Gets a bit mor technical here.

daddypine's - Task: Wiring & Electrics
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This again needs quite a lot of forward planning because a lot depends on what you are going to want the vehicle to do and, if you are on a tight budget, this also comes into play.
We actually went for the sergeant EC155 with the EC51 control module.The only real difference between the EC50 and the EC51 is that the later has onboard fresh and waste water tank sensors which we thought would eradicate any guesswork and grovelling around.
The unit basically takes care of all your 240v/hook up department, your 12v system electrics including all fusing and your leisure battery charging. The package comes with a wiring harness which is pretty easy to connect up to and fairly easy to understand once you sit and get your head round it. We bought an electrical tool kit with crimping pliers to fit ends and spades on anything you connect to it.
You could buy all this lot individually and no doubt for less money but there comes a certain piece of mind with this, plus a professional looking job with everything in one place.
If you do go for this unit, one thing that is worth watching out for is the fridge part of the equation is a bit of a mine field, talked about on page after page on the forums and nobody really seams to come up with a final or even the same conclusion.
There are a couple of cables detailed as "input supply for fridge" and "output to fridge". The input supply for fridge (disregard the description) is basically, as I understand it, a signal wire that tells the EC155 unit that the engine is running and basically to switch just about everything off, as it's illegal,I believe, in europe to travel on the road with things switched on.It also, as I understand it, switches the EC155 to charge the leisure batteries mode from the vehicles alternator. After coming across much confusion about the fridge, is it also switched off? isn't it?, I rang sergeant who basically hinted that I don't use the "output to fridge" wire (as it switches off when you travel) but to run a separate wire from the leisure batteries, through a switched,fused spur, then to your fridge (this being a compressor fridge). I did this and also bought a separate red warning lamp and fitted that just after the fuse to remind me when leaving the vehicle that the fridge was still connected. This should be fine under normal use as when you are on hook up to the mains, or when the engine is running, the leisure batteries get charged anyway. It's just when you park her up for a week or month or two, get into the habit of turning it off at the fused switch or it will run the batteries completely flat.
Now, going back to the "Input supply for fridge" cable, this has to have an "engine run" signal feed to it, which in simple terms, it needs a feed from the alternator, through a relay. Well, scared me a bit too, but on reading up about relays etc, it seems that the relay basically switches a feed on, to this "input supply for fridge" cable
when the alternator is running and switches it off when it's not. Again after more reading on't tinternet,I found that it seemed most people struggled to find that "engine run" point at the alternator or fuse box, plus, as modern vehicle electrics are on a CANBUS system, it's very easy to knacker things up and you can't just tap in anywhere. So, I found you could use a thing called a voltage sensing relay. What this does is sense when the voltage to the battery goes up slightly (when the alternator kicks in (engine run)) and saves a bucketload of time and risky tampering around trying to find the signal. I used the SMARTCOM 30 amp model. Some of the forums said this wasn't up to the job and they had had problems with them, well maybe they had if it was being used with the fridge output cable to a three way fridge, but all I wanted was an engine run signal and the thing seems to work fine. Please don't take any of this stuff as gospel cos I'm no authority on these things, I'm just putting down what worked for me.

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