Components Parts
Towing, Trailer Hitch, Trailer & Electrical Wiring.
The 'Witter' tow bar assembly was already fitted on this car when I purchased it.
If you are contemplating having a tow bar fitted then I strongly recommend that you have it done by specialist company this will ensure that it is fitted correctly . That it does not affect the electronics and
meets all the EC spec approvals mentioned on mypage 32, W168 section of his site, see link below.(use the back arrow to return to this page)As many of you will be aware the W169 has the can-bus system of electronics, your specialist company will use a wiring kit (Loom) which will be compatible with the system and will fit electronic components and inline fuses to ensure it works safely with your car.
If you live in the Sussex area (UK) Sussex Towing brackets Ltd will be pleased to quote you for the towing facilities on you Vehicle, be it for towing a small trailer or a large caravan. Tel 01903 216215 Due the electronic problems on the car I purchased which came complete with a Witter removable Tow bar and electrics I have found this company very helpful, they resolved my problem and I would recommend them for your installation.
I appreciate that many owners will have had their W169 'A' Class for many years and as a result be familiar with all the various fixtures and fittings on the car .
However history tells us that when the car is eventually sold it is quite common for the handbooks and various guides to be retained or even lost with the result that the new owner does not have the benefit of the information contained within the guides which can even include the owners hand book.
In the case of the 'Witter' removable tow bar on this car the information is just a thin booklet which can easily get separated from the main reading matter especially as it is needed a couple of times before you get the hang of fitting and removal procedures.
I will therefore include details of the working of this unit in the w169 'A' Class section.
As pre mentioned the tow ball is detachable from the tow bar assembly and can be stowed in with the spare wheel.
The unit is detached /unlocked from its housing by key, I also keep this with the bar. Where ever you keep it, keep it save because although you can fit the bar without the key, you cannot remove it without the key, so lost keys will result in the tow bar and ball being left in the housing.
If your buying a used car with a detachable tow bar and it's fitted when you see the car, enquire about the location of the keys! and make sure it detaches if that is what you want.
The bar complete with ball coupling are slid into the housing on the tow bar assembly which is securely fitted to the car.
It will be heard to click when fully housed.
When the bar is fully housed the two half round holes in the bar housing are occupied but the round pegs on the tow bar. Highlighted in Red.
Another safety feature of this unit is the green indicator will clearly show when the bar is correctly housed.
The hole in the bottom of the towing assembly can be used for the security hook/ trailer brake
Having undertaken the task. (I myself use a small trailer to save putting rubbish in the car,) the bar can be removed from its housing.)
This is done using the key:-
Insert the key into the lock situated on the side of the unit turn the key fully.
At the same time press the lock housing in towards the bar and turn clockwise, this will allow you to withdraw the tow bar from its housing.
Tow bar Electrics
I myself had a bit of a shock when I first went to use my trailer,
Instead of finding a 7 pin connection, I found a 13 pin totally new to me. I had to make enquiries as to how this could be adapted without having to change the 7 pin trailer plug fitted. although I enquired of a Mercedes outlet I did not manage to purchase the adaptor required. Halfords Stores came to the rescue and had an adaptor on the shelf at under £10.00, so that soon sorted the problem out.
Having got the adaptor, which shows that only seven pins are wired the next thing was fitting. Note the projecting lug which needs to be aligned when fitting
The adaptor is designed to fitted and removed when required.
There are lugs which prevent the adaptor being fitted incorrectly, simply offer it to the original plug having aligned the lug slot and turn clockwise to secure. Anti clockwise to remove.
I should point out at this point that when I first went to use my small trailer I have lighting problems, Non of the lights on the off side of the trailer would work I checked bulbs and contacts and although some contacts were worn and corroded they did work when tested via a spare 12 volt battery.
I then searched the lower boot area of the car for inline fuses without any success and checked as far as I could that the wires were sound and that nothing had come loose or detached. I found nothing apart from a very professional installation as far as the electronics were concerned. the wired loom had obviously been supplied as part of the towing system.
