ATC failure - Update

[Youngnick]

Recently while hitching my ‘van with, unknown to me, a discharged onboard battery, I discovered the ATC failed to energise. My local dealer’s service manager used a test rig and identified no fault with the van as all systems were operating to specification, thus correctly surmising a blown car fuse.

Rather than just replace the fuse and put it down “to one of those things”, I felt it important to identify and correct the fault if possible, there being no obvious reason for failure in a system which had performed perfectly well for a number of years.

My thanks go to the representatives of the firms listed below who provided the definitive information regarding the ATC operational failure involving a 2011 Volvo XC60 towing a 2012 Lunar Clubman which employs electrical circuitry supplied by BCA Group and equipped by an ATC provided by Al-Ko and using a standard 13 pin plug connector to a dealer supplied tow bar assembly.

Under normal circumstances when hitching up, the ATC unit will draw around 13 1/2amps when you first plug it in to do its self check ** before entering standby [when] the only power it draws is 50miliamps, this is enough just to illuminate the LED….. The ATC needs a minimum 20amp fuse for the constant 12v supply, it can spike to this if the ATC is called upon to work while towing**

We are aware that Pin 9 is not only used to charge the battery it can also be used to power the caravan when hooked up to the tow vehicle with the engine off. The Cable used is a multicore cable with core sizes stipulated by BS EN 1648-1:2012 and ….. the circuit to the caravan from the tow vehicle is protected by a 15amp fuse so this would be the maximum current drawn*** …..The battery charging drain will not necessarily be a trickle charge – it depends on the charging state of the auxiliary battery****. ….A flat battery may start off drawing high current but it will very quickly reduce the draw as charge is put into the battery***

With a current provided to pin 10 and earthed through pin 13 of the multi-pin plug, such as when the tow vehicle’s ignition is “on” it energises a [habituation] relay which then connects the leisure battery to pin 9, As the ATC also works off pin 9 the two are connected... i.e. … with the tow vehicle ignition ‘on’ it is possible for the ATC to draw current from the leisure battery.*** I tested this and found it to be correct in my case. The ATC went through its self check procedure correctly and indicated green as anticipated even with the pin 9 fuse removed.

The cause of the broken circuit to pin 9? I suggest it was a result of the caravan battery drawing up to 15 amps in addition to the ATC requiring 13.5amps producing a possible demand of 28.5 amps on a circuit protected by a 15 amp fuse, which duly failed. However, should you hitch up a ‘van with a fully charged battery to a car with a blown fuse on pin 9, the ACT will activate correctly, un-fused, as it is earthed through pin13. You may eventually notice the ‘van battery fails to recharge but at least the ACT should work initially.

In future I shall be checking the ATC not only having hitched up but also having started the engine. If I am in any doubt that the ‘van battery may be discharged, I shall not be surprised if the ATC fails to operate!

My recommendation would be a review of BS EN 1648-1:2012.

*Volvo **Al-Ko ***BCA Group **** Caravan Club Technical Department Lunar Caravans

[rogerthechorister]

What's an ATC? Is a different option 12N and 12S plugs with the usual burnout mods?

[Youngnick]

An Automatic Trailer Control unit is a standard feature on many recent caravans. Produced by firms such as Al-Ko they apply the caravan brakes in order to stabilise "sway".