C5 multiplexed Towbar lights

This is the Forum for all your Citroen Technical Questions, Problems or Advice.

Moderator: RichardW

Post Reply
The Hatter
Posts: 4
Joined: 05 May 2014, 21:32
Location:
My Cars:

C5 multiplexed Towbar lights

Post by The Hatter »

I have a 2004 Mk1 C5 estate with what looks like an original Citroen towbar and the complicated multiplexed wiring.

I have a problem with the indicators when I hitch up a trailer; the trailer indicators don't work, the car rear indicators don't work, and the car front indicators (and instrument panel lights) flash rapidly. The car is fine with the trailer lights disconnected, and the brake/side lights on the trailer work as normal.

I've double checked the trailer by hooking it up to my Xantia and that works, plus I've got a light board for a cycle carrier - that brings out the same symptom on the C5 as the trailer does; so the problem is definately with the car.

Does anyone know what the problem could be with the towbar wiring? I'm hoping there's a fuse or something hidden somewhere...
BX
Posts: 476
Joined: 30 Oct 2009, 00:53
Location: Ireland
My Cars:
x 17

Re: C5 multiplexed Towbar lights

Post by BX »

On the mk1 forget about complicated or multiplexed. There is multiplexing between the switch ( comm unit) and the BSI. In the BSI the switching is done using old fashioned relays. There is no current sensing or other complication. The only exception being the indicators which must have current sensing and must indicate a fault condition to the driver.
I would guess that your problem is most likely caused by an earthling fault on the wiring of the towbar socket. This would cause power to be returned through the indicators and/ or the brakelights when the trailer lights are connected. The earth is on pin 3 of the towbar 7 pin socket and is usually white. Check also for damage to the cable going to the towbar socket. Check also by following this cable if a relay box is fitted that might be water damaged.
I have read your post again and now am probably as confused as you are however a visual examination of the wiring might reveal the cause of the problem.
The Hatter
Posts: 4
Joined: 05 May 2014, 21:32
Location:
My Cars:

Re: C5 multiplexed Towbar lights

Post by The Hatter »

Good to hear the lights are not multiplexed and it's likely to be a good old fashioned earthing fault. I've checked all the wiring that I can see for damage and it looks OK, so I'll start pulling out trim and delving deeper. If all else fails I'll wire in a new towbar socket with a 'smart' relay.
BX
Posts: 476
Joined: 30 Oct 2009, 00:53
Location: Ireland
My Cars:
x 17

Re: C5 multiplexed Towbar lights

Post by BX »

If you are fitting a box use a universal bypass relay box similar to this one http://www.ebay.ie/itm/7-WAY-SMART-BYPA ... m20e5cab10
There are also smart boxes to connect to the can bus. I would avoid these on any make or model. The universal box is invisible to the current sensing circuitry on any car. It is better than wiring the socket direct to the car on any vehicle as short circuits on the trailer wiring cannot affect the operation of the lights on the towing vehicle. There is no programming and most of all they are cheap.
User avatar
daviemck2006
(Donor 2020)
Posts: 4989
Joined: 04 Dec 2010, 18:45
Location: Macduff
My Cars: Far too many good ones to remember. Many BL, Vauxhall, Ford, Renault, Citroen, Peugeot, and now VAG
x 487

Re: C5 multiplexed Towbar lights

Post by daviemck2006 »

My C5, 2007 vintage has only a single 7 pin socket, there it an electronic box of tricks in the spare wheel well that my last one did not have, and it had the twin sockets for the caravan. I do not know if this box is part of the towbar wiring or no, having only had the car a week. I do have a trailer board that is very old, I dont know if it works properly either! I'll have to try them and see what happens.
Skoda Karoq 1.6tdi 2018
Citroen relay camper 2012
In the family
Seat Leon 1.5tsi tourer 2019 daughter 1
C1 vtr+ 2010 daughter 2
JohnD
(Donor 2022)
Posts: 2632
Joined: 14 Mar 2001, 23:41
Location: Epsom, Surrey
My Cars: 2010 Citroen C5-X7 tourer
1998 Citroen Saxo 1.5D
2018 Citroen C4-B7
1998 Peugeot 306. 1.9D
2011 Citroen C1
x 72
Contact:

Re: C5 multiplexed Towbar lights

Post by JohnD »

If the box you've found has seven screw connectors at one end with a variety of coloured wires at the other, then what you have is a Smart7 bypass relay. That powers the lights on the trailer without over loading the car's wiring.
2010 C5 X7 VTR+Nav 160
1998 1.5 Saxo
1998 Pug 306 1.9D
2018 C4 B7 VTR+
2011 Citroen C1
Post Reply