Faulty tail lights, but no one can find problem

[cglittle]

Just had caravan serviced, and engineer said all lights work off their test unit. My car is ok, as lights work fine on other trailer. … but that’s when things get strange.

This is just the tail lights, which is quite a few, with all the orange lights along the side as well… both sides affected.
Everything else is ok, reversing, brake and indicators.

Car engine running:
Lights work on trailer, but unplug trailer, plug in caravan… the lights on the caravan don’t work. Unplug caravan, plug in trailer…. Trailer lights now don’t work.
Restarting the car seems to reset, now trailer lights work again.
Trailer gas 13 pin plug, but only uses 7



My logic is saying there is a fault with caravan that is tripping something in the tow bar electrics box…. And stays tripped until the engine is restarted.

I have one of those cheap double - ended test units that can test either the car, or the caravan and it says there’s a fault with fog light… but I’m not sure how accurate that can be.

 

[Jcloughie]

Just a thought, perhaps the extra load having lots on the side lights terminal, is causing a problem with the cars cambus system. Other than that it is above my pay grade.

John

 

[ProfJohnL]

There have been several reports of LED caravan lights not behaving with modern cars can bus systems. One of the mains attractions of LEDS is how little power they use compared to conventional filament bulbs, and the lower power draw confuses the tow cars bulb check systems

I believe Sargent Electrical Services have some solutions for this type of problem
I suggest you contact them to discuss and hopefully resolve the problem

 

[GaryB]

My previous car was a 2018 Volvo V90, I had a tow bar fitted along with a vehicle specific wiring harness from PF Jones. Everything worked fine BUT if you unplugged the caravan or trailer with the car running then plugged it back in the lights would all flash on/off in sequence and the only way to reset everything was to switch the car off then back on again. Neither Volvo or PF Jones could understand why.

A different issue to the OP but demonstrates that modern vehicle electronics systems sometimes do strange things.

 

[cglittle]

I was thinking along the same lines.
The caravan service people said the lights work on their test rig… which will just be a straight 12v feed.
The caravan has regular filement lamps… not LED
The car is a Rangerover Evoque. 4 years old, and the tow bar has only been fitted for2.

Maybe just a simple overload for the tow bars relay box? When you add up every bulb, I suppose there’s quite a few.
Next time I’m working on it, I’ll first try a direct 12v feed, Then remove some bulbs to reduce the load and see if it makes a difference.

 

[GaryB]

What electrics were fitted, a dedicated harness or a universal kit and bypass relay? Dedicated harnesses are usually a better option (but not always as was the case with mine!) but I have heard and read of the universal kits giving even more grief.

Modern vehicle electronics are way too complex for me to get my head round, in my case I just accepted it as a quirk!

 

[Woodentop]

Modern electronics require that the final state be in place before power is applied, i.e. the trailer of whatever type must be connected to the tug before the tug is started as the ECU does a whole flow of rapid tests when kicked into life.
We have just got a Skoda Karoq which only has USB-C connections, If I plug a USB memory stick containing music into the USB-C when the 'Infotainment' unit (agghhh!!) is already active it finds the memory stick happily. If however I leave the stick plugged in when I leave the car, when I return it will not find the stick - I have to unplug it and plug it in again for it to appear on the system.
Strangely our previous car, a Passat Estate, had the VW version of the same 'Infotainment' unit and that was happy to find the USB stick whatever.
The only difference is that the Passat supply to the USB was ignition switched, whereas the USB-C (and the cigarette light power outlets) on the Skoda are permanently live.

 

[ProfJohnL]

I have just reviewed this thread from the begining, and I'm not clear about the nature of the fault.

This is just the tail lights, which is quite a few, with all the orange lights along the side as well… both sides affected.
Everything else is ok, reversing, brake and indicators.

Car engine running:
Lights work on trailer, but unplug trailer, plug in caravan… the lights on the caravan don’t work. Unplug caravan, plug in trailer…. Trailer lights now don’t work.
Restarting the car seems to reset, now trailer lights work again.
Trailer gas 13 pin plug, but only uses 7
....

You haven't actually told us what happens if you plug the caravan in before you start the engine. Does the problem still occur? Or is it only if you plug in whilst the engine is running?

The amber lights are of course only meant to flash with the direction indicators. They should not be on at other times. Some long trailers do have to have amber side reflectors.

 

[Jcloughie]

I have just reviewed this thread from the begining, and I'm not clear about the nature of the fault.



You haven't actually told us what happens if you plug the caravan in before you start the engine. Does the problem still occur? Or is it only if you plug in whilst the engine is running?

The amber lights are of course only meant to flash with the direction indicators. They should not be on at other times. Some long trailers do have to have amber side reflectors.

The amber lights along the sides are marker lights and come on with the side lights. I think they are legally required and have been for quite a few years.

My van had 3 or 4 on each side.


John

 

[cglittle]

I’m not sure what happens when the engine isn’t running. Allows hooked up with car started.
I actually don’t think my lights work without ignition on. Again, I’ll test that next time I’m there.
The caravan is a 2014 Swift challenger sport 586

Confirm its amber marker lights along the side.
that’s why I’m now thinking overload, as there’s maybe 6 bulbs on each side.

the wiring for tow bar is Rangerover, not a universal kit

 

[Dustydog]

What happens when you plug in and then start engine. All the above Forumites have explained about modern electronics . As a matter of principle I always plug in engine off. Too many fumes by exhaust 😜

 

[Buckman]

Not an answer to OP's question, but if you have a Stop Start system on your vehicle and if stopped at traffic lights with foot on brake and car shuts down, do brake lights on car and caravan also shut down. I know that my DRLs switch off unless I take my foot off the brake.

