I have tested the multicore cables supplying the tow socket. The bar and electrics were fitted by a Witter agency. EARTH, INDICATORS, REAR FOG & BRAKE all supplied from the correct cable to correct pins. HOWEVER, Black & Brown (7 & 5) are dead while Brake (red 6) is also supplying lights (side, full, etc).

 

I can hear the audible indicator sound from the left hand side of the trunk and so assume that an approved box is fitted in the there. My question is would any of you know if the wiring cockup is likely to be from that box to the towing socket or between the box and its supply?

 

Thanking you in advance. 

have you actually connected a trailer board to check if working while connected

Delboy, thanks and yes. This is what alerted me to my problem so I have now pullled the wires from the socket and did my tests so I know that 5 & 7 (sidelights) are dead while 6 (brake) is fed by brake & side! The rest are fine & correct. Now I have to dismantle the back left interior to see what's in there. But then I'm at a loss because I wouldn't know if the box (assuming there is one) is incorrectly powered or if correctly powered (wired up) the output side is incorrectly wired or if the box is buggered. I have never been down this route, 123 series Mercs being so simple (primitive).

 

If you have VCDS, you can do a fault scan for the control module: if it's an OE one or OE compatible it should be accessible at address 69 and will report any fault codes detected. You could also check fuses 8, 9, 43, 44, 45 as those are the ones used by the OE electrics. The OE type kit is all plug and play connectors that plug into the connectors provided in the vehicle. Control of the pins on the socket is via CANBUS messages sent to the towing controller which switches the appropriate pin(s) locally. If any pins are dead it would be due to a failed power supply to them within the module, which could be a blown fuse or a dead controller.

Chimaera, Thank you. Regrettably I don't have VCDS and rather fear that if I did I would hit the wrong tit and bugger everything up. However, armed with this information I can visit a local trailer mob with it and  let them know to do this testing first. Thanks again. Good weekend to you.

Unlikely to get such a fault from the OEM controller, I reckon its a bypass module incorrectly wired.

 

I think the guy has only taken a feed from one rear light and shunted it to the input for the other, this saves running an extra cable from one side to the other, he has further mistaken the core for the brake light filament with the one for the rear lights, very easy to do.

 

Check the wiring connections in the back of the socket before digging any deeper, they cable is not sealed and has no strain relief.

JR, Thanks. I understand this and might even be able to check this myself. I do live in fear that we might be looking at missing cables from the front to rear so will do this in stages checking the inputs and outputs from the box gubbins at the back. I have done a deal over the telephone with a local outfit such that I do the dismantling of the interior and they'll do the rest if I can't find the source of the problem. 

It would help if you could give a better description than "box gubbins" as from that I cannot tell if you are dealing with an OE coded set up or a bypass relay, any advice will depend on knowing which it is.

 

How are the cables joined to it, ar they a manufactured loom with multi-plug connectors or single cables with screw terminal terminations?

 

If it is the latter bypass type relay it does not need any cables running to the front although that is the prescribed method.

1 hour ago, J.R. said:

It would help if you could give a better description than "box gubbins" as from that I cannot tell if you are dealing with an OE coded set up or a bypass relay, any advice will depend on knowing which it is.

 

How are the cables joined to it, ar they a manufactured loom with multi-plug connectors or single cables with screw terminal terminations?

 

If it is the latter bypass type relay it does not need any cables running to the front although that is the prescribed method.

+1 on this, and pictures would really help too.

1 hour ago, chimaera said:

+1 on this, and pictures would really help too.

Chimaera I am happy to answer these questions with photographs when I get the back end open. As it is the weather is rubbish and I'm still suffering from Covid and feel quite knackered.

On 04/02/2022 at 19:48, J.R. said:

I think the guy has only taken a feed from one rear light and shunted it to the input for the other, this saves running an extra cable from one side to the other, he has further mistaken the core for the brake light filament with the one for the rear lights, very easy to do.

 

This car has a common tail & brake light feed. J.R. If as you suspect a 7 way universal bypass relay has been fitted then neither of the side light circuits will have been connected to the relay as all is needed is the common brake feed connecting to the bypass relay.

