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The BMW light control module, commonly known as the LCM, lamp control module, or light check module, is an extremely common part that fails on the BMW E38 and E39. What is it, how does it work, and why does it fail?

Once upon a time, a light switch in a car simply completed a circuit between the battery and a bulb, or sometimes the battery to a relay which activated the battery/bulb circuit. Those days are long gone; as early as the 80's, BMW moved to an electronic module to control lights and switches. Electronics were also introduced to warn the driver about faults with bulbs. You've probably seen the "left side light out" or "right side out" bulb lights on a dashboard before. Starting with the 1994 BMW 7-series, the light module and check control module were combined into one unit, the LCM, that was then used on cars up to the present.

Shop BMW Light Control Modules

 

BMW models featuring the LCM

  • 2001-2003 BMW 525i
  • 1997-2000 BMW 528i
  • 2001-2003 BMW 530i
  • 1997-2003 BMW 540i
  • 1995-2001 BMW 740i
  • 1995-2001 BMW 740iL
  • 1995-2001 BMW 750iL
  • 2000-2003 BMW M5
  • 2000-2004 BMW X5
  • 2000-2003 BMW Z8

 

What causes the BMW LCM to fail?

Whilst generally reliable, LCM problems include:

  • headlights staying on after the car is shut off
  • indicators randomly flashing
  • high beams not working
  • Miscellaneous gremlins.

The E38 and E39 in particular are a bit susceptible to these faults as when the sunroof drains become blocked, water can seep down the inside of the drivers side A-pillar and end up dripping onto the LCM which is in the outside of the drivers footwell. As a result, water damage is the number one cause of LCM problems.

If you do have BMW light control module problems though, double check the basics before considering a LCM change.

  • Are the relevant fuses good?
  • Are the bulbs good? Even if they look fine swap from one side to the other and see if the problem follows.
  • Consider swapping the entire light assembly from side to side - they won't physically fit of course, but the cable will match so you can test.
  • For headlights, see if any applicable relay clicks when you turn the lights on & off. Sometimes other relays (such as the horn) are compatible and can be substituted to rule out problems there.
  • Physical damage to wiring looms.

If you do need a replacement LCM, the correct one can be hard to find used as you generally need the exact same part number. This becomes difficult as there are maybe 20-30 different modules across the E38 and E39 alone, accounting for different options and often with small differences across territories, and some only used for very small production runs. We have new ones on our site. Follow the link below and be sure to add your car to the "My Garage" vehicle selector at the top left corner of the page to ensure proper fitment.

Shop BMW Light Control Modules

 

How does the BMW LCM work?

Remember the BMW "check control" part of the module? That's where some of the complexity comes in. On the earlier cars the check module controls lights, brake switch and so on. On newer cars like the E83 X5, the module is coded for options like as air suspension (EHC) and tire pressure monitoring. The LCM is also used as a backup for the stored mileage of the car - if the mileage within does not match the mileage reported by the cluster, an anti-tamper dot may illuminate on the dashboard. This is done to show the mileage reported may not be accurate and is the bane of shady used car salesmen.

The internals are actually quite simple. The LCM really just consists of some power transistors (covered by a heatsink) and a micro-controller. The micro-controller contains both RAM and a custom ROM program preset for the car configuration; the controller can directly drive the outputs and monitor the check inputs.

If after exhausting all other possible options you still find yourself needing a new LCM, you can find the Genuine BMW parts on our website, here. If you have questions or comments, leave them in the comments section below. 

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Written by :
Bryan McPhail

Bryan is a longtime BMW enthusiast in Florida.


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