Fitting tow bar to an Audi and testing the electrics

[garethp]

Bought an Audi last year which has (had) an Audi tow bar fitted but the detachable ball was missing, Audi want £250 just for the ball and I've finally given up trying to find a used one.

So I've removed the Audi kit and bought / fitted a whole new assembly as it was cheaper and thought I'd test the electrics with a multimeter and then a bulb before refitting the bumper.

And then it all went wrong the tow bar electrics were already fitted and are fully integrated with the car so should be ok but I keep getting weird results on the trailer socket such a left indicator ok, right appears to be dead, tail lights then no tail lights etc. I think it's down to the bulb failure system checking the bulbs and disabling things as its detecting a 'fault' as I test one bulb at a time but I'm not sure? The dash display does tell me various trailer bulbs are faulty but I wouldn't have though it would disable other bulbs.

Does anyone have any thoughts? I'm thinking sod it, hope for the best, re fit all the bits, plug the trailer in and see what happens...

 

[Johnseat]

How old is the car, more than likely it's the bulb check causing the fault.
The Audi kit would be a full can bus enabled system which bypasses the bulb check when it recognises a tow bar fitted.
For the cost of the tow bar from Audi, you would have been better paying that extra as there's a chance the Audi system would have told you about faulty trailer lights.
What type of bypass relay did the kit come with, is it just a standard type with a 12v feed and utilising the rear lights as relay switches as these can send the current draw over the max allowed thus knocking off the lights.
You will need the can enabled bypass relay which are cheap as chips on ebay.

 

[WorkshopChris]

Does it have the new 13? pin socket they sometimes have a pin that switches when it detects a plug is inserted in to alter settings in the electronic control unit.

 

[Barlidge]

Maybe take it to your nearest friendly MOT station, they have plug in testers.

 

[Crater Creator]

My mercedes did the same thing, intermittent faults on different bulbs. I ended putting a new set of lights on the trailer and it's all fine now. My theory is that the error-check is a bit sensitive and screws things up.

 

[Robotstar5]

I normally plug an old trailer board in as it tests them all in one go and I can place the board where I can see it from the drivers seat while operating the switches.

 

[pedrobedro]

I normally plug an old trailer board in as it tests them all in one go and I can place the board where I can see it from the drivers seat while operating the switches.

That's my practise as well. A trailer board only cost about £8. Someone somewhere must have a stockpile of those towballs.

 

[garethp]

How old is the car, more than likely it's the bulb check causing the fault.
The Audi kit would be a full can bus enabled system which bypasses the bulb check when it recognises a tow bar fitted.
For the cost of the tow bar from Audi, you would have been better paying that extra as there's a chance the Audi system would have told you about faulty trailer lights.
What type of bypass relay did the kit come with, is it just a standard type with a 12v feed and utilising the rear lights as relay switches as these can send the current draw over the max allowed thus knocking off the lights.
You will need the can enabled bypass relay which are cheap as chips on ebay.

The car is a 2006 A4 and had the Audi beam and electrics fitted but the removable hitch was missing, the Audi quote for the hitch alone was £100 more than a whole witter beam and removable hitch. I had to take the bumper off anyway to sort some rust so I thought I may as well fit a new towbar while it was in bits, but I've kept the audi wiring as its part of the wiring loom on the car, I thought it made sense to test the socket before refitting the bumper and all the interior panels.

Does it have the new 13? pin socket they sometimes have a pin that switches when it detects a plug is inserted in to alter settings in the electronic control unit.

It is a 13 pin socket and I did wonder about that

My mercedes did the same thing, intermittent faults on different bulbs. I ended putting a new set of lights on the trailer and it's all fine now. My theory is that the error-check is a bit sensitive and screws things up.

I'm hoping its ok when I plug the trailer in and my problems are being caused by testing one pin at a time, its not long since I rewired that so fingers crossed its ok, if not I'll just get a trailer board.

That's my practise as well. A trailer board only cost about £8. Someone somewhere must have a stockpile of those towballs.

That's what i thought but I've been trying to get an audi ball for the last 12 months, nearest I got was one from a 2004 car which is 2mm larger in a crucial place and it wouldnt fit

So does the detachable ball have a sensor in the socket so it knows when a ball is fitted?

Completely different but maybe similar, I sorted out the electrics socket on a focus and it was throwing a proper fit unless it had a load it liked present on almost everything...

No switch in the ball socket but otherwise sounds very similar, if I can get rid of the kids this afternoon I'll put the car back together and pull the trailer out to test it.

 

[garethp]

Well that's it all sorted, plugged trailer in and everything works perfectly

I still don't know how the car knows the trailer is plugged in as there is no switch to detect a plug, it must be part of the bulb test routine.

 

[dan.taylor.1]

It detects the resistance or current draw change from having more bulbs to power via the light circuit, usually turns parking sensors off too so they don't beep like you just got rear ended constantly!

 

[garethp]

It detects the resistance or current draw change from having more bulbs to power via the light circuit, usually turns parking sensors off too so they don't beep like you just got rear ended constantly!

Thought it must be something like that, I noticed it turns the car rear fog lights off as well. I also seem to remember something about it altering the ESP settings when towing as well.

The lights on the trailer are rubber and pop open if the right part is squeezed so I had a play removing bulbs and seeing what the dash display said, I was pleased to see the bulb monitoring covers the trailer as well.

 

[addjunkie]

I can't say about this one, but you may not be so glad if you fit led trailer lights...

The small voltage used to check the bulbs can make the leds flicker, the system might not notice they are there, it might report them as failed, the indicator tell tale might not respond and some systems don't even try to light them if it thinks it can't 'see' them.

There are ways to sort this out, but it'd be nice if things just work like what they done did in the old days

Mine is like this. With cheapo light board tell tale works for the indicators, rebuilt a trailer new bits lights eyc, fitted led lights, tell tale on indicators no longer works, though all the lights work

 

[addjunkie]

As long as the lights work, im not going to worry about it unduly.

 

[gav0994]

Mine is like this. With cheapo light board tell tale works for the indicators, rebuilt a trailer new bits lights eyc, fitted led lights, tell tale on indicators no longer works, though all the lights work

You need to wire a resistor in series with the indicators on the trailer to get the tell tale to flash in the car. The LEDs don't provide enough load resistance for the car to know the indicators are flashing on the trailer.

beaten to it!

 

[gav0994]

Won't work in series, it'll just limit the current even more...

You want it in parallel, that way it can drag more current out of the car and make sensors sense stuff

indeed! been a long weekend for me...head is a bit worse for wear!

 

[Justme]

Won't work in series, it'll just limit the current even more...

You want it in parallel, that way it can drag more current out of the car and make sensors sense stuff

But stop it actually doing its job.

Just fit a better relay that works with LED's.