Towing electrics multiplex bypass relay install

[charentejohn]

This relates to an earlier thread about the standard wiring system from the manufacturer http://www.frenchcarforum.co.uk/forum/v ... =3&t=50537" onclick="window.open(this.href);return false; but is now a question about these bypass relays.

I looked at the scary wiring diagrams for the standard system, lots of plugs into sockets under the dash, and removing panels etc. So the bypass relay option seems much simpler, looks like a standard install apart from the wires going into a relay box instead of direct to the socket plus a lead to the battery pos terminal.

Any warnings or general adivice on fitting these, seems straightforward but messing with tricky electrics is a worry.
My car is a C3 2006 and is multiplexed so needs a standard system and reprogramming or one of these bypass relays.
I only do a few journeys to the tip or to buy a few bags of cement a couple of times a year so not serious towing. I can understand the full standard system would be useful as when integrated it would warn of trailer lighting problems but not a big deal for my small domestic usage.

[BX]

The bypass relay set works because the relay coils draw very little current and so do not affect bulb failure detection circuits on vehicles equipped with them. Multiplexed wiring without current monitoring has basically nothing to do with the need for these relays. However they are a very good idea for a number of reasons on every vehicle. The two main reasons are: 1 there is no extra load imposed on the existing wiring and switching (be it the old fashioned switch or the relay in the BM34 on PSA cars). 2. As these relay sets have their own fused feed, a fault on the trailer lights should not affect the vehicle lights. For example a short on the trailer may blow the fuse leaving the trailer without tail lights. However the lights on the towing vehicle continue to work.
Wiring is simple. A good earth. One or two fused feeds (depends on relay set used) direct from the battery or from a heavy duty distribution point. Connections to the various light circuits on the towing vehicle. Connections to the towing socket.

[charentejohn]

Thanks, good explanation of why it is good to fit one. I did notice the writeups by places selling them say that if you just fit a normal lighting setup, no relay just old fashioned system, that a multiplexed car could be ok but there is a risk it will not and faults will keep being flagged up.

I like the idea of the fuse being specific for the trailer too as that is always a concern when you tow old trailers or seldom used ones. That is that they will have a fault that blows a fuse in the car and you don't have a spare. As these are a relay the fault is on the trailer side leaving the car electrics separate. Like you said it would make sense to use these on any car, not expensive as I can get one for 54 Euros.
I will order my towbar and relay today then.