Towbar electrics

[CJH]

I've decided to do something about my dodgy towbar electrics. I've got intermittent, weak supplies to several of the lights, and I think I've found out why! Yuck!



So I've decided to go the whole hog and fit a towing relay instead - not really needed on this wiring from a simpler age, but it'll mean my towing electrics will get a nice fresh battery feed. So I've bought all the bits to repair the scotchlock damage and to tap the relay signal wires into new latched blade connectors at the light connectors.

But I noticed last night that all the wires I need go through this round connector (apart from the reversing lights which I think are the black+blue wires next to it).



So the Obsessive/Compulsive in me is getting the better of me. It would be far neater if there was a gizmo that could sit between this round plug and socket to give me my relay signals. The battery feed to the relay will come from this box too, and the relay will sit nearby, so it would all be nice and neat.

So does anybody know if this plug and socket can be bought new? If not, I guess I could contact some breakers to look for that section of the loom. Is it likely to be common to any other cars?

[CJH]

I've found a breaker who's got this plug and socket chopped out of an old loom, so with a bit of luck I'll be making up my own break-out adapter tomorrow.

It feels good to scratch that OCD itch!

[syncropatrick]

Mine has a similar arrangement which I only discovered when starting an engine swap and clearing out the engine bay. Might explain why lights dimmer on one side.
On the job list to re-do properly, so your post is most helpful.

[CJH]

I get about 11.5V at my tail light wires, even though my starter battery is at 12.7V, and that's without a load. Old wires, high resistance connections etc. Not a good starting point for towbar electrics I reckon. My plug and socket didn't arrive today, but I've done the rest of the installation.

Here's the relay. Input triggers on the right, outputs on the left. The yellow supply comes from a fused connection to the live post inside the black box on the bulkhead. When I trigger one of the inputs, I now get full battery voltage at the 12N plug. The relay also incorporates an indicator tell-tale buzzer - it only goes off with the indicators if the trailer indicator bulb is working.


Here's the right hand lights plug - as well as the other scotchlocs there's evidence of an earlier installation.


Here's what's left after removing the scothlocs and snipping off the old blade connectors.


A close-up of the damage. I put sections of self-adhesive shrink tube over the cuts.


Then I re-sleeved them, trimmed the corroded ends of the wires and put new latched blade connectors on. I've also added a fog light feed over to the left hand cluster.


Finally I put the rubber boot back on and popped them back in the housing.


The left hand side was easier. Only a couple of scothlocs this side.


All new blade connectors, and some heatshrink to cover the damage. The new fog light feed goes in the empty slot. My ULO bulb plates already have the fog light terminal, and someone had even put a bulb in the slot - all it needed was the feed wire from the other side.

[Robsey]

Nice to see some proper wiring rectification work being done.

I have a huge dislike for scotch-loks.
(Only to be used as a temporary, get you home solution, when you have no other choice)

I am told on other forums that I am a bit OCD with wiring,
preferring to solder my connectors on, not just relying on the crimp
and then heat shrink tubing all exposed wire ends.

In my opinion, your final outcome looks a 1000 times tidier and safer than your starting point.
Very impressive.

[CJH]

Thanks Robsey. I agree - for a permanent installation they're not ideal. There was a section of the red+grey tail light wire that nicely shows that the connection they make isn't always as good as it should be. Here's a close-up.



Evidently it's got very hot around this scotchloc connection. This is a 0.5mm2 wire, which should be good for 40-50W at 12V, so it's fine for the 10W tail light. Even if it had been incorrectly wired to feed both trailer tail lights, it should still have been big enough. Evidently though the scotchloc connection provided more resistance than the rest of the wire, and so heated up locally.

[Robsey]

Sheesh - proves our point.

I'm not sure if you have anything else in this area, but at worst it could have been a fire waiting to happen.
You'll sleep more soundly tonight knowing that you have sorted this in the nick of time.

People may say I am over-reacting but I had a fire in my first car twenty five years ago due to dodgy wiring that over-heated.
Hence my over-zealous streak now.
(I have to admit it was my dodgy wiring - hangs head in shame).
Lessons were learnt.

Note also that occasionally scotch-lock blades can cut through one or two strands of the core, further reducing the current carrying capacity.

And as a point of interest, I always smear a thin coating of petroleum jelly / vaseline on the spade / lucar connectors to hopefully keep out moisture and so slow down corrosion.

[CJH]

I'm adding a 12S socket. I'd appreciate a sense check, since wiring to a camper that already has leisure batteries could be achieved slightly differently to a normal towcar.