Edited 6 February, 20224 yr by Derbyshirebod

On 03/02/2022 at 19:58, Name said:

Now I have to dismantle the back left interior to see what's in there. But then I'm at a loss because I wouldn't know if the box (assuming there is one) is incorrectly powered or if correctly powered (wired up) the output side is incorrectly wired or if the box is buggered. I have never been down this route, 123 series Mercs being so simple (primitive).

 

It's a PITA having to take the rear panels out! I know as I've recently fitted a tow-bar to my 2012 Superb Estate.

You will have to remove the plastic panel that goes across the boot floor below the tailgate and the piece of trim that goes up the LH boot opening. Remove the LH aluminium rail in the boot. The upper D pillar trim. You might get away with leaving the C pillar trim in place. Remove the outer LH side seat panel it needs pushing into the car and the lugs are easily broken if you don't push in the correct direction! Then you can remove the panel that runs across the bottom of the rear side window. You need to remove the 3 boot luggage hooks to reveal the 3 torx headed screws hidden behind them and all the rest of the screws holding the panel before finally being able to lift the boot side panel up and out!  

 

Unless you have an OEM controller you won't be able to use VCDS to connect to the tow-bar electrics.

 

I'd agree with J.R that you've probably got a universal bypass relay fitted. I say that because you've said you can hear an audible warning from the boot. I might be wrong but if you had an OEM system then I assume you'd probably only have a dashboard warning ⚠️  light for the trailer indicators and no audible warning? As a lot of people find an audible warning some what irritating. 

 

So if it is a 7 way universal bypass relay you have in your car then I'd be inclined to think that it is probably faulty ? As it only requires a single feed from the common brake & side light feed and you say the trailer brake lights are working correctly then the relay is probably be wired correctly. The relay knows when to supply current to the side lights  or brake lights or both. The fog lights on the car also have a common fog & tail light feed and as you say the trailer fog light works then it seems the tow-bar electrics have been correctly connected.

 

The Brown & Black wires feeding into the universal bypass relay shouldn't be connected.

http://www.thomson-caravans.co.uk/advice/caravanadvice/pdf/teb7as.pdf

 

I've made several assumptions so please excuse me if the above is a load of B******s! 😉

 

Edited 6 February, 20224 yr by Derbyshirebod

The link to the wiring instructions in my last post are slightly different to the instructions that came with the same bypass relay I got with my tow-bar,  which I've scanned and attached.

My instructions tell you not to connect the tail light circuits to the bypass relay where you have a common brake & tail light feed which you do on the MK2 Superb. So make sure that the black & brown side light wires going into to the relay haven't been connected. Assuming you have this kind of relay?

Edited 6 February, 20224 yr by Derbyshirebod

If you get to the point of trying to find what wire does what this might help a little? Photo of the N/S light cluster of my 2012 Superb Estate. The wiring colours change at the plug where the light harness connects to the main loom I can't remember what the main loom wire colours where now . The two black wires you can see going into the car in the photo merge into one at the plug and join into 3 earth wires which are all joined at one point and split back into 3. The brake and fog light bulbs also double up as side light bulbs so there are 4 side light bulbs in each rear light cluster which caused me some confusion! The red & green wires are the common feed wires for the brake & tail lights and fog & tail lights and the wires that should be connected by pass relay.

Gentlemen all and, now, Derbyshire bod, thank you. This is most detailed and informative. I feel better I feel better this morning and was going to attack the rear in the full knowledge what it requires having watched a video and made notes. However the weather is awful so I suspect this will have to be delayed. Once open I'll take photographs make my observations refer to these excellent notes and see where we go from there. To learn of a common brake and side light feed makes me feel better about the probability of the wiring from the source (front) being correct at least.

20 minutes ago, Name said:

 I feel better I feel better this morning and was going to attack the rear in the full knowledge what it requires having watched a video and made notes. However the weather is awful so I suspect this will have to be delayed.

