Lights Control Module (LKM)


The LKM is a module that often breaks on E34s. When headlights suddenly quit working, or other erratic headlight behavior, the LKM is often at fault. The LKM is the big "cassette" located in the relay/module box in the engine compartment. It may take a good yank to pull it out.

Note: The LKM in the photo's are from a Euro E34 (4 relays; low beam, high beam front fog, rear fog) whereas most US LKMs will only have 3 (no rear Fogs)

I know there is a procedure on bmwe34.net...
http://bmwe34.net/e34main/maintenance/electrical/LKM.htm
but I thought I would add some pictures so that people can see exactly what fails and how easy it is to fix. As you know symtoms include low beams that turn off on their own and other sporadic behavior.

Once you have removed the LKM module from the car you have to extract the circuit board by carefully prying the edge of the case apart and sliding the connector / board assembly out.

Euro LKM


Prying the case apart


Board extracted


Other side, notice relays on both ends


Board out, relays down


Bad connections outlined in yellow


If you look closely at the above photo you will see that the relay solder connections have fractured and are not really soldered in any more. What you have to do is reheat them with a decent soldering iron and add a little more electronics grade silver solder. You need to add more solder as the flux in the new solder will help re-establish the connection so your LKM will work again. You can be anal and remove all the old solder and re-solder each connection but it's not really required. (I don't like solder suckers)

Look closely at the relays and be sure to touch up all the contacts (on both ends of the LKM)

If you haven't ever soldered before then Google it. Its not hard

http://www.aaroncake.net/electronics/solder.htm

Hope this helps some of you

Article by Byron Topp