Apparently there are two conventions (pre- and post-1998), but there is a way to wire the 12S to be compatible with both, namely:

Pin 1: Reversing lights (this will come from my towing relay above)
Pin 2: Switched feed (16A) (pre-1998 battery charging)
Pin 3: Earth (35A) for pin 4 (but also for pins 2 and 6 pre-1998)
Pin 4: Permanent Live (16A) (for caravan lights, and also for battery charging post-1998 via a changeover relay in the caravan triggered by the fridge feed)
Pin 5: Spare/sensing device (I haven't found out what this 'sensing device' is yet)
Pin 6: Switched feed (16A) (fridge)
Pin 7: Earth (16A) (for fridge post 1998)

So there seem to be 3 live feeds: one (Pin 4) is a permanent live, and two (Pins 2 and 6) are switched in the same way that the camper fridge and split charge relays work, by sensing a voltage from the alternator.

So far, so good. But my question is about the source for those three live feeds. In a conventional towcar this would be the starter battery, obviously. The only risk is that the caravan interior lights/pumps etc could drain the starter battery eventually.

In my camper, I've been careful that nothing can drain my starter battery while I'm camped up, and I want to preserve this. So I think I can make the camper's leisure circuit the source for my 12S live feeds. When the alternator is running the leisure and starter batteries are linked anyway, and when I'm parked up the 'permanent' live would use my leisure batteries. Does that make sense? Have I missed a set of circumstances that would cause a problem? The only one I can think of is if I remove both my leisure batteries from the van and then go camping. Seems unlikely.

[marlinowner]

Possible problem might be that if wired as you suggest then when ignition is on pins 2 and 4 will be linked as both will be connected to the caravan battery, so the starter and leisure batteries will be connected regardless of alternator output. Not a big problem but would mean the voltage sensing relay was bypassed when caravan hooked up and ignition on.

[CJH]

I may not have used the right terminology. By 'switched' I meant 'powered from a relay that is triggered by the alternator output'. the plan was to use a relay like this (30 to the leisure battery, 87 to pin 2, 87b to pin 6):



Pin 4 would be fed directly from the camper's leisure battery. This would keep pins 2 and 4 apart, and only connect them when the alternator is running.

[bigherb]

.

Apparently there are two conventions (pre- and post-1998), but there is a way to wire the 12S to be compatible with both, namely:

Yes you are right, wire up as post 98 but power to terminal 2 and 6 must be through separate relays. Also gives you an option for extra earths for pre 98 wiring.
Pin 5 is for vehicles with stability control all it does is connect to earth when a trailer is present and tells the stability ecu that a trailer is attached and alters the program.

Nice to see someone do the wiring properly after seeing some of the efforts with pre insulated terminals on here.

[CJH]

Yes you are right, wire up as post 98 but power to terminal 2 and 6 must be through separate relays.

Thanks Bigherb. Is there a reason it has to be two separate relays, or would a 40A version of the single relay with two separate outputs do just as well?

[bigherb]

I didn't look properly at your relay that type will do, not the type where the twin outputs are connected internally in the relay. As there must be no connection between terminals 2 and 6 when the relay is off or the fridge will powered all the time from the caravan leisure battery when the engine is off.

[CJH]

OK, thankyou. Yes, that's a tip I remember reading on this forum a while back.

[CJH]

Pin 1: Reversing lights (this will come from my towing relay above)
Pin 2: Switched feed (16A) (pre-1998 battery charging)
Pin 3: Earth (35A) for pin 4 (but also for pins 2 and 6 pre-1998)
Pin 4: Permanent Live (16A) (for cara van lights, and also for battery charging post-1998 via a changeover relay in the cara van triggered by the fridge feed)
Pin 5: Spare/sensing device (I haven't found out what this 'sensing device' is yet)
Pin 6: Switched feed (16A) (fridge)
Pin 7: Earth (16A) (for fridge post 1998)

Also gives you an option for extra earths for pre 98 wiring.
Pin 5 is for vehicles with stability control all it does is connect to earth when a trailer is present and tells the stability ecu that a trailer is attached and alters the program.

Taking on board your comment about extra earths, I'm looking to optimise the pin assignments to give maximum capacity, without affecting compatibility, so:

Pins 3 and 7: Both are earth pins, with the wires taken back a common vehicle earth. Am I right that I can bridge these two pins in the socket, so that any circuits needing an earth can use all of the available earth wires?

Pin 5: My van doesn't have stability control, so I'd like to use the brown wire as a spare core and double up one of the other wires. The 7-core 'heavy duty' 12S cable I have appears to be thinwall, with 6x21A cores and 1x29A. The 29A will be the Pin 3 earth, so the question is, which of the three live feeds could do with a double run of 21A. My recently acquired caravan has (I think - not had a chance to study it yet) the older style 12S wiring, so charging down pin 2 and internal lights down pin 4. It has a fridge that needs a 16A fuse. So I think in my case it would make sense to double up pin 2 for battery charging, i.e. to use both the blue and brown cores to pin 2 and leave pin 5 vacant.

Does that all make sense?