Trailer coupling wiring

Submitted: Wednesday, Mar 11, 2009 at 07:40
ThreadID: 125458 Views:4062 Replies:3 FollowUps:0
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The 7 wires for the trailer coupling includes the 12V positive power wire which on our van side when unplugged is live. I have only recently given some real thought to this wire and realise I don't know or understand the purpose of this wire which is obviously connected to the van DC power supply. Five other wires (and earth return) are dependent on vehicle power supply.

Can anyone enlighten me on the purpose and linkage of the heavy 7th power wire.

Greatfull for clarification on a relatively basic issue.

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Reply By: The Hob - Wednesday, Mar 11, 2009 at 08:57

Wednesday, Mar 11, 2009 at 08:57
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Maybe this will help.

AnswerID: 577251

Reply By: Flipp'n Lorry - Wednesday, Mar 11, 2009 at 17:50

Wednesday, Mar 11, 2009 at 17:50
This is basically a "trickle" feed to the battery system in the van, to provide some additional charging topup. If the batteries in van are low, you can then count on a little boost to them when towing between camps.

This can be set up to operate in 3 different ways:

1) Always on - even when car engine is disabled. This would be wired direct to the car batteries and bypasses the ignition system. This has the advantage that for short stops, you are still going to get a small feed to van which is useful if batteries are especially low in overcast conditions and/or if fridge is running. Disadvantage is that if you are staying overnight with van connected to car, then you really need to pull the harness plug out to avoid risk of being unable to start car the next morning. And continually pulling harness plug will eventually lead to loose connection problems as the connector terminals wear or get compressed.

2) On, only when car ignition is on. A relay is used from the car ignition to initialise the circuit to the van.

3) Smart wiring: with this system, the feed is always on (whether car is in use or not), but a control unit disables the feed if car batteries fall to a prescribed minimum voltage. With this system, you can leave the harness connected, have the advantage of a small top up from car to van batteries, but do not need to worry about flattening the car batteries.

In all cases, there will be an inline fuse somewhere (usually in vicinity of the car batteries) to protect the integrity of system in case of a short.

Now I am going to predict that this thread will quickly degenerate into a debate as to the advantage/disadvantages of an Anderson plug setup, but here are my comments for what they are worth.

I have never had an Anderson plug. for following reasons:
a) The additional expense, and just something else to go wrong
b) Never had flat batteries in caravan anyway
c) I am inclined to view that Anderson charging does represent a potential risk to overcharging the van batteries and reducing their service life.

Many have a contrary view, so you can bet this will be a long and interesting thread as a result of your simple question.

AnswerID: 577252

Reply By: Bobrovin - Thursday, Mar 12, 2009 at 03:51

Thursday, Mar 12, 2009 at 03:51
Hi Swagy
I agree with everything Phil has said above including the anderson plug comments and would add that unless you have had a second battery added and specifically asked for the wiring to the 7 pin plug I doubt that you have any power going to it. When a tow bar and plug are fitted by dealers and the like they do not automatically add in this power wire.
On my van I have pulled the wire out of the back of the plug and taped it up so I do not have a live plug laying around when disconnected from the car.
AnswerID: 577253

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