Comms with emergency tensioning error U116787 (w166-24)

weeve

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ML350 2013 3.0 W166 (24)
Hi all
Firstly an apology.. sometime lurker, first time poster now I have an issue I can’t fix because I know so little.. I know I know worse type of forum user. Sorry...

In my slight defence I’ve only had 1 merc and only since lockdown mk1 and (sigh) this is my second issue. The first was a nice big engine fault) which was the NOx sensors but which somehow were covered by Mb even through the car is a 2013..prob as only done 43k miles. Note to self. Older complex cars will do your head in even though they are lovely when the work ..Anyhoo... I digress..

I have the orange ‘pre-safe inoperable error warning’ at start up. It has come and go for a month+ Now it just comes on all the time.

Icarsoft gives me the error as per title ‘communication with the left front reversible emergency tensioning retractor has a malfunction. The message is missing’. The code is linked to the instrument cluster (presumably as it’s not getting the message..)

I think I could do with a new battery (drops to 11.6 or so after couple of days) but the car starts okay and there aren’t other faults that stay current so before I drop 200 squid plus on a battery only to have the same issue I’d love some insights from others if you don’t mind.

The Mrs reports her seat belt does not do the pretension pull as we start anymore (mine does) so I’m reasonably confident the issue is not just the battery (maybe). Is there a fuse dedicated to this that could have gone with a bad voltage supply? Possibly just a lose connection? (I guess I’d need to take the seat belt trim off to check) but my car knowledge is based on Dr. Google rather than any skill whatsoever. Pointers most welcome. Thanks.
 
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robin.large

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I have no idea what the problem is but if it were me?
Check all connections to seat. Unplug then plug in?
I would start the engine with only driver and no seat belt.
Check for code?
Then plug in drivers belt only. Check codes?
Then add passenger. Check codes.

If code thrown on start up then likely module.
If no code then could be sensor - check if seat sensor or belt sensor

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weeve

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Thank you

Replugged under seat connex
Cleared fault code with engine off
Pre safe inoperable returns with ign. on
Fault code returns as as current fault as per above under ICluster
Testing pax seat belt shows no pretensioning action
Driver seat belt works fine.

Got to be a connection, fuse (?), sensor or the unit itself has gone... but was working temperamentally without strain so hopefully one of the first three. Problem is I’ve no idea what to do next. Guess remove trim and replug the connection to the unit which I assume is bottom of the pillar
 
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weeve

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Hi. - just done another scan. Still only the one issue but I noticed the smart scan only picked up the

-RevTR.RF Right Front Reversible Emergency Tensioning Retractor (A76/1)

and not the Left one

Its on the manual scanner list but when I try to enter it it tells me it doesn’t exist. Is this correct? Surely not right? Perhaps indicates there is no power to it or it’s broken? Would have thought that would be seen as a fault on the scan of the left side.

Maybe module is dead? Wondering Is the module part of the whole seat belt assembly? So would need whole new seat belt?
 

ajlsl600

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As a starting point. Do you have a number of low voltage codes? Or is yr belt pretension only issue flagged up. One thing, these cars hate low Batts, so be sure that's OK.
There Wil be something /in belt system that is not reporting its status, hence error look at wiring there, anything disslodged, maybe under seat. That area is where I would start.
Also. What scanner are you using?
 

alexanderfoti

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It cannot communicate with it, for the reasons you have mentioned. Check the fuse supply for it and its power. They do fail though so its likely it is the unit itself.
 
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weeve

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thank you for the replies.
Only code I have
Icarsoft mb2
My challenge is finding the wiring or the correct fuse. Any pointers ... ? The wiring all under the B Pilar trim or any under seat?

Re fuse box... bit lame I know but I only have dr. Google to help (the joy of living in a rural location) and can’t see it. Have a list from Mercedesmedic but nothing jumps out as a seat belt fuse
 
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alexanderfoti

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Only code I have
Icarsoft mb2
My challenge is finding the wiring or the correct the fuse. Any pointers ... ? The wiring all under the B Pilar trim or any under seat?

The fuse boxes should still have their fuse legends inside them? Engine bay, end of dash and rear of car
 
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weeve

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Just back from work and its pitch black here now so was unable to check. I think I’ve found it on a diagram of the battery fuse box Fuse 33 and 40A.
Assume I can just test it by swapping the right (fuse 35) and left tensioner fuses? Don’t have a spare one... tomorrow’s job before work then. Can’t see how a 40A fuse would blow though unless I crashed.... think this may be red herring. Wiring or unit fail I fear. If it really is in the prefuse box next to battery it may be impossible to get at without removing seat or employing my child and his tiny hands ....(joke.. he is busy in the coal mine tomorrow)
 
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alexanderfoti

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Just back from work and its pitch black here now so was unable to check. I think I’ve found it on a diagram of the prefuse box in the engine Fuse 33 and 40A.
Assume I can just test it by swapping the right (fuse 35) and left tensioner fuses? Don’t have a spare one... tomorrow’s job before work then.

Yes, that will work :)
 

alexanderfoti

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40A seems a bit high for that though, do double check the fuse position vs the diagram
 

robin.large

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To blow a fuse you first need a power problem. As this would most likely be the tensioner then it's the tensioner I would suspect.
Testing fuses is easiest with an ohmmeter or multi meter.
If it were a sensor you'd likely only see trouble codes once the seat was occupied or the belt plugged in.
The lack of visibility/Comms seems to point to the unit/module failing. This isn't a common fail, but they do fail.

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weeve

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Yes ... indeed. Actually edited my post above as you were replying. Seems to be in the battery prefuse area.. so my guess is impossible to access without seat out and a red herring anyway. What bothers me before I drop into the dealer and hand him a kidney is it was temperamentally working... and was fine for the past two weeks after a few hiccups. I’m old school and worry it might just be the wiring but I’m prob grasping at straws.
 
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mattkh

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Yes ... indeed. Actually edited my post above as you were replying. Seems to be in the battery prefuse area.. so my guess is impossible to access without seat out and a red herring anyway
Does the heater in the seat work. The 40A fuse could be for the heater in the seat and the same circuit may use the belt tensioner.
 
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weeve

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View media item 3485
Seems it’s solo as per diagram
No seat heaters in car I don’t think. If there are I’ve never noticed but then again not had it long...

I’m being very greedy with my questions, sorry. But does anyone know if the module and seat belt assembly are one thing. Just so I know if I need to give them a lung as well as a kidney if this has died a death.
 
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