Has anyone solved S/W211 failing rear interior lights/ambient?

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MJJ

Active Member
Joined
Jan 13, 2020
Messages
135
Location
Wiltshire
Car
'09 S211 E220 CDi manual - 197k miles and counting
Hi all,

I am struggling to solve a minor, but annoying problem I am having with my '09 S211, and wondering if anyone else has solved this. Googling shows a few instances of this being a problem for others, but no shared solutions.

So, the rear seat interior ambient, and main 'door open' lighting only works maybe 1% of the time. Reading lights work fine. No combination of rear dimmer button, or front overhead console button presses springs it to life, but occasionally, it comes on of its own volition and stays working for a few minutes. All other interior lighting is working perfectly.

Given the lighting occasionally works, I am ruling out bulbs and wiring. Investigating wiring diagrams in WIS show the lighting is controlled by two modules, 'Overhead Control Panel Control Unit' and 'Rear Signal Acquisition and Accumulation', i.e. the rear SAM.

I have scanned the car for faults with iCarSoft, and the only persistent fault is on the rear SAM module - P9078 Communication with the component H3 (alarm signal horn) is interrupted. iCarSoft shows an 'Upper Control Panel' which may be the same as the Overhead Control Panel in WIS, that has no errors.

My gut feel is a faulty/failing rear SAM, but if anyone has managed to solve this previously, please do share how.

Thank you,

Martin.
 
Hi Martin
I have a similar issue on my S211 in that no interior lights work at all (using switches or automatically) neither do the alarm sensors, auto-dimming command display, wipers, and lights.
Did you ever get yours sorted - Thanks John
 
Hi Martin
I have a similar issue on my S211 in that no interior lights work at all (using switches or automatically) neither do the alarm sensors, auto-dimming command display, wipers, and lights.
Did you ever get yours sorted - Thanks John

John,

I diagnosed my issue to be down to the thin cable that runs from the front overhead light assembly (with the buttons for lights, sunroof etc), to the rear-seat equivalent. It carries CAN messages, but I have not replaced as I think the headlining needs to come down. I live with it.

Your issues sound significantly different, and possibly all related to the ambient light sensor in the car. Interior lighting does not illuminate in daylight (on my car at least), and the various auto-lights and dimming are likely relying on the same source. The wipers issue puzzles me as it is nothing to do with lighting, but makes me think the fault is with the whole front overhead light console as perhaps the wipers sensor is fed by the same module. Have you read the fault codes that the car has stored?

Martin.
 
John,

I diagnosed my issue to be down to the thin cable that runs from the front overhead light assembly (with the buttons for lights, sunroof etc), to the rear-seat equivalent. It carries CAN messages, but I have not replaced as I think the headlining needs to come down. I live with it.

Your issues sound significantly different, and possibly all related to the ambient light sensor in the car. Interior lighting does not illuminate in daylight (on my car at least), and the various auto-lights and dimming are likely relying on the same source. The wipers issue puzzles me as it is nothing to do with lighting, but makes me think the fault is with the whole front overhead light console as perhaps the wipers sensor is fed by the same module. Have you read the fault codes that the car has stored?

Martin.
Hi Martin,
I have sorted mine and everything is singing and dancing. The problem was the negative (ground) to the overhead panel which was obviously damaged or not properly grounded. I discovered that the OHP was only receiving approx. 3.5 volts and on using the live wire (black and red I think) and the rearview mirror mounting bracket as a ground, the voltage shot up to 12.55 volts. I checked all the ground wires in the passenger footwell but these were all tight. I decided to cut the original negative and made a new connection to the rearview mirror bracket and everything is working brilliantly.
John
 
I thought I would come back and post a solution to this, albeit a year later. Lockdown 3 motivated me to try and solve this. I got my son on the case with an oscilloscope so we could probe the CAN wires to the rear seat overhead light unit, and it turned out there was nothing wrong with the wire feeding it.

Bought a used replacement complete light unit, fitted (2 minutes work), and all working again. My son dismantled the original non working light unit, and could not see any blown components anywhere so still no idea where the fault is in the original.

However, used replacement has been working perfectly for a few weeks now, so I am happy to consider it fixed.

Martin.
 

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