You're not the only one at the mercy of the weather 😊 as I've no longer got a garage to work in! 😊 I've got repairs to carry out to the door wiring, I have all the stuff I need but trying to solder in a gale and intermittent rain isn't my idea of fun! lol 

 

Not that it's going to apply to you as it's been done already, the drivers side indicator wire will need to have a spur taken off it and taken over to the passenger side of the car to connect to the bypass relay. All the other wires that have to be connected to the relay run along behind the lower boot panel. The bypass relay needs a + feed which I took off the auxiliary socket on the left hand side of the boot instead of running a + feed all the way from the front of the car. I've been looking but doesn't look like I took any photos of where I put my bypass relay? I've placed it directly under the upper boot panel under the grill to make sure it's easily audible. 

 

I found a photo of the harness to loom connection and you can see the wiring colours have changed. You'll be able to see looking at the connection which colours wires should be connected to the bypass relay without the need to start opening up the loom to look.

 

I had a lot of pain and grief with my rear lights after installing my tow-bar. Though in the end it turned out to be nothing to do with the tow-bar wiring! DEL80Y and J.R. were most helpful 😊 Others have helped me so I think it's only right to share what I now know if it can make someone else's life easier!

10 hours ago, Derbyshirebod said:

As it only requires a single feed from the common brake & side light feed and you say the trailer brake lights are working correctly then the relay is probably be wired correctly. The relay knows when to supply current to the side lights  or brake lights or both.

 

Can you explain that to me please.

 

Do you mean that the vehicle uses a common filament for both rear light and brake light and the voltage on the common feed wire is reduced for the rear lights giving a lower wattage output and increases to 12v when the brakes are applied?

 

If so then the bypass relay must be monitoring and reacting to that voltage.

 

The one I used I'm pretty sure did not have that feature, my car uses seperate filaments (in my view all vehicles should) if someone were to try to wire mine to a vehicle with the common feed they would initially not e able to find enough cables to connect to, they probably scratched their head and did the best they would with the results that the OP has been left with. That is par for the course for many so called professional installs in my cynical world and why I always DIY.

 

My relay is sold as Canbus compatible which is a meaningless nonsense term anyway but it does have very high impedance inputs which take only milliamps from the circuits they are monitoring so does not throw up bulb monitoring system warnings for overcurrent.

On 04/02/2022 at 19:48, J.R. said:

Unlikely to get such a fault from the OEM controller, I reckon its a bypass module incorrectly wired.

 

I think the guy has only taken a feed from one rear light and shunted it to the input for the other, this saves running an extra cable from one side to the other, he has further mistaken the core for the brake light filament with the one for the rear lights, very easy to do.

 

Check the wiring connections in the back of the socket before digging any deeper, they cable is not sealed and has no strain relief.

 

2 hours ago, J.R. said:

 

Can you explain that to me please.

 

Do you mean that the vehicle uses a common filament for both rear light and brake light and the voltage on the common feed wire is reduced for the rear lights giving a lower wattage output and increases to 12v when the brakes are applied?

 

If so then the bypass relay must be monitoring and reacting to that voltage.

 

The one I used I'm pretty sure did not have that feature, my car uses seperate filaments (in my view all vehicles should) if someone were to try to wire mine to a vehicle with the common feed they would initially not e able to find enough cables to connect to, they probably scratched their head and did the best they would with the results that the OP has been left with. That is par for the course for many so called professional installs in my cynical world and why I always DIY.

 

My relay is sold as Canbus compatible which is a meaningless nonsense term anyway but it does have very high impedance inputs which take only milliamps from the circuits they are monitoring so does not throw up bulb monitoring system warnings for overcurrent.

 

Yes J.R that's exactly it! As you said the brake light and fog lamp bulbs are both single filament and work as side lights with a reduced voltage. When the brakes are applied the relay must detect the higher voltage and in turn supplies a voltage to just the trailer brake light circuit. When the relay detects the lower voltage for the side lights, the relay the supplies a voltage to just the trailer side light circuit.

