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A while ago I had an email from a chap called Roy. I thought it was going to be an easy one to sort out but never expected what the eventual issue was.

Rather than try to explain all the steps, which is difficult sometimes trying to diagnose without actually being hands on, Roy did a fantastic job, so I thought I’d simply post the email exchange…..

Hi Simon,

I have a problem with the drivers side lights on my 1997 Bailey Unicorn caravan.

I bought a new BMWW X3 in 2020 and had a BMW tow bar and electrics fitted in the factory.

I towed the caravan with the X3 for the first time this year. After driving using my headlights for about 70 miles the drivers side lights on the caravan stopped working and I got a message on the dash telling me “trailer electrics were overloaded. Trailer not or only partially illuminated, switch off unnecessary electrical consumers in the trailer and have problem checked by your service engineer”. I switched off the fridge but it made no difference. 

I bought a 13 pin caravan plug and short piece of 13 pin cable and checked the wiring with a multimeter. I connected the black probe to the white no. 3 pin and the red probe to the black no. 7 pin and I was getting a reading on the multimeter, I then checked the brown no. 5 pin which feeds the drivers side but I didn’t have a reading (I have checked for continuity on the brown cable and that is okay). 

I continued to our destination, it was daylight, I had nearside lights on caravan and car but no drivers side lights on caravan.

When we came to leave the caravan site I connected caravan to car and all the lights were working on the caravan but after about 10 miles the nearside caravan lights went off and remained off. I found that if I tried to connect the 13 pin plug the nearside lights on the caravan came on and then immediately went off.

I took the car back to BMW and they can’t find  a problem on the car.

I took the car to a caravan dealer and connected up to one of their vans and the lights on their van all worked.

Is this a problem you have come across as I do not know and also the caravan dealer doesn’t what the problem could be.

Kind Regards.

Hi Roy

Not heard of this before, but I think I might know what may be happening.

The number plate lights are usually connected to one side or the other…. near side running lights or off side running lights. The slight load increase because of this ’trips’ the electronic load sensor in the BMW and it shuts down that circuit. It should reset when you shut off the engine and start it up again.

One quick temporary solution would be to remove one of the side running light bulbs from the orange marker lights running down the side of the van.

A longer term solution would be to swop out the incandescent bulbs for LED ones. These have a lot lower power draw. Just changing over the tail lights and number plate lights should do it.

Let me know how you get on



Good morning Simon,

Thank you so much for your very prompt reply.

I shall take your advice and change the bulbs for led ones. I will let you know how I get on but it won’t be for a couple of months as the van is stored for winter.

Thank very much.

Kind regards. Roy

A short while later…

Hi Simon,

Just an update. I collected my car from BMW this afternoon, I asked why if there was no fault found, why I had a reading on my multi meter for the nearside but not the offside. He said that this did sometimes happen with multi meters. He got his meter that they use for MOT’s and this showed that there was current on both sides, he also did a lamp test which showed the same result. This proves that it must be a problem with the caravan so with the help of your articles I will check all the running light electrics on the caravan. Incidentally, the front sidelights and the marker lights are all led units but all the backlights are incandescent bulbs so I will take your advice and change them. Can I replace the incandescent bulbs with led bulbs or will I have to change the whole units?

The light units are available from Bailey’s.

Kind Regards


Hi Roy

Never known a multimeter to behave in that way…. you can get a voltage reading, but when you try to pass current across the connection if the connection is high resistance, the voltage will drop. That’s why testers usually have a mini trailer board with bulbs in to enable them to test the voltage when the circuits are drawing current.

The existing tail lights can simply be updated  with LED bulbs. I replaced all mine …. https://caravanchronicles.com/2021/09/09/finishing-off-a-few-jobs/ I included details for a company I used on this page. I have had mixed results buying LED replacement bulbs in the past so asked some of my readers for their suggestions for companies and this one was suggested by many people. I’ve used them a few times and the products they supply are IMHO the best quality I’ve come across.



Hi Simon,

Thank you for replying to my query so promptly.

