I'm having more trials and tribulations trying to sort the 7S wiring socket on my VSE.

I bought a split charge kit from Land Rover (STC8875) which is listed as fitting all P38's from 1995 to 2002.

The instructions when looking at our GEMS car make perfect sense, you remove a bung from the pink plug under the fuse box and slide in a new contact and insert a 5A fuse into slot 35 in the fuse box.

When looking at the fusebox for my 2002 P38, there is already a wire in the pink plug and Fuse 35 is a 10A fuse for the fan.

After a brief chat with Marty last night, there is another way to wire it in but due to the kit it won't charge the battery in the caravan so I'd like to avoid this route if at all possible.

I've also found a 4 pin connector behind the rear left tail light. If you remove the plastic panel beside the subwoofer it's hanging there. From what I can see the 4 pins are:

Red/Green - Reversing Light
Pink/Red - Permanent Live
White/Orange - Switched Live
Black - Ground

Anyway, my question... does anybody here have a THOR car with the white 12S towing socket fitted? Does it just run through a bung in the floor into the connector behind the tail light or does it still have split charge and a volt sensing relay at the front of the car too? I really cannot find much information about the later cars online at all.

David.

I would assume there's two ways of doing it. Either just connect to the dangling connector meaning you probably won't have anything connected to Pin 2 of the white socket so it won't charge a caravan battery, or connect the extra bit under the bonnet so it has the ability to charge a caravan battery while the engine is running. The GEMS diagram shows a couple of spare ways on the pink plug, one of which is fed with permanent power from fuse 35, however, as you say, the Thor diagram is different. However, there is a permanent live output shown as spare on pin 3 of C0570 (the grey plug) fed from Fuse 29, although the face view of that connector shows it to have a Red/Black wire in it which the diagram shows as going nowhere.....

The split charge kit from LR won't charge the caravan battery either, that's part of the problem.

In the 12S socket from LR it's designed for pre 1998 wiring, from post 1998 wiring the blue cable cannot be used and if it is used it can damage the car and caravan electrics unfortunately. I'm wiring mine into a single 13 pin socket but the blue cable that was used to charge batteries on the pre 1998 systems can't be used which is the main reason I want to figure out how it was done on the post 99MY cars.

David.

Right, I've been doing some more looking into this tonight and checking things with the multi meter.

As per the picture below, from left to right, the wires are:

Black - Ground
Green/Brown - Reversing Lights
Pink/Orange - Switched 12v Feed from Fuse 8 on the BECM
White/Orange - Permanent 12v Feed from Fuse 14 on the BECM

So, in a nutshell, that plug in the rear has all of the feeds that I need, I've checked them with the meter tonight. The reverse lights are fed 12v when the car is put into R, the switched 12v feed becomes live with the ignition on and goes off with it and the permanent live is permanently live.

At this point I was thinking that I've cracked my problem and that all I need to do is make up a loom from that plug to the socket and all is sorted. HOWEVER, the switched 12v feed is fused at 30A on the BECM and appears to be a 1.5mm cable. The permanent live is fused at 20A and is a much bigger cable.

I'm a bit worried about putting a possible 30A load on such a small cable.

David.

I've never owned a car with twin electrics so this is all a bit theoretical for me but by looking at the diagram here https://www.westerntowing.co.uk/blog/caravan-12s-wiring-diagram/ it seems to me that you have everything you need except for a battery charge feed on the blue wire (pin 2). I'm not sure why a fridge would be powered from an ignition switched supply either (surely it will start to defrost when you aren't driving?) but that must be a caravanner's thing. You've got the other feeds in the connector you've found behind the LH rear light. I've used that connector to provide power to a reversing camera and a pair of fag lighter power sockets in the boot with a changeover switch so they can be either ignition switched or permanently live. As you say, the wires, and fusing, are too feeble to use to charge an additional battery in the caravan unless it's only a little one (and from my limited knowledge of power in a caravan I know that usually they are hoofing great big leisure batteries).

For the blue wire to charge a battery, the way I would do it is use a voltage sensing relay (or just a blocking diode if you can live with the 0.8V drop) and run a separate cable, preferably 6.0mm, directly from that to pin 2 on the socket. That way you can have the caravan battery in parallel with the vehicle battery when the engine is running but disconnected when it isn't. My car still has the split charge relay that plod installed for an auxiliary battery in the boot and that takes it's trigger from the sense terminal on the alternator so operates (or did) when the alternator is charging.

