Bulb Failure Warning Problem - PeachParts Mercedes-Benz Forum

sjh · #1 11-15-2010, 02:37 PM

Greetings,

I've bought a xenon HID conversion kit for my '90 w124 300D. I had my doubts but I've been very impressed. After a few more weeks to make sure it works correctly I'll post a thread on it. Here's the problem.

The power for the HID unit is drawn directly from the battery via a fused connection. One of the three prong connectors that use to connect to the headlights directly now plugs into a wiring harness that drives a relay for the HID unit. The plug for the other connects to nothing.

Now the bulb failure unit (BFU) does not detect the filament from the bulb that is not plugged in. I like the BFU and would not like to simply disable or ignore it. Is there any way I can generate a dummy load or in some other manner keep the BFU from signaling the presence of the missing headlight?

Thanks.

The Good Doctor

Jeremy5848 · #2 11-15-2010, 03:48 PM

An interesting challenge. The question is, how does the BFU work? If it works by sensing the current drawn by the headlight, is it smart enough to know the difference between a headlight and a taillight or does it just need to see some current and doesn't really care how much?

Putting in a resistor to simulate the ~5 Amp load of a headlight bulb would be a waste of energy as well as a potential fire hazard, since it would get rather warm. I suggest experimenting with a small bulb (taillight or whatever) connected to the unused connector in the three-prong plug. See if the BFU is happy with that and if so, substitute a 50 Ohm 5 Watt resistor for each headlamp and let us know how it works.

Jeremy

compu_85 · #3 11-15-2010, 04:40 PM

Look in the wiring diagram and find the pins for the left and right low beam headlights. There will be an in and an out on the bulb monitor. Remove the pins from the socket and connect them.

-Jason

sjh · #4 11-15-2010, 05:45 PM

Quote:

Originally Posted by compu_85

Look in the wiring diagram and find the pins for the left and right low beam headlights. There will be an in and an out on the bulb monitor. Remove the pins from the socket and connect them.

-Jason

GREAT. I think you are saying if the current does not flow through the relay it wont be able to monitor the bulb status. And I also think you are saying that if the leads are not connected (both in & out) to the relay it is not affected.

Now I need a wiring diagram for a 1990 w124 300D. I'm sure they are about. I'll go look. If someone wants to assist my search I'd be appreciative.

Thanks again.

Tpawlik · #5 11-15-2010, 06:06 PM

Since you solved Dr_SJH's problem so efficiently, maybe you can diagnose mine. My bulb/lamp failure light comes on randomly. It came on this morning after about 10 miles. It always resets when I shut off the car. I drove it home, 38 miles and it never came on once. It sometimes goes weeks before it comes on again.

compu_85 · #6 11-15-2010, 06:09 PM

Tpawlik, your problem is probably a loose lamp socket. In the morning when things are cold resistance might be higher and just enough to trip the system.

Yes, the lamp monitor looks for no connection at the other end when power comes into it. If you never send power into the lamp monitor, it can't tell that a lamp is faulty. That's why the idiot light doesn't come on until you try to switch on the defective lamp.

-Jason

sjh · #7 11-15-2010, 06:33 PM

Quote:

Originally Posted by Dr_SJH

Now I need a wiring diagram for a 1990 w124 300D. I'm sure they are about.

One should not quote themselves. Just doesn't seem right. Anyway here is a link for the complete wiring diagram -

http://www.w124performance.com/service/w124CD1/Program/ETM/ETM.pdf

It is a 77 MB file so it will take some time to download.

Billybob · #8 11-15-2010, 08:17 PM

Quote:

Originally Posted by Jeremy5848

An interesting challenge. The question is, how does the BFU work? If it works by sensing the current drawn by the headlight, is it smart enough to know the difference between a headlight and a taillight or does it just need to see some current and doesn't really care how much?

Putting in a resistor to simulate the ~5 Amp load of a headlight bulb would be a waste of energy as well as a potential fire hazard, since it would get rather warm. I suggest experimenting with a small bulb (taillight or whatever) connected to the unused connector in the three-prong plug. See if the BFU is happy with that and if so, substitute a 50 Ohm 5 Watt resistor for each headlamp and let us know how it works.

