Thread: Check Trailer Light

The Check Trailer Light message is displayed when the trailer ECU does not detect a complete circuit on the indicators, Brake or tail lights during a cold or warm test. Cold test are every couple minutes and a failure will display the message until a successful test. The L322 does not test for load, and LED lights should satisfy the test – but they will flicker.

If you have a faulty earth in the trailer plug on your car, it is possible the lights will operate by earth through the ball/chain and the test will fail. Check the lights without coupling the trailer to the car so the only connection is the plug.

I found an issue with mine doing similar a while back, turned out to be a connection issue with the plug – one f the indicator pins just didn’t fit tight and occasionally gave the message with vibration/bumps. Spread the pin and all good.

I fitted LED tail lights to my trailer and now get a flicker in both turn and tail, as soon as I plug in . If I turn tail lights on, no flicker, just steady red tail.
When indicating, sometimes a faster cycle/flash, some times normal????? Someone said put a resistor across the indicator feeds? But didn't know what resistance to use.
Only very occasionally do I get "check trailer light".

Not a real concern, but I want to know why?

Hello Don.

The trailer ECU sends a 2V test pulse to check for a complete circuit – this is known as “cold” testing. If you get the message occasionally, I would think there is a poor connection and the message is valid, unlike D3/4, BMW based system checks for a complete circuit – not load. So LED lights with nothing added will function but you get the test pulse flicker when they are not on (they are cold).

When you turn the tail lights on, 12v is provided (the light operates as normal) and the ECU uses this constant 12v as its “warm” test.

D3 *****************************
The system in D3 (only checks the indicators) expects the load of a 21w globe – plug in LED lights and the car does not recognise a trailer at all - hence the need for a resistor. The resistor provides the load the car is looking for, but the LEDs will still flicker. If you fit a resistor for D3, you will get rid of the dash message but the LED unit will still flicker because 2volt is enough to make them do that.
*************************** Back to L322

The resistor on its own will not actually do anything usefull for L322 - it will just provide a positive result for the tests everytime which is not what you want but the flicker will still continue. The car recognises a trailer when a complete circuit is provided. So the only problem is the flicker. To solve this you need a shunt box, I made a couple myself for about $30. There are a couple of commercially available units available.
It is a simple circuit that has:
1- a relay or mosfet to turn the light on – so the 2v pulse does not make it to the light
2- a small resistor to complete the circuit prior to the mosfet for the cold test.
Down side of the shunt box is you loose cold testing of the trailer circuits – it cold tests the shunt box. However, warm testing will still work but only if your circuit cuts the resistor out when the light is turned on. I don’t believe all the commercial units allow for this. I have a detailed write up at home I can PM over.

*********** Back to your issue
From what you describe Don, you have an intermittent issue with one of the indicators – this is as expected. If the L322 trailer ECU detects a problem with an indicator during warm test it informs the instrument panel which then informs the LCM that an indicator is out and the LCM does the rapid flash on the side that has the problem. So if you are getting a rapid flash sometimes, at the point when it is happening the car is failing a warm test for at least one indicator that you are using at that point - the car knows there is a trailer because it sees the other circuits – it is including the trailer indicator in this equation at this point – get out and check, I bet it is the trailer. More than likely it is also the source of your message – this will be picked up by the cold test. Most likely cause will be the connection at the back of the car.

Originally Posted by donrover0

I fitted LED tail lights to my trailer and now get a flicker in both turn and tail, as soon as I plug in . If I turn tail lights on, no flicker, just steady red tail.
When indicating, sometimes a faster cycle/flash, some times normal????? Someone said put a resistor across the indicator feeds? But didn't know what resistance to use.
Only very occasionally do I get "check trailer light".

Not a real concern, but I want to know why?

Mine (07MY) did all of that until I fitted a magic box. I think it depends on whether or not your foot is on the brakes and if the lights are on or not. From memory I would get normal speed indicators unless the park lights were on, then they would flash fast. They would also flash fast if sitting at the lights with my foot on the brake.

In summary, get a magic box to rectify the problem. Mine is permanently wired and mounted in the spare wheel well with a simple switch to turn it on and off.

Thanks Harlie,
I get fast flash on both LED indicators; and have checked that they are working.
But not always????? Occasionally normal flash ( and they are working then too).
I might have access to an incandescent bulbed trailer to try.

Originally Posted by donrover0

Thanks Harlie,
I get fast flash on both LED indicators; and have checked that they are working.
But not always????? Occasionally normal flash ( and they are working then too).
I might have access to an incandescent bulbed trailer to try.

Which is exactly what mine did.

I'm sorry but there is a problem which should be easy to fix - a shunt box just hides the issue - if you don't want to investigate, that's fine - give up and move on.

I tow a car trailer with LED and no box often, they work perfectly they just flicker. So did my camper before i added the box. My box trailer & boat trailer, I converted back to standard incandescent - because the testing is a useful feature to have. Only reason I haven't converted the camper is because there is no suitable replacements for the nice flush mounted tail lights, so it got a box