I was actually planning to use that connector (the permanent live one) to feed a 12V plug in the boot, exactly to use with the fridge. Mine draws max 6-7A, honestly I believe it should be fine.

The switched cable feed I was planning to running to besides the LPG tank and run an old Viair compressor is sitting in my garage doing nothing ... that draw is the one I am having concerns with (I think the comp draws 21 or 30 cannot find exact info).

Our caravan fridge can draw 10A when it's running so it's on a switched live to protect the tow car battery. As long as you don't open a fridge door, it'll stay cold during a power cut for a long time so it's not an issue when you stop the car for an hour or two during travels.

The permanent live is for the caravan water pump and lights so that you can use them when you stop if needs be.

One of the main problems is the wiring loom for the P38 from LR is for caravan electrics Pre 98, they changed in 98 but still used the 12S socket. I'm trying to wire mine into a 13 pin socket when they changed again. Instead of having Permanent 12v, Switched 12v and Battery Charging, they changed the electrics in the caravan to include a habitation relay which divides the Permanent 12v between the fridge and the battery charging so you could in theory end up with one hell of a current draw on the switched 12v now.

Anyway, I sat down with a pen and paper last night and drew a new circuit diagram and with a bit of modification to the LR kit I've bought I now know how I can get it all wired in :)

David.

I fitted a 13 pin connector to our red one - and ran two new feeds for it, rather than using that connector - because the wiring is a bit weedy.

I actually fed them both through accessory switched relays, so there is no permanent 12v to the caravan when the vehicle is switched off. That way regardless of what was plugged into it (some caravans have their own VSRs to only switch on when they detect alternator output), if it was left connected and parked up - there was no chance of the starting battery being drained.

I've finished this tonight :)

I used the Land Rover kit for the Volt Sensing Relay, double relay and the wiring so I've not used the wires in the boot for anything other then reverse as I just didn't trust them to be heavy enough.

The kit from LR has a permanent live to the rear and 2 switched ones. I've removed one of the switched ones and have ran an additional earth into the boot as well so it's wired as if it had genuine 13 pin electrics now which is ideal as it'll charge the caravan battery, operate the reverse light on the trailer which it didn't do before, and I want to fit a charging system to the trailer as well because I can then charge the 12v winch battery too now the car is configured properly.

One VSE down, one to go! At least the next one will be simple now I know how to do it.

Thanks to everyone for the pointers :)

David.

leolito wrote:

I was actually planning to use that connector (the permanent live one) to feed a 12V plug in the boot, exactly to use with the fridge. Mine draws max 6-7A, honestly I believe it should be fine.

The switched cable feed I was planning to running to besides the LPG tank and run an old Viair compressor is sitting in my garage doing nothing ... that draw is the one I am having concerns with (I think the comp draws 21 or 30 cannot find exact info).

I wouldn't run a compressor off of that...

Whilst the switched feed comes off a relay in the BECM and ultimately F8 (Which is 30A) it also is the switched feed to the radio, HEVAC, cig lighter etc. I'd definitely not want to hang 20-30A off it being pulled through the BECM.

I'd possibly use it as a trigger wire for a relay to drive the compressor off a newly run (probably 6mm2) fused feed directly from the battery terminal.

dhallworth wrote:

Our caravan fridge can draw 10A when it's running so it's on a switched live to protect the tow car battery. As long as you don't open a fridge door, it'll stay cold during a power cut for a long time so it's not an issue when you stop the car for an hour or two during travels.

The permanent live is for the caravan water pump and lights so that you can use them when you stop if needs be.

One of the main problems is the wiring loom for the P38 from LR is for caravan electrics Pre 98, they changed in 98 but still used the 12S socket. I'm trying to wire mine into a 13 pin socket when they changed again. Instead of having Permanent 12v, Switched 12v and Battery Charging, they changed the electrics in the caravan to include a habitation relay which divides the Permanent 12v between the fridge and the battery charging so you could in theory end up with one hell of a current draw on the switched 12v now.