Jeremy

I thought I'd read somewhere that the BFU detected a bulb out condition by sensing a current draw imbalance between two related bulbs, i.e. both turn signal bulbs, both head lights, both tail or brake lights. With one of the related bulbs out, the imbalance is detected and the indicator light illuminates. That is why people experience problems with dissimilar bulbs. It would stand to reason that if this was the case the BFU may not detect a bulb failure if both related bulbs are out; I don't have a car here right now so I can't verify that. In the OP’s present configuration we don’t know if the BFU has detected a sub-threshold current or a current imbalance, because the modified lighting

With the OP’s predicament it sounds like there is either a single controller or a single HV supply (with outputs to two, a left and a right bulb) as opposed to twin HV supplies one for each of the two bulbs?

The single original connection is actually used only as a control feature to operate the power relay supplying current directly to the HV supply/ies? If so perhaps the easiest solution is dual relays one for each lighting unit, each controlled by the original connector, that would require a current flow through each of the headlight connections and in turn through N7.

Alternatively it would seem that if both OE headlight connectors where attached to the single input Siamesed so to speak, they would both be electrically connected and both have current flow through N7 eliminating the bulb out condition, no?

Jeremy5848 · #9 11-15-2010, 08:59 PM

Quote:

Originally Posted by compu_85

Look in the wiring diagram and find the pins for the left and right low beam headlights. There will be an in and an out on the bulb monitor. Remove the pins from the socket and connect them.

-Jason

The schematic of the BFU shows a common "in" for the left lamps, another for the right lamps, and a third for a "K" group of lamps (maybe turn signals/brakes?). I do not see a separate "in" and "out" for each individual lamp -- there aren't enough pins.

Disconnecting a lamp from the BFU should have the same effect as a lamp failure. (I would be happy to be proved wrong.)

Jeremy

compu_85 · #10 11-15-2010, 10:27 PM

Looking at the diagram I see this:

Power comes from the headlamp switch, goes through the combination switch, exits as either high or low beam, goes into the fuse box where the power is split for each side of the car on each beam (4 fuses), then those 4 inputs go to the lamp failure unit, then exit and go to the headlamps.

Here's how the power flows through the lamp monitor:

Code:

                   LH LL RH RL Fog
16 pole plug pin | 1  2  3  4  5
8 pole plug pin  | 2  1  4  3  6 & 8

Join the pin on the 16 pole plug to the pin on the 8 pole plug and you have bypassed the lamp monitor. Note that the fog lights have one in and two outs as they share one fuse before the lamp failure unit.

-J

arcticathlon · #11 04-22-2011, 10:28 PM

Quote:

Originally Posted by compu_85

Looking at the diagram I see this:

Power comes from the headlamp switch, goes through the combination switch, exits as either high or low beam, goes into the fuse box where the power is split for each side of the car on each beam (4 fuses), then those 4 inputs go to the lamp failure unit, then exit and go to the headlamps.

Here's how the power flows through the lamp monitor:

Code:

                   LH LL RH RL Fog
16 pole plug pin | 1  2  3  4  5
8 pole plug pin  | 2  1  4  3  6 & 8

Join the pin on the 16 pole plug to the pin on the 8 pole plug and you have bypassed the lamp monitor. Note that the fog lights have one in and two outs as they share one fuse before the lamp failure unit.

-J

compu_85

I jumped those pins directly on the lamp failure unit, and the light still comes on. the wiring diagram that was posted in earlier threads matches the pin combination you show, but i am not sure if this is the case for my 1995 E300d W124. also, from a different thread, it appears that there are 2 version of the lamp failure units, in 1995 E300d's so i don't know if that changes things even more.

what can i test, or does anyone else have a wiring diagram for my 1995 W124?

compu_85 · #12 04-23-2011, 12:53 AM

Hmm, that's strange and not the result I expected. I might have to fiddle with this on my car.

-J

arcticathlon · #13 05-02-2011, 09:41 PM

got it figured out. i purchased a N7 light unit on ebay, and received the old style, my original was the newer revision. I cut the traces to the pins for the high and low beams and that did the trick. not sure why soldering a wire did not work.

Cheers

sjh · #14 05-03-2011, 01:22 AM

Are you saying that you basically bypassed the unit for the high/low beams?

That is did you directly connect the in pin to the out pin and then cut the internal connections so that current would flow into or out of the unit from the leads you jumped?

arcticathlon · #15 05-03-2011, 02:01 AM

first i tried the jumping method, but that still lead to the light being illuminated on the dash.

then i tried just cutting the traces to remove the High/low from the entire fail circuit completely.