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Discussion Starter · #1 ·
Hi,

I just replaced my headlight bulbs with 90/130W units, plus 100W fog lights, on my MY86 300E and now the yellow defective bulb indicator comes on.

I read in my Bentley MB E-Class Owner's Bible 86-93 that this is a normal occurrence, but they also mention that the bulb warning module can be modified to work with the higher wattage bulbs.

Anyone has any ideas or heard about this modification? I'm pretty good at electronics, so I'm qualified to open up the module and thinker with it.

How does this module work anyway, how does it sense a defective bulb? does it measure the current consumption of each bulb? Maybe it's a matter of replacing some of the sensing resistors inside the module (?) so as to correct the current consumption reading.

If anyone has a schematic of the module please post it, maybe we could figure this out and possibly helping a lot of other MB'ers with this common problem.

I really don't want to resort to the warning bulb pulling solution.

Thanks.

Alex
 

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changing to higher wattage bulbs

Don't do it! You will melt the tiny wires which are not designed to carry that wattage! This is an old story told by many who thought they could improve the terrible o.e. headlight output. The original headlights for your car, original on every Mercedes in the world except America, are called "Euro" headlights. They have all glass front lenses with a different reflector. They run 80/100 H4 bulbs without alteration. To use higher wattage bulbs requires relays and heavier wire. During the 1980's, the head of the D.O.T. felt that all imported cars should come with the same type headlights as American ones. Car makers like Volvo, Peugot, Saab, Mercedes, etc., had to have plastic headlight doors made to fill in the space where the large headlights were designed to fit. P.
 

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Discussion Starter · #3 ·
Hi Pkmaven,

My head lamp lenses are Euro style (with H4 bulbs), so they don't have the melting problem that the American ones have. In regards to the "tiny wires" I'm not sure to what you are referring, as the wiring that comes from the fuse box seems to be #14 gauge wires up to the monitoring module, and from the module on to the head lamps it seems #16 wire. I've check it out myself, these are the yellow, white, yellow/black and white/black wires. Since the wire run is not very long these wires should be fine. I found that most problems seem to develop on the bulb connector itself, as they tend to overheat when the contacts between the bulb pins and the mating sockets are loose. It's always a good idea, even on stock bulbs, to periodically check for connector tightness, as any heat that develops there is a voltage drop and basically is waisted current that's not making it to the bulb.

As to the module, since I posted this tread, I've taken it apart to inspect it, and I found that it senses bulb current based on very low resistance heavy wattage resistors that are wired in series with each of the bulbs to be monitored. Also inside the module there are 4 ULN2455A IC's (among others) which sense the small voltage drop that develops across the shunt resistors when current is drawn trough it, and this way it knows when the bulb is functioning. If the bulb opens, it no longer draws current, and the module will turn on the warning indicator on the dashboard.

If one would follow your (perfectly valid and logical) suggestion of using relays, in Mercedes cars you would loose the bulb monitoring function and would get a continuos fault indication every time you turn on your head lamps. This happens because the relay coil doesn't draw enough current and the sensing circuit will see it as an open (burnt) bulb condition.

On the other hand, the problem with higher wattage bulbs seems to be that since they draw more current than the stock ones, a somewhat higher voltage develops across the series resistors, and this seems to confuse the sensing circuit so it turns on the warning indicator even tough the bulbs are fine.

As to the series resistors in the head lamp monitoring circuits, they aren't "tiny" either, both the LO and HI beam resistors are made of what seems to be at least #10 or #12 heavy gauge wire, so they should be fine to leave as-is for 90/130W bulb usage.

What really needs to be done is to re-calibrate the sensing circuits so as to eliminate the fault indication without impairing the bulb monitoring function. Someone is mailing me the spec sheets of the ULN2455A IC's, so I'll be hopefully able to figure out how to re-calibrate the unit for 90/130W bulbs. The sheets should arrive next week and I'll be getting back into this.

Alex
 

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