Anyway, I sat down with a pen and paper last night and drew a new circuit diagram and with a bit of modification to the LR kit I've bought I now know how I can get it all wired in :)

David.

How does the habitation relay work? I thought the separate 7S pins for fridge and caravan battery charging were supposed to have separate feeds from the vehicle battery.. Ideally you want it set up so that the fridge will run from the car and the caravan battery will be charged at the same time?

Car
battery
feed 1 > socket > fridge
feed 2 > socket > caravan battery

If there's only one feed or the feeds are connected in parallel too close to the caravan (too far to the right in the above excuse for a diagram), then due to the caravan battery being so much closer to the fridge (in wire / resistance / voltage drop terms) than the vehicle battery it's likely that the caravan battery will lose some of it's charge (rather than be charged) due to the fridge drawing most of it's power from the caravan battery? To charge the caravan battery there'll have to be 13+V at the caravan battery even with the load of the fridge but with a single feed there might only be 12v unless the feed is rather thick for little voltage drop under load?

With a separate feed the only difference thickness of feeds could make is how quickly the caravan battery gets charged and how cold the fridge could get, both independently..
Does the habitation relay thingy have some sort of voltage sensing relay that only connects the caravan battery to charge above a certain voltage at the caravan, some sort of DCDC converter, just some sort of double throw relay which continually switches between charging the battery and powering the fridge, etc?

The original 12S socket had:

Switched 12v
Permanent 12v
Fridge

The 13 pin electrics socket only has:

Switched 12v
Permanent 12v

The habitation relay in the caravan is supposed to divide the permanent 12v between the fridge and the charger apparently.

Now I’ve got my socket wired I’ve tested it on the caravan this evening and the electrics on the caravan behave exactly the same as they do behind the L322 with factory 13 pin electrics so I seem to have got it wired properly thankfully.

David.

I've had caravans on and off since 1990, in recent years I've never bothered wiring up a 7S socket (intended and even fitted the double socket bracket on the towbar but never got around to it). My most recent caravan is still an old one 2004 model... so (being post '98) you reckon it will have the divider? I wonder how such divider works... I think I'd prefer the older spec wiring with full power to both the fridge and battery charging..

Thought car / towbar makers fitted: Older vehicles before easy production of voltage sensing relays - 2 wires from the car, one from the battery, one from the ignition switch. Newer vehicles since easy production of voltage sensing relays - single thick wire from the battery to near the 7S socket, voltage sensing relay disconnects the fridge when car battery voltage is below around 12.7V (car side of the socket).

Maybe check voltage at the caravan battery when the fridge is running to make sure it seems to be charging or on a longer drive the caravan battery might be nearly flattened? Not so much a problem these days when everyone stays on a site with 240v electric and caravans have built in battery chargers but in the old days if intention was to do some touring around, no 240v on sites so no battery charger so relying on the car charging the battery up... the old way might have been better, might even be better these days?

Someone had tampered with wiring on one of the caravans I owned, the fridge and battery charging wires were joined in the caravan, this did lead to the caravan battery being almost flattened on longer trips despite my having run a hefty wire from the vehicle battery to the 7S socket until I sorted it.

Sorry for the partial hijack - thanks Marty for the heads' up! I was going to add 12V and USB charger to the rear cig lighter also, so I think this is enough to fill up F8 in the BECM's capacity. I'll run the compressor separately with its own cable and fuse. We'll shoot you an email later on today or tonight!

Hi Simon,

Charging was one of the things I checked. The L322 has factory 13 pin electrics on it so I checked the behaviour in the caravan whilst connected to that. All of the systems in the caravan work when the car is switched off, when the car is running only the 12v lights, water pump and fridge work internally and the voltage at the battery terminals is 13.5v.

I then connected the P38 to it so I could check everything worked on that after I'd added my 13 pin socket. All of the behaviours in the caravan are mirrored, the fridge is working and the battery terminals are reading 13.8v so both appear to be charging the battery as well.

I've used the official LR Split Charge Kit to wire mine in but instead of running 2 12v cables from the front to the rear I've just left one of them disconnected as it's not needed. The 13 pin socket has an extra earth cable in it so I've ran an extra earth out of the socket as well but other then that it's been a fairly easy task.

David.