550 FACTORY TRAILER COUPLING WARNING LIGHT

[The Smudger]

Hi all!

OK, I have bought a new family car, 2009 m/y W211 estate E320. It has the factory trailer coupling fitted, and when extended it has a warning light on the dash "TRAILER TOW HITCH CHECK LOCK MECHANISM".

The hitch appears to be locked, and seems to work OK. Any thoughts or ideas please?

TIA,

Steve.

 

[BTB 500]

I assume there's a sensor or microswitch to detect when the hitch is locked. As a first step I'd be looking for that and checking the wiring to it and any connectors. That's assuming it really is locked of course - do you have any operating instructions etc.?

 

[MJJ]

Agree with @BTB 500 . There must be a microswitch somewhere that communicates lock status to the trailer module.

Simple things first, the hitch may have sat unused for years and perhaps the microswitch will start working with a bit of usage again. I would be deploying and folding the towbar several times and see if that frees up the microswitch at all.

If that does not work, perhaps some of the below helps from looking on the various tools available to us DIY-ers.

Looking at the parts diagram, there does not seem to be any separate switch - so I assume built into the pivoting hitch mechanism.


Looking at the wiring diagram in WIS, what I assume to be the switch (S142) is shown, and includes its own three pin wiring connector S142X1 prior to feeding into the main trailer module via a 4 pin socket with one pin unused.




So, I would be looking under the car to see if there are any 3 or 4 pin wires leaving the pivoting hitch mechanism, and then unplugging/cleaning anything you can. Then, it would be worth trying to find the trailer module (in the boot, behind one of the side covers I expect) and looking at the 4 pin connector plugged into the trailer module.

I see you are in Wiltshire. If you are anywhere near Malmesbury, you are welcome to bring it over and see if iCarSoft diagnostics can tell you anything more.

Martin.

 

[The Smudger]

Gentlemen, thank you so much for your replies! And Martin, yes i am near Bradford on Avon so not a million miles from you, so good to know thank you.

Can i get access to the EPC and Schematics? Schematics would be very helpful i am a technician 30 years mainly JLR/BMW but did work for MB 24HR for a while, am now a technical trainer freelance but mainly for BMW.

Thanks again chaps!

 

[MJJ]

Gentlemen, thank you so much for your replies! And Martin, yes i am near Bradford on Avon so not a million miles from you, so good to know thank you.

Can i get access to the EPC and Schematics? Schematics would be very helpful i am a technician 30 years mainly JLR/BMW but did work for MB 24HR for a while, am now a technical trainer freelance but mainly for BMW.

Thanks again chaps!

Glad it helped. So, if you were to search eBay for 'WIS ASRA 2019', just as a suggestion, you would find several options available to you. I imagine, given that these tend to cost a whopping £6 or £7, these are not exactly what you would call licenced copies. However, someone I know well (cough...) has had successful experience and usage with such a purchase.

I have also seen mention on this forum that joining the Mercedes owners club gets you access to the toolset also, but as I have not yet joined that club, I cannot comment for sure.

Martin.

 

[BTB 500]

IIRC MBOC used to have online WIS/EPC access but this was dropped.