W203 states rear lights aren't working when they are actually fine.
Thread Starter
Newbie
Joined: Sep 2017
Posts: 13
Likes: 0
C180 Kompressor Avantgarde SE
W203 states rear lights aren't working when they are actually fine.
Not a big issue but every time I start the car I get nag prompts stating that both auxiliary rear lights are on and I should check them. They aren't, the main bulbs are working fine. Strangely I get a slightly different message if I do remove the main bulbs but then it just simply states that the rear auxiliary lights are in use, no mention of checking. I've tried resetting the fault using decent garage software but it doesn't work. My battery or alternator or possibly both are a bit crap (I occasionally get that prompt too) and I suspect that will be the issue but I'm a bit skint at the minute and need to leave it a while. Could it be anything else?
MBWorld Fanatic!
Joined: Dec 2003
Posts: 3,275
Likes: 236
From: BC, Canada
2003 C230K Coupe Orion Blue
The car uses current draw on the circuits to determine if a bulb is out. Sometimes incandescent bulbs will have a higher than normal resistance before they actually fail.
The solution is to either replace the bulbs, or add a resistor to draw slightly more current on a given circuit. No need to go crazy here - you just need to change the value enough to fool the car computer.
If the car thinks the bulb is still on, that is a separate issue. But sometimes these cars can do wacky things. Have you disconnected the battery before? If done improperly (reconnecting the negative cable before positive), the front SAM can be permanently damaged.
The solution is to either replace the bulbs, or add a resistor to draw slightly more current on a given circuit. No need to go crazy here - you just need to change the value enough to fool the car computer.
If the car thinks the bulb is still on, that is a separate issue. But sometimes these cars can do wacky things. Have you disconnected the battery before? If done improperly (reconnecting the negative cable before positive), the front SAM can be permanently damaged.
Thread Starter
Newbie
Joined: Sep 2017
Posts: 13
Likes: 0
C180 Kompressor Avantgarde SE
The car uses current draw on the circuits to determine if a bulb is out. Sometimes incandescent bulbs will have a higher than normal resistance before they actually fail.
The solution is to either replace the bulbs, or add a resistor to draw slightly more current on a given circuit. No need to go crazy here - you just need to change the value enough to fool the car computer.
If the car thinks the bulb is still on, that is a separate issue. But sometimes these cars can do wacky things. Have you disconnected the battery before? If done improperly (reconnecting the negative cable before positive), the front SAM can be permanently damaged.
The solution is to either replace the bulbs, or add a resistor to draw slightly more current on a given circuit. No need to go crazy here - you just need to change the value enough to fool the car computer.
If the car thinks the bulb is still on, that is a separate issue. But sometimes these cars can do wacky things. Have you disconnected the battery before? If done improperly (reconnecting the negative cable before positive), the front SAM can be permanently damaged.
MBWorld Fanatic!
Joined: Dec 2003
Posts: 3,275
Likes: 236
From: BC, Canada
2003 C230K Coupe Orion Blue
The SAM (signal acquisition module) is a PCB that is attached to the fuse box. The W203 has 1 in the front and 1 in the rear, both attached to their respective fuse boxes. In your situation, I would check both. The lights you're having problems with are in the rear of the car, but my understanding is the front one is usually the one that fries when the battery is improperly reconnected.
The other option is to figure out which relay drives the auxiliary lights and swap positions with another identical relay. Relays can go bad but it's extemely rare.
The other option is to figure out which relay drives the auxiliary lights and swap positions with another identical relay. Relays can go bad but it's extemely rare.
