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Thread: 12v Wire through bulkhead or rear fuse box?

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  1. 12v Wire through bulkhead or rear fuse box? 
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    Really struggling with my 2016 Alhambra. Have just fit a towbar and now moved on to the electrics. I have a universal 13 pin kit which has been easy enough to fit but I have come unstuck with the voltage switching relay. The kit requires a permanent 12v feed and I have spent over an hour trying to find a way through the bulkhead to connect directly to the battery but have had no luck as yet. Does anyone know of the simplest path through? I did notice that the fusebox in the boot area is supplied by a fairly heavy gauge cable, would it be safe to take a live from this if there is no way through the bulkhead (bearing in mind that it will need to run the caravan fridge and battery charger during towing) or is the current to this box moitored?
     
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  2. Re: 12v Wire through bulkhead or rear fuse box? 
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    What current does it need to be rated at? Is there general consensus on where to take the supply to

    General rule for wiring is that as long as you don't change the upstream fuse near battery to a higher rating nothing will melt, catch fire, etc. If you connect towbar feed to boot and upstream fuse blows you'll need to uprate existing supply cable and fuse (hence can use same firewall path). Or run a parallel one like you're trying to now...

    EDIT: Original loom probably has a supply for tow-bar addition of course. I'll see if I can spot that in wiring diagrams...
     
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  3. Re: 12v Wire through bulkhead or rear fuse box? 
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    So looking at the wiring diagram for a standard socket at Trailer connectors in Europe - Wikipedia it is pin 9 that you're looking at, which is a 2.5mm cable so probably good for 20A-25A depending on routing through open air, etc.

    On the Sharan wiring diagram that runs into a 2.5mm that goes to position 5 on fuse bank SD with a 25A fuse. SD fuse holder is of left of boot as you look in from outside.

    Does that seem to match the Alhambra?

    EDITED: Ooops, was looking at wrong vehicle. Edited.
    Last edited by VAG-Abound; 21-02-2020 at 03:36 PM.
     
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  4. Re: 12v Wire through bulkhead or rear fuse box? 
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    Quote Originally Posted by VAG-Abound View Post
    What current does it need to be rated at? Is there general consensus on where to take the supply to

    General rule for wiring is that as long as you don't change the upstream fuse near battery to a higher rating nothing will melt, catch fire, etc. If you connect towbar feed to boot and upstream fuse blows you'll need to uprate existing supply cable and fuse (hence can use same firewall path). Or run a parallel one like you're trying to now...

    EDIT: Original loom probably has a supply for tow-bar addition of course. I'll see if I can spot that in wiring diagrams...
    Recommended 20A fuse, the cable to the fusebox in the boot is more than big enough but just don't know if theres anything between that and the battery that would get upset by tapping into it
     
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  5. Re: 12v Wire through bulkhead or rear fuse box? 
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    Quote Originally Posted by rob7 View Post
    Recommended 20A fuse, the cable to the fusebox in the boot is more than big enough but just don't know if theres anything between that and the battery that would get upset by tapping into it
    Based again on the Sharan platform again I don't think there is. It looks like a direct feed from 80A fuse SA4 at the engine to the SD fuse holder.

    The VAG trailer wiring then takes SD5 fused at 25A via a relay (switched by the trailer control module) to the trailer pin 10 switched supply, which is what I think you're trying to set up. So battery to trailer current is unmonitored.

    SD6 is fused at 30A to feed the trailer control module which in turn supplies the pin 9 unswitched supply, so it may be monitoring current usage on that line. This module connects to the vehicle control via the CAN bus, and so can in principle disengage either pin 9 (directly) or pin 10 (via relay control) if battery condition gets too low while parked or driving.

    In short, if Alhambra has same set up taking a feed from SD5 will give you what you want.
     
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