30 Dec 2008
Reaction score
United Kingdom
Hi All. Does anybody know anything about towbar wiring? Owing to the recent writing-off of Mrs. Avocet's Nissan X-Trail, we no longer have a tow car in the family, so I'm about to try and stick on on my beloved Alfa 164. This is a 1989 ca, so "CAN" isn't really a problem, but it does have a brake light bulb failure warning light feature (however those worked in the late 1980s)!

What do I need to wire it up? I have a 7 pin socket and plenty of wire, but other than that, I'm a bit of an electrical numpty!
Sponsored Links
I prefer to use the correct 7 core cable, pass that into the car boot area via a grommet and then use Scotchlok connectors (the only time I would use these is inside the vehicle) to connect up.....I use a separate earth connection via a ring terminal direct onto the bodywork. most connections can be made courtesy of the offside rear light, but the cable should be long enough to reach the N/S indicator and number plate lamps. You'll need a connection into the rear fog lamp as well.
Is the existing wiring conventional or ribbon cable?
I use a 12v tester sharpened to a point so I can pierce the cable insulation for testing, rather than cutting in in to it.
John :)
Thanks John. It's all very conventional round wire to all the rear lights (separate wires at that). None of this new-fangled CAN stuff! The only thing I don't know how to handle is the brake light bulb failure warning system. I've heard that they can go mad when you plug in the extra load of a trailer board? Apparently there's some sort of relay or electronic gizmo that you have to fit in order to overcome that?
I think I'd connect things up as normal and give it a go first......for sure, you can use the current going to the stop lamp bulb to energise a relay coil (which would take less current than a bulb) but then you'd need a live to go across the switching terminals of the relay - a bit of a faff!
So long as all the stop lamps come on when the brake pedal is depressed I wouldn't worry too much.
More of an issue, I find, is to wire in an indicator repeater lamp on the dash - despite instructions, sometimes they work, sometimes they don't.
Maybe a call to Towsure or similar would help - they will have come across this before.
John :)
Sponsored Links
It's a few years since I last did one but it had a bleeper which I wired in the boot (as the required visible/audible indicator warning) instead of messing about with a light on the dashboard.
Some cars have a space for a trailer turn signal light already built into the binnacle for that very purpose, your handbook will probably tell you. Some people go to a lot of trouble wiring up a special light when all you have to do is pop in the five pin heavy duty relay and put a bulb into the empty hole in the dash.
Last edited:
There was a unit called a Lucas 2TU there are some I see on ebay which was designed to get around the BMC cars of the time that had duel level brake lights designed not to dazzle at night, it had relays for indicators and brake lights to remove load from car rear lights, and also included a fuse.

Today I would used the Durite 0-727-14 relay or similar, there is a version with built in fuse if required.

The 7 pin socket has changed what pin 2 does, it was interior light in caravan unless using vacuum brakes when it was vac warning, then caravans started to have electric operated reversing hitch so it became reversing light, then the illegal rear guard fog lamps became a legal requirement so then pin 2 became rear guard fog warning.

This pushed the reversing light, brake warning, and power to a second plug, now both combined to a 13 pin plug.

There was also special flasher relays, two types, the two pin type you fitted a second of higher amp rating can't off hand remember how wired, but the new relay would not flash unless correct load, the other type had three pins and was replaced with a 4 pin model so there were two warning lamps one for caravan and one for car.

There were other options, which copied the larger bosch type, at that time having a buzzer to show when caravan indicators were working was not permitted, it had to be visible, I don't know if that is still the case? There was also a warning light relay that measured to current and if enough flashed the warning lamp, as an add on where original flasher worked OK with extra load.

Split charging never really worked very well, however some modern caravans have electric anti snake devices and need power, also in some cases the fridge also works from car. The fridge and split charging are not permitted to work from same supply, but want both to only switch on with ignition, so in some caravans there are relays in the caravan so the non ignition supply is switched on when the ignition supply is made live.

Today it is getting more complex, with the switching off of alternator causing a problem with anti-snake device so the old voltage dependent relay can't be used any longer, we are back to blocking diodes to ensure when car is started current is not drawn from caravan battery. But with your old car that should not be a problem. However it has resulted in a new device, the DC to DC inverter for charging the caravan battery.

I think for 164
Maximum towable weight 1300kg (that would be 2866 lbs).
Max. vertical load on towing hook 65kg (143 lbs).
where the X-trail was 1500 to 2000 depending on model so hope it is man enough?
Brilliant, thanks! It's a completely old-fashioned electrical system, apart from the bulb failure warning light. (No telltale on the dash for towing, just a tail and brake light bulb failure system on the car itself). I've bought a TEB7AS 7-way universal bypass relay, which is designed for cars with CAN systems, but I think it'll probably work with my bulb failure system too. I'll find out in the next week or two! Unfortunately, (see separate post) I have more pressing matters to attend to though. As of last night, I have an alternator not working!

Yes, it's an old 164 (Series 1) and weighs about 1400kg. To be honest, it's not a great tow car, but I'm only really looking at towing a sailing dinghy which is a very light, unbraked trailer. Maybe 2-300 KG with all the gear in it. I have a much bigger twin axle trailer which we used to to w with the X-Trail but that would be way too much. The tail definitely wags the dog with that!
If it's of interest to anyone, I ended up getting one of these:

(If the link doesn't work, it's a TEB7AS 7-way universal bypass relay). One side of it connects to all the wires you'd usually use that feed the tail lights plus power and earth. The other side of it goes to the trailer socket. It's designed for modern cars with "CAN" systems and I guess it just takes a tiny signal from each of the lighting wires, but uses its own internal relays to feed a heavier current down the appropriate wire to each trailer light. However it works, it seems to also bypass the bulb failure warning light on the dash, so I'm a happy bunny!
Links in this post may contain affiliate links for which DIYnot may be compensated.
Sponsored Links