Strange electrical fault when towing

Have you looked at similar threads? I don’t think it’s that unusual.

No i havent.... i thought i was just unlucky.

When trying to explain the problem to the towbar fitters, they didn't seem to understand, and basically blamed the caravan. 

If its a common problem, has there been successful solutions or workarounds?

Have you checked that the vehicle software was updated after the tow bar was fitted.

I assume that would be done by the tow bar fitter?

if that hasn’t been done, then I would expect none of the lights to work, or that it wouldn’t work on the trailer…. Which it does?

Hi, Back in 2007 we had a towbar fitted to an Audi A6 Avant, and I witnessed the testing of the lights using a trailer light board.

A short  time later while heading to Plymouth ferry port I noticed the side lights on the van were not working-on hitch up all worked fine. 

The fitter came to the port with a replacement relay which we hoped would solve the problem, but alas ,no.

A stressful time followed and our proposed overnight drive, we had two young Grandchildren so best time fir us, had to be abandoned.

After we returned and the fitter made enquiries of the relay supplier it was decided that the problem was too many lights!

We had a Hobby and there were 14 “marker” lights which was causing the relay to overheat and trip-we had by this time realised that turning off the lights for a period would give a temporary fix-and the solution was an older more basic/less sensitive relay.

Thats what i was thinking.... too many lights. 

There's a white light at the front of the caravan... 3 or 4 amber marker lights, the standard tail lights, a high up white/red at the back and the license plate light

5W apiece? Quite a load.

Costly to replace all of those with LED equivalent bulbs.

I've got an older version of the car relay coming from ebay as I type... so ill try that. 

If that doesn't work, i might try to do something clever with a couple of regular automotive relays and the permanent 12v vehicle feed coming through the plug. 

I have a similar problem with a 2019 Swift and a brand new Kodiaq. Lights sometimes work fine on first hitch up, then later (30 or 60 mins) the car flags an error "Check LH trailer indicator" - or RH, but always indicator. Looking at the van, the appropriate side lights flash instead of the indicator, so there is an indication to other road users. Un-plugging and plugging can cure the problem for a while. Once home I tried the trailer board lights and they are fine.

After a bit of research, I discovered that Swift sell a replacement fuse/distribution panel for the van, a simple matter to fit. It seems that the modern Swift vans with LED lights do not really draw enough current to convince the car that the lights are working. You can wire a small resistor into the circuit to force more current draw, but this is likely to get warm/hot. The Swift panel has a cooling fan built in. Never had the problem with the last car, a 2014 CRV, it probably wasn't so sensitive.

I'm waiting on a reply from Sargent about the problem before I commit to a £150 panel, but I'm pretty sure that's the route I'll have to take.

By the way, the old multimeter is useless to check the LED circuits, no circuit between the lights and earth pins on the socket. Had me puzzled for a while before I realized that's the nature of the "diode" bit in LED!

You are correct that the low wattage led lights on Swift and probably some other caravans fool the tow car into thinking there is no caravan attached. I contacted Swift who referred me to Sargent who manufacture a small plug in module which increases the wattage rating of the caravan lights and immediately cured the problems. I had to quote the caravan chassis number which got me a £10 discount (total cost only £39 in 2016). Fitting the device in a Swift involved lifting up the front nearside bench seat cushion and locating some coloured plugs (which power all road lights) you have to separate them and then plug the device in between them. It was impossible to get it wrong as the plugs are coloured coded and are different fits.