It is a pity that Witter Towbars had not informed the owner in their small handbook that inline fuses were fitted and stowed in the battery compartment rather the hide them away, in this case down the far side of the battery.
Had I purchased the unit and had it fitted no doubt the fitter would have tipped me off where the fuses were but as it was fitted on the car before purchase I did not have that opportunity. However a visit to Sussex Towing Brackets Ltd Graham, soon discovered the problem, tucked away and out of site in the battery compartment of the car were two inline fuse carriers, two 15amp fuses in one and one 15amp in the other, just one fuse was blown which was what was causing the lighting problem, fuse replaced and every thing worked OK.
Although this system is linked into the can-bus electronics on the car I got no indication of the fault on diagnostics tool
Let me explain the problem I did experience:-
1.There is every chance that the electrics on the trailer which were in poor condition caused the fuse to blow.
2.This then resulted in a message on the instrument cluster on the car saying;- 'TURN SINGLE REAR RIGHT' and a bulb with an !, indicating that there was a problem with that bulb, it could be a bulb contact, wiring problem or broken/burnt out element, although when I checked it was working normally along with the front and mirror strip bulbs.
3. However when the off/side indicator was operated the warning lamp in the instrument cluster operated at twice the normal speed. Which is an indication that a bulb has blown (gone defective)on the off/side of the car, the trailer electrics were not connected to the car when this occurred.
4. Initially it was thought the 'trailer module' had blown, this is an complex electrical device situated in the rear off/side locker, which when unplugged nullified the trailer electrics and the car's indicator warning lamp worked normally, so I now knew that it was a problem connected to the towing electronics and not the car itself. A new module which was a very expensive item did not cure the fault, however replacing the 15amp mini spade fuse did.
So if you have a towing bar and electronics on your Vehicle and suffer this fault, check and replace any burnt out fuses in the inline fuse carrier's before doing anything further. The fuse carrier with the single fuse is a standard spade fuse as it fitted in the main fuse layput. The fuse carrier with the twin fuses are a mini spade which I understand are now commonly used in Japanese made cars so should be readily available. Doing this check first when problems with trailer lighting arise will save you money and a lot of agro.
I will be contacting Witter Towbars to highlight the deficiencies in the manual, after all there is no point fitting a removable tow bar and following all instructions if your trailer electrics do not work, and you as the owner are unaware of the inline fuses and their location!
However having considered the situation and the possibilities that it was the trailer electronics that caused the problem I have now bought a new lighting board £36.00 from Halfords, that worked out cheaper than buying the replacement rear lights with combined indicators, high visibility light and cable to re-wire the trailer, it will also allow me to remove the lighting unit when the trailer is not in use, thereby prolonging the life of the lighting unit.
Do bear in mind also that the tow bar and electronics on your car are now included in the MOT, if they are not working correctly or your tow bar is corroded or badly fitted you car should not pass the MOT, so getting the job done by a professional is in my opinion money well spent. Although this was included into the UK MOT it has now been dropped but it is still an offence to use a trailer without the lighting required by the road traffic act /vehicle building regulations which includes brake,and indicator lights .
The weather cap on the main unit needs to be lifted and supported while fitting the adaptor, once in position the cover rest on the adaptor.
I recommend you remove this after use, the main plug weather cap can then be lowered and the protective plastic cover replaced.
Remember also to fit the tow bar housing blank/cover.
This will leave the rear of your W169 looking neat and tidy and the covers will prevent the ingress of water and road grim.
I have not greased the ball on my unit nor have I lubricated the tow bar housing, if you decide to lubricate yours then keep it in a container/bag to prevent contaminating the spare wheel and boot area.
However a visit to Sussex Towing Brackets Ltd Graham, soon discovered the problem, tucked away and out of site in the battery compartment of the car were two inline fuse carriers, two 15amp fuses in one and one 15amp in the other, just one fuse was blown which was what was causing the lighting problem, fuse replaced and every thing worked OK.
So if you have a towing bar and electronics on your Vehicle and suffer this fault, check and replace any burnt out fuses in the inline fuse carrier's before doing anything further. 
