trailer lights,again

[racebmwm3]

Originally Posted by srmmmm

The tech at the dealership worked with me several years ago to research what was going on with the use of LED trailer lights. This should address several points brought up through the course of this thread:


The BC (body computer) checks the amperage load in all bulb circuits to evaluate the presence of a burned out lamp. The BC knows when a trailer harness has been plugged in at the adapter and tests those lighting circuits as well. Since the LEDs draw such a low amount of current, the BC interprets them as burned out incandescent bulbs and tells the flasher unit to use the "rapid pace" interval as a signal to the driver that a turn signal lamp is not functioning.


I tried using additional LED signal circuit resistors that are available from several trailer lighting manufacturers, but found they deteriorated badly after a year's worth of lake immersion. So I installed a small, incandescent red marker light in series in the turn signal circuit on each trailer taillight and it cured the problem.


When the taillights are on in conjunction with the headlights, there was enough of a current load that the BC knew the trailer taillights were working.


As for the periodic flash observed in the LEDs when the X5 is turned off, that is the signal passed though the body harness by the vehicle alarm system. The LEDs are so sensitive that they are able to light up briefly when the alarm test signal is emitted. I haven't tested it, but I believe that if you disconnect the trailer harness from the vehicle when it is locked and armed, that the alarm siren may go off on the X5 letting you know that someone is trying to steal your trailer.


2002 X5 3.0 361,300 miles (Regularly towing 1300 pound PWC & trailer)

2014 428i 63,900 miles


2004 325i sold at 123,600 miles
2001 325i sold at 66,000 miles


1970 Firebird under restoration - engine dyno run in August

Interesting. Gonna try the latter right now.

Since they are LEDs, I wonder if I should be concerned about current draw over time? Else, I'll just disconnect the trailer from the X.