Going into limp mode with trailer on - intermittent
2015 Ssangyong Rexton NA

When towing HW trailer the vehicle goes into limp mode, doesn’t do it with other trailers and the trailer in question doesn’t have any issues when towed by other vehicles. Brings all trailer lights on before going into limp mode
Problem added: May 29, 2022 (1 month ago)

Ssangyong Rexton 2015
Variant: NA
Problem Category
Electrics, ECU, Warnings & Lights

Replies and fixes

Dont know if this applies or is now relevant as it was a few years ago

Does your car go into limp-mode every time you tow your caravan after about 5 miles or so, you clear it by cycling the ignition switch and it happens again after a few more miles and so on? if so, I may have the answer to your problem. I have been reading on the web that some caravan owners have experienced this problem with many newish cars. When taking their car to the local franchise, the vehicle diagnostic equipment says that there is not an engine management fault and everything appears to be programed correctly within the cars electronics and ECU, in other words, they cannot find anything wrong which would be likely to course this to keep happening.

Before reading on, it might be a good idea to read one of my previous posts titled "Future use of pin 12 within the euro plug" this will give you a clearer insight as to what I am going to explain next.

In recent years motor vehicles have become very sophisticated electronically and vehicle ECU's control just about everything within a modern vehicle, these vehicle ECU's are now so sensitive that any slight difference to any standard programed tolerance can course the vehicle to go into limp-mode. When connecting a heavy trailer such as a caravan to a modern vehicle, the vehicle, like vehicles manufactured in years gone by will require more fuel, more air and more power will be demanded from the vehicle engine as it is pulling more weight than it would be solo. In past years this has not been a problem as cars were not ECU controlled or ECU controlled cars built a few years ago, the ECU's were not as sensitive as they are today. We now have a problem with some modern cars and over the next few years I believe this problem is going to become worse unless the vehicles ECU knows when you connect your caravan to your tow-car so it can make engine management changes ETC to allow for the towing of a heavy trailer preventing the vehicle switching into limp-mode. So in these modern times, how can the vehicles ECU detect when a trailer is connected to a tow-car? Some modern vehicles can detect a coupled trailer automatically by sensing additional bulbs and extra weight on the rear suspension ETC when a trailer is first connected to the vehicle, there should be no limp-mode problems with vehicles with this type of detection. We now get down to the route of the problem with the said vehicles that are repeatedly going into limp-mode when a caravan is connected. The ECU's on many modern vehicles built within the past few years are programed to detect a connected trailer through pin 12 within the euro plug, this is causing problems as some tow-bar fitters are not connecting relevant wire from the vehicles ECU to this pin. This maybe due to them fitting universal seven-way bypass relays to power the trailers road lighting or fitting none franchise tow-electric kits that are not compatible to use with the pin 12 detection function. It may mean that you may have to pay twice and have your current tow-electrics replaced by a genuine franchise tow-electrical kit that is solely manufactured for your vehicle alone, this maybe the only way of curring the limp-mode problem. To this end, if your vehicles ECU needs to know that you are towing by detecting through pin 12, you must address this in order to solve the problem.

WARNING; You must link pin 12 to pin 3 within your caravans euro plug so it can signal your cars ECU through pin 12 otherwise the detection will not function. Your tow-bar fitter may of wired your tow-electrics correctly but your system is not detecting due to your caravans euro plug has not had pins 12 and 3 bridged together, this may only be the problem, so I advise you to check this first.
Answered May 29, 2022 (1 month ago)

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