 

In the 1st photo you can see I've got the side lights on, though the 501 12v 5w  4th side light bulb isn't in view. Initially I found it confusing and that's why I wondered had I wired up the tow bar wiring incorrectly? Though I hadn't! 

 

In the 2nd photo are the side light bulbs I replaced when I was having all the problems with the dash board bulb warning messages which resolved the problem. The larger bulbs are the common side light/brake light bulbs. I didn't replace the fog light bulbs.

 

The TEB7AS Universal Bypass Relay seems to be fairly popular and I assume they can all be wired the way I have mine wired? Though the link I 1st posted didn't say anything about only connecting the brake light common wire to the relay. Hence why I scanned and uploaded the instructions I received with my TEB7AS bypass relay.

 

 

Edited 6 February, 20224 yr by Derbyshirebod

Right, done it and thanks again Derbyshire bod along with others. 'Done it' meaning that I pulled the back of the car to bits snapping off various plastic lugs in the process. I can only assume that the bloke that took his apart and made the video had done it so many times that all his plastic detente bits and clips were broken off and missing anyway. So to report upon my findings the relay box was tucked away and having removed it I have found that everything from the drawings and pictures supplied comply. The BLACK BROWN and GREY are not used and capped while the RED, YELLOW, GREEN,  and BLUE are all connected correctly into the harness having followed Derbyshire bods rear light cluster picture and comparing it with my own, and wiring. The green, as was suggested, has been connected to an individual line leading to the driver side. I was disappointed that mine is also connected to the auxiliary 12 V socket in the back minus a fuse so I think when I put it all back together I will install the relay inside that that covered storage space along with a fuse holder so that in the event it all ****s the bed again I can change it without dismantling the car. So the verdict is that the relay is malfunctioning? Correct? I don't see how it works anyway with this sidelight and brake feed being common.

 

EDITING: If it's a question of the relay recognising resistance have I unnecessarily taken the car apart because the cause is my fitting LEDs in these rear clusters? 

 

FYI

harness black & blue goes to Blue (reversing light),

grey & white to Green (fog)

brown to 2 x Black (common),

purple/grey (my eyes) & black to Brown (lights),

black & red to Red (brake & lights)

black & white to Yellow (indicator)

Edited 6 February, 20224 yr by Name

Get rid of those nasty scotchlocks terrible things that should never be used !! 

41 minutes ago, Name said:

Yes the exact same bypass relay as I have. Though if you've not got the grey wire from the bypass relay connected to the cars blue wire to the reverse bulb then the trailer reverse lights won't work. Not that many trailers have reversing lights. The way the relay works is quite simple really. With only the common red wire from the brake  circuit when the car side lights are on then it detects the reduced voltage and only supplies a current to the trailer board side light circuit. When it detects the higher voltage when the brakes are applied it only supplies a current to the trailer board brake light circuit. When the relay detects the higher combined voltage from both brake and side light circuits the relay then supplies a current to both the trailer board brake and side light circuits. This is a much better system than the old way of splicing into the cars wiring loom and there isn't a voltage drop as the bypass relay has it's own voltage supply so you don't get dim or flickering bulbs on the trailer board lights.

 

I don't see why you think taking a feed from the back of the auxiliary socket is a problem? That's what J.R and I have both done. The auxiliary socket circuit is protected by it's own fuse in the car fuse box.

 

As long as you've used LED ERROR FREE CANBUS BULBs in your rear light clusters I think you should be okay? Otherwise I think you would have been getting bulb warnings on the dashboard? As in the photo. But yes maybe a good idea to put the original bulbs back in and see if the trailer board side light do work correctly before splashing out on a new relay ! Which are about £30.

 

 BTW I did say disassembling the boot cover panels was a PITA ! 😅😅

 

 

 

 

Edited 6 February, 20224 yr by Derbyshirebod

27 minutes ago, nige8021 said:

Get rid of those nasty scotchlocks terrible things that should never be used !! 

There's not really that many alternatives other than to cut and solder. It's not like these particular connections are subject to the elements. Agreed they are cheap and nasty.