[Closed] Fitting tow bar wiring to canbus-using a multimeter...advixe please!.

Sorry mods not sure if this goes in the chat forum as it's for tow bar for bike rack use 🙂

This was the bit that put me off doing this myself - I know next to heehaw about electrics so decided to play it safe and get someone to do it all. Mine is a 2014 Peugeot Partner.
All sorted and working but there is no reverse light action on the rack...works on car but rack doesn't even though it has a reverse light fitted.
I was after a swanneck ball but the place I went only had removable ones so didn't go for that.
I think they fitted a relay bypass for the electrics.
The rack I have is 10 years old and has a bolt sticking out at side of rack mount which means the rack doesn't sit flat, but it is 'good' as it means the bike tyres are sitting higher than the exhaust so no risk of exploding due to exhaust gases hitting them.

Sorry, bit of a longwinded story.

Canbus is normally a twisted pair and I doubt if there is anything on the network at the rear of the vehicle - unless you have a top of the range model with a rear camera (maybe)? Been a while since I fitted a tow bar though.

All you need to do is ID the wiring back from the lamps and work out where the left and right split is. I use a test light and not a multimeter.

And don't use scotch loks - solder and heat shrink every time.

I'd be surprised if it hasn't got a trailer wiring connector hidden behind one of the tail lights.

I wired a towbar on a C5 and just used the wiring diagram and took the positive off the power point in the back (used scotchlock connectors sorry Marko and they have been fine for 4.5 years) only use the towbar for a bike rack though.

The problem with wiring towbars isn't to do with canbus.. its to do with the bulb failure warning systems that many new cars are fitted with.

Back in the day, all you had to do was wire the towbar socket into the relevant lights and everything was good to go. Doing that now is very liable to make the bulb warning system throw a hissy fit.. Hence the need for a bypass system(relay)

Any decent multimeter will be perfect for working out the wiring.. Peugeot Partners and Citroen Berlingo's are fairly straight forward to do. There's nothing that you really need to look out for.

Similar to Falkirk mark above - fitted a towbar earlier this year. Took a spur from each of the tail light sockets back to a central point - behind panel on rear left of boot.

Used spade connectors to write it into the input side of the relay, output of relay straight to the 7pin for the towbar electrics. Only mildly frustrating part was running a new fused live back from battery( take off fuse right next to battery) rear to the boot.

Easy job over the course of a morning. O

Pretty much as per Scruff, only I took my live from the power socket in the boot on my Focus and put the relay in the first aid box.

If it's canbus it depends on the car as Marko said, some run the network cable to the rear light cluster (which probably makes it more hassle than it's worth to bodge), others just look like a normal wiring loom and a bypass relay works just fine as it draws so little current.

DickBarton - Member

All sorted and working but there is no reverse light action on the rack...works on car but rack doesn't even though it has a reverse light fitted.

7-pin electrics don't have a reverse pin. I'm guessing somewhere in the system you've got something intended for a 7-pin system. Really annoys me as the boat trailer is about 20ft long and the car has really good reversing lamps but I can't see what's behind the trailer if it's at an angle!

Have you got the tow bar yet? I looked at fitting one to my 3 series and it was cheaper to get it professional fitted than i could get the parts to do it myself. I dropped it in on the way to work and it was done in a couple of hours.

Yes, 7-pin electrics. 2 indicators, fog, brake and the normal rear light(s)...that is 5 or 6, so reverse could be 7th...so what am I not understanding? I suspect it is something dead obvious...I'm assuming the brake and rear light are 2 bulbs but 1 cable (split to go left and right) or are they all wired individually (which makes it 7 so no reverse)?

Yes, got the tow bar and wiring kit already, was quoted 300 fitted but all the kit only cost Me 100 so happy to have a go

Had CANbus-comptable towbar electrics "suppied and professionally fitted" (skoda main dealer) to our yeti a few years ago. Sent the body ECU into a fit, that ultimately required a new body loom, ECU and entirely different towbar package.

I seriously doubt anyone could do a worse job.

IIRC 7-pin has no reverse?

Yes, 7-pin electrics. 2 indicators, fog, brake and the normal rear light(s)...that is 5 or 6, so reverse could be 7th...so what am I not understanding? I suspect it is something dead obvious...I'm assuming the brake and rear light are 2 bulbs but 1 cable (split to go left and right) or are they all wired individually (which makes it 7 so no reverse)?

if you dont earth any of those they wont work.... the missing pins your earth.

Gotcha, ta.

Tail lights are split across two pins and then there's the earth:

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It does depend on the car, tbh, but my Volvo's OEM towbar electrics are glorious. Literally, plug and play.

You get a loom with extremely detailed fitting instructions and proper plugs on the end, which plug into various pre-existing sockets in the boot. No wire cutters to be seen and it all worked perfectly afterwards.

I suspect Citroen might not be quite as considerate here but it may be worth a chat with the dealer to see if they have anything vaguely approaching this.

edit: Doh! I skim-read the original post and missed that the OP had already got a wiring kit. Oh well, my comment still stands, even if it might be too late 🙂

Regarding reverse light, you want a 13 pin TEK. This is also better sealed than 7 pin type. I would recommend vehicle specific electrics rather than the universal type as no need to locate the correct wires or solder or invalidate your warranty.

You can't permanently break a CAN bus with a multimeter, as they are protected against short circuit and being connected to battery voltage.

However, you don't need a multimeter to find a CAN bus either, as they are very obvious, being made up of a twisted pair of wires, that very often are yellow (CAN High) and Green (CAN Low). if you get it backwards, all that happens is the BUS fails temporarily, so just try with the wires round the other way.

In some cars the airbag wires are yellow as well.....(Alfas for example).

I suspect Citroen might not be quite as considerate here