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Old 02-17-2003, 04:26 AM
Posts: n/a
Bulb failure warning light problem


My bulb failure warning light has shown some strange behaviour. For some time it has come on intermittently (when I have sidelights / headlights on), often soon after starting the car in the mornings, and then switching off after 5 or 6 miles. Now it turns on when I have the lights on, and stays on. But there are no problems with the lights - all bulbs are working at full brightness.

I have checked the bulbs and they seem to be correctly seated. The bulb holders appear to be clean and dry, and the connectors properly connected (though I admit I have not made very careful checks for corrosion). I've also checked the fuses, and they all seem OK.

Is there some sort of control module that might be at fault? The owner's manual suggests that there is such a module, but does not say where it could be.

Help much appreciated!


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Old 02-17-2003, 07:52 AM
Senior Member
Join Date: Sep 2002
Location: New Jersey, U.S.A.
Posts: 2,398
License plate lights are often overlooked when checking exterior bulbs. Also, look closely at the taillight bulbs and replace any that appear dis-colored.
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Old 02-17-2003, 08:48 AM
Registered User
Join Date: Jan 2003
Location: Dedham, MA
Posts: 58
What model and year? As indicated in another post, in my 1992 300D, I also had what I thought was false indication, but eventually found it was the license plate lamps. I had simply overlooked them.
But a couple of years later, the routine state inspection found the license plate lamps were not working, and the lamp indicator did NOT come on. The technician tried to replace the bulb but found the bulb was OK. Instead, the wiring was at fault -- there was a break in the ground circuit to the lamp housing. I never found the ground failure, but did enough testing to conclude the circuit was open some place between the lamp and the ground point, probably in the length of cable that flexes as the trunk lid opens and closes. That looked like a bear to get at and check/replace. (It turned out the car was in an accident and totalled before I resolved the issue.)
These circumstances differ from yours but the point is that ground failures may result in false indications, or may be a source of intermittent failures.
1992 300E with ASR
35 years of Diesels until now!
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Old 02-17-2003, 09:01 AM
Posts: n/a
Thanks for the responses. And sorry, I should have said what vehicle.

It's a 1998 Merc C240 (Petrol 6 cyliner), about 78K on the clock. Number plate lamps are OK, even trunk / boot lamp! In fact I can't find any defective lamps. Perhpas it is a problem with wiring that I can't see at the moment.

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Old 02-20-2003, 01:54 PM
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Just for feedback...

Checked all bulbs carefully this afternoon. One tail light bulb was rather discoloured. Removed and replaced. Also cleaned the rear lamps' contacts. Now warning lamp worked as it should. Thanks for advice!

In passing, one other lamp had a badly pitted area (a little hole, in fact) in the base contact, so it probably wasn't making a good connection anyway. I replaced that too. I guess that slipped through the quality control...

Regards and thanks for input.
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Old 02-20-2003, 06:31 PM
Ashman's Avatar
Service Advisor
Join Date: Sep 2000
Location: Sherman Oaks, CA
Posts: 4,748
I was about to say sometimes a worn bulb, even though it still lights up, creates a voltage irregularity which can trigger the warning lamp.

'92 300CE - Sold
2004 C240 - C7 Wheels
2015 ML350 - P1, Pano, Ash poplar wood, Sport Dinamica interior, Running Boards Keyless go.
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