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How difficult is it to fit a Towbar?

Discussion in 'Electrics, Vacuum, Ignition and ECU' started by Brian23, Mar 14, 2019.

  1. Brian23

    Brian23 Senior Member

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    I have a 2005 Eclass Estate and I am looking to fit a towbar. How difficult is it to fit one on the driveway and will the car automatically accept the electrics or do you need other accessories for the electrics?
     
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  2. M80

    M80 Senior Member

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    I don't know about the 211 but I've done a couple of 203's.
    On those you remove a rear cross member and the tow bracket goes back in its place. I would look at the kits online and the drawings are likely to show how that might be the case.
    Online instructions may even be available.

    As for the electrics, my 639 has the oem kit and I see no advantage to having that.
    A towing relay like this
    Universal 7 way bypass relay electrics TEB7AS Towing electrics Tow Bar Electrics | eBay
    isn't difficult to get your head round.

    Basically there is often a fuse at the rear that will supply the trailer through the relay and outlet.
    The relay picks up the signals from say the indicators and passes the power to the trailer, fed from the fuse.
    Advantage here is that a trailer fault will take out the supply fuse and the car is left to function normally.
    You can also run a sounder to the front, this will bleep with the indicators as a reminder, I just left mine at the rear.
     
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  3. rorywquin

    rorywquin Senior Member

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    The last car I owned with a tow bar (2008 Landcruiser) had to have a fancy OEM wiring loom for the trailer lights -- AFAIK all to do with he modern electronics in the car.....
     
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  4. davidsl500

    davidsl500 Senior Member

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    Tow Trust have fitting instructions on line and suggest 120 mins to fit.
     
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  5. ajlsl600

    ajlsl600 Senior Member

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    as long as maker electronics not involved towbar fit is easy enough on the drive weather permitting .i must have fitted at least ten on my various cars .HOWEVER it does seem that car makers are hell bent on making this as difficult as possible for D.I.Y along with almost anything else, recommend you use this place and others to find out more and maybe experience of someone who has managed to defy the car manufacturer and dealers . thats verging on a national pastime on here
     
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  6. OP
    Brian23

    Brian23 Senior Member

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    Thank you. I will look at this. Brian
     
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  7. OP
    Brian23

    Brian23 Senior Member

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    Thank you, great, I have downloaded instructions.
    Brian
     
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  8. OP
    Brian23

    Brian23 Senior Member

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    Having looked into this it seems that:- For those vehicles equipped with a check - control system, it is necessary to use a wiring kit with a bypass relay or a specific wiring kit.
    What is this?
     
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  9. Jim2

    Jim2 Senior Member

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    I fitted a towbar to my w212 saloon on the patio at the back of the house,,,the mechanical part is not difficult, once you have the rear up on axle stands for easy access. For the electrical part, I had the Son-in-Laws help ( he is an IT Techie, and just love's modern car electric's ) He identified a live feed at the back and hooked it up to a by-pass relay, and it's working fine. I have fitted a lot of towbars over the year's on different makes of car's. The last one ( prior to the w212 fitting ) was on a 2014 Skoda Octavia, and for this I used the same type of by-pass relay, only difference was I took a live feed from a fusible link at the battery terminal, and ran it back through the length of the car. worked perfect too.
    But as ajlsl600 said....Its much more complicated on modern cars that in the past...electrics were much simpler, you just spliced the cable from the trailer socket directly into the corresponding wire on the rear lamp.
    If you do it once, you will not have any problems repeating the process in future.
    You can avoid all this if you opt for the Mercedes dedicated towbar wiring harness, which plugs directly into the box, and is connected to the internal wiring canbus.
    Good Luck with it..
     
    Last edited: May 7, 2019
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  10. OP
    Brian23

    Brian23 Senior Member

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    Hi thank you for the info. When you say that the Mercedes dedicated towbar wiring harness, which plugs directly into the box. Which box do you mean,
    Brian
     
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  11. M80

    M80 Senior Member

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    On the Viano it had what I beleive would be oem trailer elctrics. described on the VIN as 'E57 Electrics for Trailer Coupling Socket'.
    2 relays / conection units sited below the o/s rear light cluster, this was for the 2 sockets rather than just the 1- 7 pin socket so a little more complicated.
    Anyway it was / is the biggest abortion of an instalation I've seen and if this was done at purchase, as I believe, I'm not sold on the quality.

    I've only towed 7 pin with it but could never see what it did that a more simple and cheaper relay such as the common TEB7AS has done for me in the past. That picks up power from the 15 amp fuse in the n/s light area, picks up signals from lights and runs trailer lights. Any trailer faults would blow the 15 amp fuse leaving the car lights unaffected.

    There was no additional dash indication that a trailer was on the back, there were no alarms on the vehicle aside from a bleep at the rear on using indicators, that the TEB7AS allows for anyway.
    Had I carried on using the Viano the wiring would have been replaced and connections soldered, as is my preference.
     