Re.the multimeter, I bought a 13 pin caravan plug & a small length of 13 core cable. I have used this with my multimeter to check the current, I have tripled checked the continuity of and all is good. After “witnessing” there is current coming through at the BMW dealers, the dealer was checking using the 13 pin socket on the car I wonder if there is a problem in the 13 pin plug connection supplying current to the o/s caravan lights. I will start checking in that area first. When I took the van in for service earlier this year I had difficulty connecting the 13 pin plug and they fitted a different make for me, I.have checked the wiring in the plug and it is all okay. While I am at it I will rewire the plug.

Re the led bulbs, thank you for that address, I will order a set of bulbs & replace all the incandescent bulbs with led’s. Strange that they fitted incandescent bulbs at the back and led on the side & front, probably cost came into it.

Thank you once again for your knowledge and advice, I will let you know the outcome.

Kind Regards 

Hi Roy

Where the 13 pin cable comes into the caravan and terminates there is usually a small fuse board for all the road lights. This is a good place to check the voltages when connected to the vehicle…. as you should be able to pull each fuse and check (with the lights on of course) the voltage being supplied to the fuse from the vehicle then with the fuse inserted the voltage with the circuit ‘live’ and under load. I would not expect more than a 2 volt drop. If it’s more then possibly time to take the 13 pin plug apart.

Hope this helps


A few months later…

Hi Simon, I wrote to you about 4 months ago asking for your advice re. no offside road lights on my caravan and I thought that I would give you an update.
The caravan is a 2017 Bailey Unicorn and I tow with a 2020 BMW X3 diesel, the car has a factory fitted tow bar and 13 pin electrics. The first time that I towed with the car road lights all worked when I connected up but after about 15 miles the off side lights went out and a message came up on the dash telling me that the electrics on the car were overloaded and to speak to my dealer. They stayed off for the rest of our journey but when we came to return home the lights all worked for about 10 miles then again the off side lights went out and I got the message on the dash. 
I wrote to you and went through all the items you suggested and everything was fine, I even changed the 13 pin plug just in case there was a connection problem. 
I felt sure that the problem was with the car and not the caravan so I took it to my BMW main dealer. The technician checked everything whilst I was there and everything was was working fine, but when I got home and connected car to caravan I still did not have road lights on the off side.
My caravan was due for a yearly service so last week when I took it in I asked the caravan dealer if they would check it out, he said that they had heard of this problem. I got a call a few days later, and the workshop manager said that they thought that they had found the problem. He had spoken to an electrician and he seemed to think that I had been running with the lights on automatic, which was quite probable, he said that it has something to do with the smart alternator. His advice when connecting car to caravan was to switch of the engine and switch the light switch off, connect the 13 pin plug to the car, start the car engine then switch the headlights on (don’t use the automatic headlight function when towing). When I collected my caravan I went through the suggested procedure and the lights were fine. I took the car and caravan for about a 30 mile run and the lights stayed on all the time. Hopefully this has sorted out the problem. 
I had read on your site about the different problems people are having with smart alternators and as far as I am aware I haven’t come up against any of those, fingers crossed. I think that cars have become far too complicated and even the technicians are baffled with them.
Thank you for your advice Simon.

I’m so glad you got back to me with the results…. and even more pleased that you are all sorted!

Your email was quite timely…. I have just received an email from someone towing with a VW Toureg and he is having the same issue…. and as both auto light systems in the BMW and VW are made by Bosch….. I’ll bet it’s the auto light again.

I think this is a bit of important info, so with your permission, I’d like to write a brief article about the problem and steps taken to resolve it.



Hmm….. I have auto lights on my VW Amarok but don’t use them…. I can feel a little testing in the near future!

So…. it was the Auto Light function on the BMW. I have checked on my VW Amarok and this doesn’t happen, I’m waiting for the gentleman that emailed me about a similar problem on his Toureg to get back to me.

If you have a BMW with Auto Light function and this has happened to you can you post in the comments as it will help anyone else when searching on Google for a similar problem.

I get a few of these type of problems emailed to me from time to time and the results never really make it into the blog. As some of them could be useful for other’s with similar problems do you think this type of posting would be useful? Obviously permission is required for me to repost and I did confirm with Roy if I could tell his story… I was going do one of my usual articles, but them thought actually the emails tell a better story just as they are.

Let me know what you think.