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Discussion Starter · #1 ·
I have a 1990 300E 2.6, and a few weeks ago the external lamp out warning light came on and will not go off. Wen I turn on the ignition switch, and the warning lights (under the speedo area) go on as expected. I start the vehicle, and every warning light goes off EXCEPT the external lamp warning light. I have checked and every bulb is lit (including the third brake light and the rear license plate lamps). I do not have to turn on the lights or step on the brake pedal - the warning lamp just is always on from the moment I start the engine.
I replaced the control unit (Light Lamp Control Module 126 542 01 32),but no change. Is there something I need to do to reset this with the car's computer? Any suggestions? Thanks!
 

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'95 E300 DIESEL, '91 600SEL, '92 600SEL
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2 things.

1) Verify that the bulb holder is making solid contact with the bulb. Sometimes you may need to pull the fingers back a tad. Ditto on checking the plugs that the bulbs or light assemblies plug into.

2) Always use quality OSRAM/Sylvania OR Philips bulbs. No exceptions

Don't overlook the license plate bulbs and the high mount brake light

Also wouldn't hurt to use some electrical contact cleaner such as Deoxit. And no, WD40 is not suitable
 

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Discussion Starter · #3 ·
Thanks for the input. I have not replaced any bulbs in quite a while, and I did take all of them out and cleaned them and the sockets with very fine sandpaper (400 grit), and also sprayed them with electrical contact cleaner. Unplugged the rear lights sockets and cleaned with contact cleaner as well. All the lights are functional. I have had the warning light come on in the past when I stepped on the brakes or tuned on the parking or headlights or turn signals, and found a bad bulb which when replaced turned off the warning lamp. But this is different - the warning light never goes off after I start the engine - no need to step on the brakes or turn on the headlights. That is why I thought the control unit may be the culprit, but the replacement did not do the trick.
 

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2014 G550, 2000 SL500, 1995 E320 Cabriolet, 1980 TR8
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1,722 Posts
You must have a parasite current draw in one of your lighting circuits. It is just enough to trigger the lamp-out module and it has to be in a circuit that is activated with just the ignition switch. These can be difficult to track down.

The brake light circuit is one that is powered from just turning on the ignition switch. A suggested above (posted while I was writing :) ), it could be a faulty brake light switch that has developed a high resistance short across the contacts. The easiest way to check this is to unplug both rear tail light assemblies. Does the light go out? If yes, replace the brake light switch.

If that doesn't work, there is a high resistance fault to ground somewhere else in one of your lighting circuits. Did this problem happen after a rainstorm or a car wash?
 

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300te 4Matic, 1992, 223,000 miles. W447 Vito Sport Long 116cdi, 2018, 60,000 miles.
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Was the lighting module a brand new one.......!? Do you know anyone whose car you could check it on and confirm the replacement is good?!
 

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1989 W124 260E
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6,854 Posts
Are you using normal 12 volt bulbs ,, and that no fancy bulbs have ever been fitted in the past .Special can bus bulbs use very little voltage and the sysem thinks that being that the draw is so small, that the bulb is faulty . -light comes on
 

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1976 BMW 2002. 1991 250TD. 1995 E320. 2018 Honda Africa Twin
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You must have a parasite current draw in one of your lighting circuits. It is just enough to trigger the lamp-out module and it has to be in a circuit that is activated with just the ignition switch. These can be difficult to track down.

The brake light circuit is one that is powered from just turning on the ignition switch. A suggested above (posted while I was writing :) ), it could be a faulty brake light switch that has developed a high resistance short across the contacts. The easiest way to check this is to unplug both rear tail light assemblies. Does the light go out? If yes, replace the brake light switch.

If that doesn't work, there is a high resistance fault to ground somewhere else in one of your lighting circuits. Did this problem happen after a rainstorm or a car wash?
Also, don't forget that the light in the glovebox and the overhead switches are also "powered" when the ignition is turned on.
 

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'92 300TE 4matic 280,000miles, '92 300TE 4Matic 'Ice Blue Metalic' 101,000miles
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'Eric the Car Guy' has a great vid on hunting down parasitic circuit draws. You'll have to have your exterior lights on though, if that's when the exterior 'lamp out' light comes on.

9 out ot 10 times on my cars it was that friggin' high brake light or a license plate bulb......followed next by a headlight issue. Although one time it was a bad ground clip on the rear light strip....just the connector that had not been crimped well at the factory.

Kevin
 
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