W124 unknown bulb out? - PeachParts Mercedes-Benz Forum

scottmcphee · #1 10-05-2009, 10:53 AM

I was getting bulb out indicator on the dash, and only after doing all the head scratching, and pulling connectors in pairs, cleaning up the fuses, AND replacing the bulb out module with a spare I had kicking around... clued in here and LICENSE plate lights DING! Replaced it and problem went away.

Until today. Now, I'm getting same symptom as original poster. As soon as the car is started (light switch position = OFF, car parked) I get the indicator bulb on the dash. No lights are supposed to be on, how is this possible?

First thing that comes to mind is put the original bulb indicator module back in and see if it goes away. I'll do that tonight.

scottmcphee · #2 10-05-2009, 02:27 PM

I re-read the thread... the OP says failure indicator comes on soon as starting the car... same as me.
Then says adjusting the metal bits in a rear light cluster solved it. But none of these lights are "on" when the car is started.

Does this mean:
1. the bulb failure module "reads" some lights even before they are being used? Maybe ones in the rear (where a driver can't really see the problem while driving, as he could with, say a headlamp out.)

2. the bulb failure module remembers or "latches" the failure, after using the light and module notices an outage of an important bulb, say, a brake light bulb, and doesn't let you forget until the problem is resolved?

At least one of these has to be true, and possibly both. If after starting the car and you haven't used any lights yet for the drive cycle, and you get a failure indicator. ...Right?

Thanks!

scottmcphee · #3 10-05-2009, 02:45 PM

I found this in a related thread:

Quote:

Originally Posted by Sixto

It might be a bad relay if the bulb is on all the time. The bulb should be off when you first start the engine then come on when you switch on the circuit with the bad bulb, then off when you switch off that circuit. Brakes are an exception. If a brake bulb is bad, even intermittently, the warning light will stay on from when it first senses the bad bulb until you switch off the engine.

Sixto
87 300D

Same car as me, too!

So Sixtoes says it latches, but only for brake lights. It sounds as if the latch would be cleared when the key goes off, and not re-latch until brake is used again. ... or detected intermittent... So does this mean it can detect even if the light is not used, all by "itself"? Maybe. And that supports the OP's post and my observation of when it comes on, and what fixed the OP's problem.

This makes me want to check my brake lights, before suspecting and yanking the bulb failure module itself.