When I start my 300D and turn on the headlights, the defective lamp indicator turns on. I have been told that an improper bulb could have been installed causing a change in resistance thus triggering the indicator lamp. I don't think this is the cause. Is there a relay that controls the defective lamp indicator that could have become defective?

Have you checked license plate bulbs, rear fog bulb and all marker lights?

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Terry Allison
N. Calif. & Boca Chica, Panama

09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)

Incorrect bulbs can cause this issue, as suggested check all bulbs. The license plate lights and rear window brake light is often overlooked.

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'86 300E 5 speed
'71 Triumph TR6
'46 Cushman Scooter
'41 Ford 9N tractor

Yes, I have checked all lamps including license plate, fog, and running. I am stumped. I have heard there is a multiple pin relay that checks the resistance and triggers the warning lamp if there is a discrepancy. I would like to locate this relay and just for kicks replace it from one of my other 300's.

Do you where this relay is located?

Thanks.

I have posted this several times, but what the heck:

When you remove the inner taillight covers to gain access to the bulbs, you will notice two metal strips that run lengthwise. These can loosen due to vibration over time and throw off ground contact with some of the bulbs, generally the double-filament ones...they appear to be dim.

If you press a finger against the metal strip next to one of those bulbs and notice that the bulb illumination changes intensity (i.e., gets brighter), then you've hit on your problem. You can probably look at your dash indicator from back at the trunk and notice that the indicator has gone off.

Continue to press along the metal strip...you should hear faint "snaps" where it reseats. Problem solved...hopefully.

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2009 ML350 (106K) - Family vehicle
2001 CLK430 Cabriolet (80K) - Wife's car
2005 BMW 645CI (138K) - My daily driver
2016 Mustang (32K) - Daughter's car

^ I'll give the bulb trick a try tonight. I am having the exact same problem in my TE. Replaced all bulbs and it just stays on all the time when the lights are turned on.

I'll let you know if I have any luck with that.

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2016 Monsoon Gray Audi Allroad - 21k
2008 Black Mercedes E350 4Matic Sport - 131k
2014 Jeep Wranger Unlimited Sahara - 62k
2003 Gray Mercedes ML350 - 122k

Additionally, as I understand the system reads differences in resistance. A US vs OEM bulb will measure differently. Replacement in pairs may help.

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Terry Allison
N. Calif. & Boca Chica, Panama

09' E320 Bluetec 77k (USA)
09' Hyundai Santa Fe Diesel 48k (S.A.)

Corrosion also can cause a lower than spec measurement.....check for any dimmer lights and run a piece of fine sandpaper over the connectors....mine was acting up and it was a slightly corroded bulb socket....took me forever to find!