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  12. Jim2

    Jim2 Senior Member

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    Hi Brian.. on my W212, in the boot, at the drivers side, when you remove the vertical carpeting, that's where the SAM unit is. I have attached some images to show you how and where it goes. OK? I think that if you use the dedicated MB tobar wiring harness, it's basically just plug it into the SAM unit. If you will use this harness, I think that when you purchase it, the supplier will pre program it for your particular car. For me, the By-pass relay works fine, no pre-programming needed,as I only pull a light trailer. But if you are planning on towing a Caravan etc the MB one is what you need. If you are going to tow a caravan bear in mind that automatic transmission will need additional cooling, in the form of a larger capacity cooling radiator, and possibly change of cooling fan. You will have to check this out yourself, as I have not done it ( yet !!! :)
    Good Luck and if I can help in anyother way, Just ask, OK?
     

    Attached Files:

  13. Jim2

    Jim2 Senior Member

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    In modern cars, the electric system is called a CANBUS. Basically, ( and very basically at that ) it's a bit like the human nervous system. Everything is linked and communicates with each other via the on board computer. So when you add / subtract in the system, the system has to be programed to recognise the changes. In some cars, something as simple ( formerly ) as replacing a bulb, now mean's programming the system to " accept" the new bulb. For simple towbar electrics, the CanBus is by passed ( with literally ) a bypass relay. This relay is fed electrically without "Disturbing" the CanBus, and all the tail light functions are run via the relay...again letting the CanBus "sleep". So the peace and tranquility is preserved. !!!
    The 2nd option, is a wiring kit which is an extension of the CanBus, and integrates into it ( with a little programming ) And off you go, So these are the main differences.
     
  14. M80

    M80 Senior Member

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    Is there an advantage to the 2nd option?
     
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  15. Tony Dyson

    Tony Dyson Senior Member

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    I’ve been looking into the options of adding a towbar to my 2014 C220 for a while now and as expected, finding definitive information from MB is akin to pulling teeth, they are keen to fit one but not so forthcoming in specifying exactly what I’ll get for my hard earned, what I’ve found out from various sources in the meantime is the following;
    1. The benefits of adding an OEM Trailer Recognition Control Unit over a DIY 7 Channel Multiplexer Relay (Bypass Relay) are dependent on the following
      1. Vehicle model and year, benefits include and are not limited to the provision of lighting supplies to the towed vehicle, disabling the rear Parktronic Sensors, adjusting the Stability Programme, adjusting the driving profile for an auto gearbox, instrument cluster indication for a connected trailer, audible indication when indicating.
    1. I am led to believe that dependent upon vehicle model and the Year of manufacture and unless a towbar is ordered as a factory fitted extra, there are two options of rear SAM unit and Fuse & Relay Box that will be fitted to a vehicle, one that is prepared and programmed for the addition of a Trailer Recognition Control Unit and one that is not, a simple check of a rear Fuse and Relay box will reveal the presence, or not of 6 fused supplies indicated for the Trailer Recognition Control Unit, one of which is further identified for the Oil Cooling Fan Relay.
      1. If the existing SAM isn’t prepared for the addition of the Trailer Recognition Control Unit then a replacement SAM will be needed as again I am led to believe MB will not allow dealerships access to some parts of the SAM’s programming, and there is no WIS bulletin for this procedure, perhaps if someone with current access to WIS could confirm this?
    In summary, if I had a manual gearbox I would have no hesitation in moving forward with a DIY installation as I have done in the past but without the modified driving profile and additional control of the oil cooling fan relay, I would be far more confident with the OEM Trailer Recognition Control Unit in my particular case.
     
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  16. Jim2

    Jim2 Senior Member

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    For my need's the 7 core by-pass is sufficient, but if you are thinking about towing a Caravan, the for sure you will need the MB Kit. It is much more complicated. See the post by Tony Dyson above, he has all the facts ( or nearly all the facts, as he says himself.....getting info from MB re. towing, is even more painfull than getting teeth extracted without the benefit of a anaestetic) But he has a lot of the details and I think that you can figure it out from there. One of the main thing's to take into consideration if your car is a manual or an automatic...if it's auto, then it's more complicated.
     
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  17. bembo449

    bembo449 Senior Member

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    ive recently fitted one to a ford focus , kit came with everything required and didn't even need to drill any holes !
     
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  18. Auto Addict

    Auto Addict Senior Member

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    As I understand it, if a towbar is fitted to a Euro 5 or Euro 6 car, unless programmed correctly, the car electrics will drain the caravan battery whilst towing, as the main car battery only charges to around 70%, to allow for regenerative braking.

    I could be wrong of cause.
     
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  19. Tony Dyson

    Tony Dyson Senior Member

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    More complications, but not unique to MB, a quick looksee on EBay and Amazon produces Trailer Recognition Control Units for most of the Major Brands.
     
  20. Auto Addict

    Auto Addict Senior Member

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    Agreed, after long discussions with Witter about this problem, I had my towbar fitted by one of their approved dealerships.
     

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