 

[otherclive]

Not an answer to OP's question, but if you have a Stop Start system on your vehicle and if stopped at traffic lights with foot on brake and car shuts down, do brake lights on car and caravan also shut down. I know that my DRLs switch off unless I take my foot off the brake.

My cars leave brake lights on. To annoy people like me who may be behind.

 

[Buckman]

My cars leave brake lights on. To annoy people like me who may be behind.

Generally I apply the handbrake so engine starts anyway which is why it took me nearly 6 months before I noticed this abnormality. Initially I thought that my DRLs had blown.

 

[ProfJohnL]

I have always found the National Trailer Towing Association NTTA to be a pretty reliable source of regulatory information about trailers which includes caravans.

Reading this page from their web site makes no trailer sides with amber lights other than turn indicators. Side markers should be showing red to the rear and white to the front.

The only mention of amber is about side reflectors. not lights.

However I have also now found this page from Croner ( who publish a wide range of interpretations of legal requirements in many disciplines, which does contain some references to illuminated amber side markers.

 

[otherclive]

For anyone not spending all weekend in front of the TV this might help while away the hours 🙈
Road Vehicle Lighting Regulations

The Road Vehicles Lighting Regulations 1989

www.legislation.gov.uk

 

[Jcloughie]

For anyone not spending all weekend in front of the TV this might help while away the hours 🙈
Road Vehicle Lighting Regulations

The Road Vehicles Lighting Regulations 1989

www.legislation.gov.uk

I think this section covers marker lights, but it will take a little examination to see if it comes under obligatory or optional.

I would guess obligatory otherwise why would caravan manufacturers go to the expense of fitting them? The first van I had with them was a 2011 Unicorn.

John

 

[otherclive]

I think this section covers marker lights, but it will take a little examination to see if it comes under obligatory or optional.

I would guess obligatory otherwise why would caravan manufacturers go to the expense of fitting them? The first van I had with them was a 2011 Unicorn.

John

Thank you for the extra refinement, although I had assumed anyone interested in reading about marker lights would have no difficulty scrolling down the index.

 

[cglittle]

I think I may have found the problem if someone could confirm something.

I’m not at the caravan right now, might go see it tomorrow or the weekend, but I would like to know what size the bulbs are for the tail and marker lights. Will either be 5 or21W…..

If the latter, then no wonder it doesn’t work.
I found the invoice for the tow bar fitting which is dated just shy of 1 year ago… so hopefully there’s a years warranty on it.
The invoice stated the model number of wiring loom/relay box, and searching that online gives maximum load for each socket pin.

pins 5 and 7 for the right and left side lights respectively are both max. 52W.
if all the bulbs are only 5W then I’ll have to think of something else

 

[otherclive]

Be unusual for 21w as side markers, mine were 5w

 

[Jcloughie]

I think I may have found the problem if someone could confirm something.

I’m not at the caravan right now, might go see it tomorrow or the weekend, but I would like to know what size the bulbs are for the tail and marker lights. Will either be 5 or21W…..

If the latter, then no wonder it doesn’t work.
I found the invoice for the tow bar fitting which is dated just shy of 1 year ago… so hopefully there’s a years warranty on it.
The invoice stated the model number of wiring loom/relay box, and searching that online gives maximum load for each socket pin.

pins 5 and 7 for the right and left side lights respectively are both max. 52W.
if all the bulbs are only 5W then I’ll have to think of something else

Side lights are normally just 5 watts, and I think the marker lights are LED, so very little. (but I may be wrong).

I think the same as others. Plugging in the van AFTER turning on the ignition might confuse the car electronics. Just try plugging the van in first and see what happens.

John

 

[Jcloughie]

Thank you for the extra refinement, although I had assumed anyone interested in reading about marker lights would have no difficulty scrolling down the index.

I looked out of curiosity and found that so tried to be helpful. Sorry.

John

 

[cglittle]

even if the markers are 5W, a couple at 21, and the rest at 5 is still exceeding the max.

starting the car after plugging in also doesn't work. There may be an instantaneous flash of the lights, but thats it.
When travelling to and from sites, we would have stopped at services, and the lights didnt come on.

On my paeticular caravan, there is the regular tail light... which ill take at 21W. There are 3 x amber side markers at 5. A white light on the front corner, which may also be 5, and a high level light at the rear... with a clear lens front, and red rear. This could be 21??

There is no LED road lamps on the caravan, but there is inside.

 

[Jcloughie]

even if the markers are 5W, a couple at 21, and the rest at 5 is still exceeding the max.

starting the car after plugging in also doesn't work. There may be an instantaneous flash of the lights, but thats it.
When travelling to and from sites, we would have stopped at services, and the lights didnt come on.

On my paeticular caravan, there is the regular tail light... which ill take at 21W. There are 3 x amber side markers at 5. A white light on the front corner, which may also be 5, and a high level light at the rear... with a clear lens front, and red rear. This could be 21??

There is no LED road lamps on the caravan, but there is inside.

Do your marker lights look like this:



John

 

[cglittle]

Do your marker lights look like this:

View attachment 4641

John

No. They’re Swift original parts. I’ll get a better photo when I check the sizes….

You can see the 3 side markers, but not a clear shape or style.