W164 Rear Light Layout

[PaulMason]

Morning all, been chasing a dashboard message on my newish to me ML. Get a fault saying brake light left and another for brake light right. "Some" brake lights do come on, being the 2 inner, lower lights in the cluster. With the taillights off, they illuminate, with the taillights on, they get brighter. Is this the right light for brakes for a UK spec car. In some Google searches and you tube videos, it looks like the brake light is the upper most light? However in some, it looks like it is th lower inner so want to check before I proceed. I am wondering whether the rear SAM (which I know has been replaced at some stage) is the issue. Unfortunately I don't know anyone with a W164 to check in person

 

[AndrewOl]

This series of MB will switch to using the fog light if it detects a fault on the brake light circuit. It is also common for a fault to be reported when a working lamp is in place. This is due to the fault detecting circuit being very sensitive to higher than expected resistance. So check wiring and clean contacts. If that is good try a different brand of lamp. My 2006 GL rear lights wouldn’t work with Halfords lamps. Had to get Osram lamps believe it or not for it to work!

 

[PaulMason]

This series of MB will switch to using the fog light if it detects a fault on the brake light circuit. It is also common for a fault to be reported when a working lamp is in place. This is due to the fault detecting circuit being very sensitive to higher than expected resistance. So check wiring and clean contacts. If that is good try a different brand of lamp. My 2006 GL rear lights wouldn’t work with Halfords lamps. Had to get Osram lamps believe it or not for it to work!

Thanks Andrew. I am utterly at a loss as I thought it was the top light and have checked continuity of the board on the malp unit and thats ok. I've even made a harness to take the board out of the equation and have cleaned the contacts too. I haven't tried different bulbs mind so I'll make that the next try to fix

 

[PaulMason]

I also checked for 12v on the wiring and the 4th contact up on the connector to the unit (purple on the Os and pinkish on the NS) seems to have no voltage so potentially a SAM issue hence wanting to check what is meant to come on