I am trying to wire a towbar on a 2004 Mercedes E
If you want to wire a towbar, I'd say, you can't wire a towbar, as a towbar is meant to be used to pull trailers, right? But, had you said I am trying to fit and wire a car trailer socket for my 2004 Mercedes E Class so I can plug a trailer plug for the trailer's taillights, etc. I would have understood what your needs are. Now, to fit and wire a socket to your Mercedes you need an testing tool costing less than UD$10.00. This tool it like a small screwdriver with a tip almost like a neddle and is used to punch into live sheaths of wires to see which wire is conducting (has power). This small tool is usually of plastic contruction and has a removable cap at the other end and has a length of wire with a small aligator clip at the other end of the wire. This aligator clip is clamped on a negative ( - ) surface, such as any exposed bolts or metal part of the engine or body of your car. The cap of this tool can also be unscrewed
to replace the small 12V light if it ever blows (fuses out). The overall reason of this tool is to be used to very carefully (and I mean it) pierce into any sheathed wires the check if any current is present when a particular action is being performed. The tool can be purchased at any reputable speed shop. Now, to begin with wire finding and assembly of the socket you need to first locate the braided (wrapped in a plastic tape) cable at the back part of the car, usually as close to the rear body (boot). After finding this cable (it has all the wires that control the tail lights, brake lights and reversing lights wrapped into 1 bundle), you now have to prepare a similar length of braided or prepacked cable that has wires inside expecially designed and coloured for use with most cars. In this care, if your Mercedes has different color wires, do not worry about it because you can mark them with a texta or special sticky take and write on them the different colors so that you wont become confused. Note that most speed shops sell rolls of cable specially for wiring car trailers. Just purchase a short length, depending on your requirements, unless you are to wire the whole trailer, I will discuss the objective of fitting and wiring the car with a trailer socket. Next, you may have to drill a hole where you want the cable to exit near the towbar (closest to the towball) and then you may have to decide where to screw the socket on the car, either on the lower part of its body or on a already welded plate especially fitted for the purpose of fitting a socket. This plate is usually closest to the towball. Now, lets start using the special tool to detect the current and noting down which wire is for brakes, tail light, reverse, etc. Go into the boot and locate the bundled cable. If it has a sheath, carefully cut it open and expose the 5 or 6 wires, depending on what country you are, each is different. Once you cut the sheath about 4 inches or more and have exposed the wires, try and separate them a little apart so you can work on them one by one. Now, here you need a helper to turn the parkers (lights) on, press the brake pedal to activate the brake lights, turn the ignition on but do not start the engine and put the gear selector into reverse (automatic or not), etc. Next, remove 2 or so inches of sheath from the purchased cable, spread the 5 or 6 wires apart and... if you have a wire stripper, so much the better, otherwise use a knife, being carefully not to cut or strip the copper strands in each wire. If you do, you'll have to start again. So be careful. if strands are not broken or snapped, you'll get full power to the trailer lights. If they're stripped or cut, the wires may get hot and probably cook themselves out at the point where the strands are missing. Remember, all strands must be twisted before use. Now put this aside for a moment and, with a helper to give you a hand for a few minutes, you first ask him/her to turn the lights on, then punch each exposed wires from the sheathed cable you found in the boot of your car and, with the poined part of this tool note which one has the current or power flowing that turn the car's tail lights on. Note the colour of this wire. You have a choice that you either strip this cable right away and then connect the twisted exposed copper wire, apply a solder with a soldering iron or make a good twist around the exposed wire in the boot that has the current flowing, mark this as (eg, Orange - tail lights) tail lights and then tape it with black automotive plastic tape, or you have the other choice that you puch all of the wires, locate their currents and which color is which for the following lights. I'd stick with the first option if I was you. So, next, ask your helper to depress the brake pedal and then again punch the sharp tip of the tool into the sheath of any of the other remaining wires (in the car boot that is) untill you locate the wire that has current to the brake lights. Once located (it is usually red colored), carefully strip it about half an inch and then bind the red wire from the purchased cable and either you solder it or you make sure it is a good wrap-around wirework and then wrap around this one as well with automotive insulating tape.
Repeat the process with finding the reversing lights wire in the boot by punching on the sheathed wire, carefully strip it, note the color and then bind around it a stripped and twisted wire from the purchased cable that has the same color, solder it if you have a solder. if not make sure the connection are wrapped around and tight, then tape the connection. After you've done all the wires (whatever wire is leftin the purchased cable, you either leave it unconnected or pull it out of the sheath and discard. Remember that there is usually a black wire running along into the mercedes sheathed cable. This wire should be considered as an Earth, or negative ( - ) wire. It should be also used because sometimes there is poor conduct between the car and trailer. Having this black, or negative wire connected as well will make sure you wont have a failure in connection with the car's electricity to that of your trailer. Usually, after turning the parking lights on or applying the brakes to activate the tail lights and this black wire doesn't have any current (the light on the tool does not come on after you have been punching every wire in the boot cable), it demonstrates that the black wire is negative. Now, after you have linked all the wires to that of the car wires and have insulated each of them, bend the purchased cable sideways with the car cable and retape it for a final insulation and to make sure the purchased cable stays bonded with the car cable. Pass the purchased cable through a small hole that is large enough to pass the cable but the right size to fit a grommet to seal out any dust. Now, after you have decided where you want to fit the socket, you now have to also decide if you want a round 5 pin socket or a flat 7 pin socket. Again, depending on your needs and use, I would recommend a 5 pin socket. Okay, having written down the colors of the wires used for parking lights/headlights, tail lights, reverse lights, left right indicators signals, etc., you now have to wire the socket before ataching it yo the body of your car or the plate of the towbar. Please take your time studying the wiring connection instructions that came with the socket packaging or box. Once wired, have your helper help you with turning the lights on, pressing the brake petal, turn the ignition on but don't start the engine and put the gearshift into reverse, turn the left or right signals. You can test all this by either clipping the aligator clip to an exposed chassis or clamp the clip directly behind the socket and on the black negative wire ( - ) then touch the socket pins with the pointed tip of the tool. If the tool's light comes on as each pin is touched and it is the correct sequence as per wiring instructions that came with the package and everything works accordingly, you can now install the socket to the car body or towbar plate. Now, you also have to make sure that the plug on the trailer is also wired accordingly to the pins of the socket on the car by orientating and checking if the pin that is for, say braking, is also the braking pin on the trailer plug. If this is too much for you, an automotive electrician is a better option because all you need do is make mistake and you'll cook your car's wiring.
Thank you. Michael. Automotive mechanic since 1973.
on Oct 05, 2009