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Discussion Starter #1
My rear turn signals on my '08 are misbehaving. When I turn on the left or the right turn signal, the appropriate side in the front flashes but both sides flash in the rear. (All four have regular filament bulbs and, until recently, worked normally. Nothing has been intentionally changed on the car.) Any thoughts?
 

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Not enough information.

Do the brake lights work normally?

Do the rear turn signals flash both sides while the brake lights are on?

Do any of the other rear lights show any illumination when the turn signals are on?

Are the grounds for the rear lights clean and tight?
 

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Discussion Starter #9
Further Information:
The brake lights and the emergency flashers work normally. When the brake lights are "on." the turn signals function normally in the front and in the rear. When the brake lights are "off," the front turn signals function normally but both lights flash simultaneously in the rear.
When the turn signals flash in the rear, they are the only lights that illuminate in the rear.
I removed the electrical connectors from the bulb holders. There is continuity between the brown wire on each connector and bare metal on the body. If there are other grounds to check, I don't know where they are.
Does that point us anywhere towards the problem (and solution!)?
 

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If this was a Canadian 450 I would suspect the Canada E-Box.

The US and Canadian 451 must have something similar, because this version's rear lights are wired somewhat like that of the Canadian 450s. I suspect that electronic unit.
 

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Further Information:
The brake lights and the emergency flashers work normally. When the brake lights are "on." the turn signals function normally in the front and in the rear. When the brake lights are "off," the front turn signals function normally but both lights flash simultaneously in the rear.
When the turn signals flash in the rear, they are the only lights that illuminate in the rear.
I removed the electrical connectors from the bulb holders. There is continuity between the brown wire on each connector and bare metal on the body. If there are other grounds to check, I don't know where they are.
Does that point us anywhere towards the problem (and solution!)?
First, a continuity test is not necessarily valid for a circuit, a voltage drop test is a better test. That said, since all the other functions do seem to operate correctly I do not think a ground fault is the culprit.

The turn signals "seem" to operate correctly when the brakes are applied because they are using the same bulb. The SAM is properly powering the brake lamp opposite the flashing turn signal. Which brings us to the sticky bit.

According to Mercedes wacky wiring diagram the rear stop/turn lamps are powered directly from the SAM. If the diagram is to be believed the output to each respective turn/stop lamp is either turned on solid when braking or flashed when turning, as is appropriate for the situation. The same SAM outputs also supply power for the turn signal relays that power the front and side turn signals. Those relays are operated by two separate flashed ouputs from the SAM that turn the relays on and off and they appear to be operating correctly. Very bizarre. Disclaimer: the wiring diagrams are not always completely accurate thus throwing suspicion on any conclusion based solely on them.

Summary. The rear turn/stop lamps are powered directly by the SAM. The two outputs are not shorted to each other or to the center brake lamp circuit externally from the SAM since the rear turn signals work normally when not braking and one flashes when braking and turning. There is no ground fault since all the other rear lamps work normally.

Conclusion. The most likely suspect is an internal fault within the SAM. This is not a diagnosis, this is a result of a thought experiment and can not be relied upon without actual testing on the vehicle. In other words, I could be full of sludge but it appears the SAM is flashing both rear turn/stop outputs when either turn signal is active. The problem is masked with the brakes applied since it is also correctly activating the output for the brake opposite the commanded flashing turn indicator.

If this were my car the first thing I would do is scan the SAM for any fault codes. You may not have that capability. Then I would disconnect the battery to put SAM into a coma in the hope that if he is acting up he will awake with a better attitude. During the time the battery is disconnected I would unlatch the SAM and swing it down to inspect the top side and wiring connectors for any signs of water intrusion or corrosion.
 

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Discussion Starter #13
Thanks Rusted. I put SAM to sleep for a bit over an hour. Unfortunately, when I woke him up he was just as grumpy as before. (I didn't look inside the SAM since I wouldn't know what I was looking at or how to fix anything.)
So, Tmater gets a vacation until I can get a chance for a dealer visit. I'll post the results once they're in.
 

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Discussion Starter #14
Well, I have a resolution, embarrassing as it may be. The dealer traced out everything and found that I had the wrong (single filament) bulbs. They swapped them for the correct ones and all is well.
Geez, it never occurred to me to check the bulbs I was given when I bought replacements. Shame on me, lesson learned.:redface:
 
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