'87 420SEL dash light stays on
 

Trying to diagnose the problem.

The dash light indicating a failed bulb stays on. I checked all the external bulbs (tail lights, front turns, headlights hi/low, fog etc.) Can't find any bulbs that are burned out. Am I missing something? does this also cover inside bulbs?

Thanks.
-Martin

The bulb failure circuit looks for an excessive voltage drop across the bulb sockets, which would normally occur when the bulb filament blows, but certain other things can cause it, too. You can get a voltage drop from corrosion and a poor connection at the socket, sometimes even if the bulb still lights. Try cleaning up the contacts where the bulb holder twists in, as well as the supply line where it meets the bulb holder contacts. Inspect the supply line in the exact spot where the bulb holder contacts hit it to determine if it has a pit due to electrolysis that creates a high-resistance connection where the bulb holder contact hits it; i.e., the bulb holder contact doesn't really physically contact the supply line very well due to the pit. If you can't correct this by bending the bulb holder contact so as to apply slightly more pressure, you will have to repair the supply line by cleaning it up thoroughly, until it's shiny, and filling the pit with solder. Do it quickly, with the corner of a well-tinned, big soldering iron, so you won't melt the surrounding plastic. Use only rosin core solder, NOT acid core. Then file the solder down smooth.

Also, a poor ground at one of the bulb modules can cause an excessive voltage drop.

Also check your license plate lights.

Have you recently replaced any bulbs? Mismatched current draw will trigger the bulb out indicator light. When does the failed bulb indicator light? Does the bulb light when you turn on the parking lights, head lights, high beam, brake or turn signals. That is your clue to which bulb is defective. If it lights immediately after start up without tapping the brake or turning on any monitored lights the sensing black box may may be defective. FYI, the monitor does not notify you of defective interior light bulbs. Be sure to check the high mounted brake light too. Mark

Thats right. I have a "regular" bulb in the right front signal/parking light and occasionally it will trigger the bulb out warning light and flash fast. I dont know where to get German bulbs for it around here. :confused:

Fast flashing is indicative of a non-functioning bulb caused in your case by poor contact---clean the bulb and socket as instructed in an earlier post. This site has the correct bulbs for your car and Pep Boys sells Sylvania bulbs that are correct for your car (Osram Sylvania manufactures OE bulbs for Mercedes and if you look at the Sylvania bulbs you will find "Osram" engraved on them). I have found that by replacing bulbs in pairs you will have fewer problems. Although perhaps overkill, for the low cost of bulbs I replace all the minor bulbs at the same time to save labor and future headaches. You can also bet that when one bulb burns out the other is soon to follow. Mark

It could be the 'long' relay inside the fuse box. Also try this

1) remove the light bulbs in pair, tail lights, stop lights, low/hi beam, parking etc. Start the engine and the 'light bulb failure light' should go off at some point with each removal. The last pair you removed would be the problem. I think you need to press the brake for the stop light.
2) If you remove all the lights and it still stays on then it could be the relay or wiring.

Hope this help.

Have you checked the third brake light?

Thanks. There are no Pepboys in these parts... closest one is about 110 miles away in Knoxville. I will try to locate Sylvania bulbs though.

That was it! Now I have to figure out how to take the housing off without breaking the tabs.

Thanks to everyone for their help.

-Martin