havent. i guess its an earth issue . any common areas people know of?

 

[television]

These things work on the total current consumption, if a lamp has gone a bit high in resistance it will put the lamp on.

If it is not on until you press the brake pedal,then its a brake lamp, dont forget the number plate lamps

 

[Legoman]

ok cheers , ill have a nosey tommorrow , see if i can sus out the culprit

 

[roadhog]

I'm no expert but I think there's more to it than just total consumption.
Mine was on when I got the car even though all lights were working. Previous owner had fitted a 5W bulb at the back when it should've been a 10W one. It would seem that the system compares 'sides' and puts the light on when there's an imbalance? I ended up taking every single bulb out and checking it was the correct one as.
I've just fitted higher wattage headlamp bulbs and the warning light stayed out.

 

[MIW615]

anyone else find the blown bulb indicator is a bit tempramental? , mines telling me i have a blown bulb , but i havent. i guess its an earth issue . any common areas people know of?

I had a similar problem with my E-Class when I first bought it, for a while I went round in circles. I pulled every bulb out and checked the wattage, but all were correct. In desperation I replaced all the rear bulbs including number plate bulbs and problem solved - that was 9000 miles ago

One other thing I may be wrong but I had a feeling that it could have been linked to when I pressed the brake pedal so I replaced the brake light switch as well

 

[jberks]

exactly what it checks I'm not sure. I suspect it is current as comparing sides runs the risk that both bulbs have blown so you'd get no warning.

I've had it come on with dirty bulb contacts. The bulb was right, it was new (i'd replaced it in an attempt to sort the warning) and it was working seemingly ok but the warning light kept coming on. Eventually I cleaned and retensioned the contacts and was amazed how much brighter this side suddenly was than the other. I hadn't been aware how much it had dulled over the years (and it was only 5!)
Did the other side and the warning light stayed out.

 

[television]

These thing can only work on current sensing, as JBerks says, one bad holder and the lamp comes on, its quite simple, poor contact, the resistance goes up and the current down. Poor contact holders also generate heat, heat weakens the springs in the holders.
Never use sand paper as that will remove the flash coated nickel. just a non drying switch cleaner.

Bulb filaments do erode away with use and age, a couple of these and the lamp comes on

 

[MIW615]

These thing can only work on current sensing, as JBerks says, one bad holder and the lamp comes on, its quite simple, poor contact, the resistance goes up and the current down. Poor contact holders also generate heat, heat weakens the springs in the holders.
Never use sand paper as that will remove the flash coated nickel. just a non drying switch cleaner.

Bulb filaments do erode away with use and age, a couple of these and the lamp comes on

Probably also worth checking all the earthing points

 

[wireman]

The sensor unit is in the relay box along with most of the other electric stuff.
As to what it does:- It only measures current draw on each circuit. The comparator reference is derived from the battery so lower volts means it senses at lower currents, it has to do this since the battery voltage is not constant especialy when there are lots of loads swithed on.
As television says the lamps filaments do get thinner with use and the unit detects this as a lamp fail, all the lamps errode and the small drop is sufficient to trip the relay. Replacement of just one lamp will not always get you in the clear, lamps are not precision devices and the worn/erroded ones will still bring on the warning.

The only cure for persistent erroneous indications is replace all the lamps on the vehicle but since there are lots of lamps on the side light circuit it is most likely that only those need be replaced.
If you do put fresh lamps in make sure they are of decent quality and the correct power rating. Given that the wiring and holders are good this should cure the problem, it did on mine.
There as Television says are contact cleaners and lubricants available they all work well on conacts but beware some plastics do tend to melt or disolve when you splash them with these solvent based products, this can destroy the device you wish to clean.
My preffered substance as a protector is Waxoyl, I have not yet destroyed anything with it and some electronic assemblies which I have protected with it (pole mounted at 100' on the lancashire coast) have behaved impecably despite 10 years in salty winds.

 

[philharve]

Hi All

I agree with all the comments and advice given above.

I have found the lamp failure sensor to be reliable. There have been a handful of occasions where I began to suspect it but in every case the fault was either the lamp or its holder. Corrosion or water ingress can create a high resistance that triggers the sensor. A good switch cleaner or water displacing lubricant often works wonders. I've had a failure in a nearly new bulb and the filament looked fine under casual inspection. Testing with a multimeter told a different story. Under a microscope I could see the filament had actually fractured but the gap was invisible to the naked eye.

It is my understanding that the sensor works by measuring the current drawn and this is converted to a volt drop which is used by a comparator to assess the state of the circuit. If the circuit feeds current to lamps A and B and B fails(say), the current drawn is halved(approx.) as is the voltage dropped. This current/voltage change triggers the sensor.

It is my understanding that the sensor will not indicate a lamp failure if the circuit is not drawing current. For example, a single headlamp failure will go undetected by the sensor until the headlamp circuit is activated. Furthermore, the sensor only assesses the state of the lamp circuits after the car is started; it doesn't possess a memory. The are no fancy microprocessors involved, just relays and current/voltage comparators.

REGARDS

Phil