Indicators Tell-Tale

[DrZhivago]

I am rewiring my towbar and van from two 7-pin connectors (12N + 12S) to a 13 pin one.

I am puzzled by the requirement for a tell-tale for the caravan indicators. From the Caravan Club website https://www.caravanclub.co.uk/media/1022790/towbar-wiring-mo.pdf :-

"It’s a legal requirement that the towcar ‘shall be equipped with an operational tell-tale for front and rear indicators (including any rear indicator on the rearmost of any trailers drawn by the vehicle)’ (Road Vehicle Lighting Regulations, 1989.) ...... This tell-tale can be audible or visual."

That is vague to say the least. Where is the working of the lights to be detected? I consider these are the options for looking at this circuit, in order of increasing trustworthiness :-

1) Detecting the voltage at the tow-car socket
2) Detecting the voltage at the trailer plug
3) Detecting the voltage at the trailer indicator fittings
4) Measuring the current going to the tow-car socket (or anywhere downsteam of it)
5) Detecting light in the fitting itself by a photocell

Of these, (1) is almost useless because it would not even detect a detached connection lead. (2) and (3) would not detect a blown bulb. Measuring the current (4) could be fooled by an earth leak somewhere downstream, and I cannot believe that the complexity of (5) is used much or at all by caravan makers. In any case there is only one spare core in the 13-pin socket* (one fewer than in the older 12N+12S !) and that seems to be meant for detecting if a trailer is present, not for sensing the indicators.

I design and make my own electronics and could do any of the above, and it looks like (4), but I am curious as to what most set-ups are. (4) can be done within a CANBUS set-up, but as CANBUS hardly existed in 1989, what did older cars do to meet this law? Do I get the impression that this law is not always followed? The CC website says : "Note that this tell-tale function is sometimes neglected when the tow bar wiring is fitted" Don't ask me what arrangement my car had previously

My own car has CANBUS, but as I am not prepared to pay the hundreds of pounds for the maker's towbar CANBUS wiring/module/software kit, nor to rip out half the interior trim which my workshop manual states is necessary to install it; I'm going my own CANBUS-independent way.

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* Many suppliers of cable skimp with only 12 cores anyway, such as these :-

http://www.towsure.com/12-core-towbar-cable-per-metre?nosto=productpage-nosto-2
http://www.thetoolbox.uk.com/epages/es137744.sf/en_GB/?ObjectPath=/Shops/es137744/Products/MP305
At least the latter tells you what the sectional copper area is, even if some cores have less than half the Caravan Club recommendation. But this (more expensive) 13-core seems to meet the spec :

https://www.westerntowing.co.uk/acatalog/13-core-euro-cable-1elc9.html

 

[thosoneill]

Try P F Jones Ltd for a dedicated wiring kit - I'm sure you'll be surprised at the low cost

 

[Raywood]

Basically the law requires you to be warned if the indicators are not working. You can either have a visual warning or an audible one but you must have one or the other.
I have paid the extra for the makers wiring but we have a new car and were worried about the warranty. If yours is out of this then possibly different attitude is justified. It may depend on what make and model of car you have but there was no problem with ripping out the interior trim.
Before we changed the car we had a convertor which even for a good branded make cost only £28 and worked perfectly. Frankly I did not think it worthwhile changing the plugs on the car given the costs involved.
I would check with a few firms as to the cost if you still want to go ahead but non specific systems are a risk.

 

[chrisn7]

I can understand not using the car 'manufacturers' towbar as the fitting of these is usually farmed out anyway. Most cars already have a 'trailer' socket pre- installed ready to plug in to the required trailer module. A short length of additional harness is then required from the module to the 13 pin plug. To save cash you may well find a module on ebay - these two may well cost far less than you think and then everything will work as it should including the indicators/telltale. You will have to remove some boot trim but certainly not 'half the interior'

 

[Woodentop]

If you have a manufacturers towbar fitted or a fully compatible o.e.m. it will come fitted with a 13pin socket and the appropriate connect to plug into the car. When the ECU is told about the towbar it will monitor the current drawn and if a bulb fails the flash repeater on the dash will not light (even though you can probably hear clicking) and you may even get a general warning light.

Note that if you use a manufacturer's towbar and your car is of European origin it will likely come without battery charging and fridge power wiring - you must tell the installer (preferably when booking the install) that you require that to be installed as well.

 

[VicMallows]

Traditional trailer wiring wiring on cars with conventional filament bulbs detected the current to the trailer indicators and sounded a bleep in time with the indicator flash.
Wiring simply spliced into the rear light wiring in the boot with IDC parallel crimps. Simple, reliable, and worked (still works in my case) well.

I find the loud (even though sounder is in the boot) bleeping very reassuring that the 'van indicators are actually working when changing lane on a motorway. I would have less confidence in noticing some visual indicator on the dashboard during a critical maneuver.

 

[Hutch]

DrZ, if your two 7 Pin plugs are working fine, then just rewiring into a 13 pin socket is easy just follow the wiring diagrams and make sure the extra earth wires are connected, no need to rewire the whole car.

 

[DrZhivago]

Thanks for the replies.

.. To save cash you may well find a module on ebay - these two may well cost far less than you think and then everything will work as it should including the indicators/telltale.

Well I already have 95% of the job done (as I said electronics is a hobby anyway) but have left a space in my box of tricks (which is mostly full of relays) for a small extra circuit board for the tell-tale. I'm just interested to know how the tell-tale is generally implemented.

Traditional trailer wiring wiring on cars with conventional filament bulbs detected the current to the trailer indicators and sounded a bleep in time with the indicator flash.

Thinking about it, this has got to be the way to do it. I shall monitor the indicator current by passing it through a silicon diode, detecting the voltage arising across the diode, and amplifying that to drive a tell-tale buzzer up front.

if you use a manufacturer's towbar and your car is of European origin it will likely come without battery charging and fridge power wiring - you must tell the installer .. that you require that to be installed as well.

That would be me

Try P F Jones Ltd for a dedicated wiring kit

Interesting, and I'd go for it if I was not DiY-ing. I would have like to seen more technical info on how it worked and how it is installed than their website gives.

Incidentally, this website gives a lot of useful technical info :-

https://caravanchronicles.com/guides/understanding-caravan-and-tow-car-electrics/
The guy obviously knows what he is talking about, although I think he is too dismissive of split charge relays IMHO. He thinks a simple ignition-controlled relay is good enough, but I would rather that the auxiliary loads were kept switched out until the engine is actually running.