2001 740i dimmer issue

**MWBURG** · 01-24-2025, 05:28 PM

Yet another dimmer switch issue but with a twist. 2001 740i, non-original cluster. Car on, lights on, but dimmer switch does nothing (neither cluster or center stack). Leave the lights on, turn car off, then back on and dimmer works (Cluster and center stack) and keeps working unless you switch the lights off. At that point you have to repeat the procedure of turning lights back on, turn car off then back on and dimmer works again. Tried with another cluster from a 2003 x5 and had same result.

Could this be an IKE or LCM coding issue? Fixable with PA Soft? Suggestions for further diagnosis?

thx. mwburg

**purplecty** · 01-24-2025, 05:43 PM

Originally Posted by MWBURG

Yet another dimmer switch issue but with a twist. 2001 740i, non-original cluster. Car on, lights on, but dimmer switch does nothing (neither cluster or center stack). Leave the lights on, turn car off, then back on and dimmer works (Cluster and center stack) and keeps working unless you switch the lights off. At that point you have to repeat the procedure of turning lights back on, turn car off then back on and dimmer works again. Tried with another cluster from a 2003 x5 and had same result.

Could this be an IKE or LCM coding issue? Fixable with PA Soft? Suggestions for further diagnosis?

thx. mwburg

I believe that is usually an LCM issue

**Mayorchuck** · 01-24-2025, 08:54 PM

Correct. LCM provides the power for the dimmer. More specifically, Pin 16 on x10117, the largest of the 3 LCM connectors.

This is the kind of oddball glitch that a battery nap will often correct. I would try that first myself. With the car off, wait about 10 minutes to make sure the NAV unit is shut down. Remove the battery cables for about 10 minutes. I usually tough the cables together before I put them back on the battery. On a side note, in testing your switch, you probably used your dimmer more than I ever did. Ha.

**MWBURG** · 01-25-2025, 07:18 PM

tried battery nap but no change. can't understand the logic of the dimmer working after an ignition OFF the back On cycle other than some kind of voltage spike that occurs during ignition on AFTER light switch is on. Lights have to be on during the ignition OFF/ON process to get dimmer to work....??????? Stumped...

**Mayorchuck** · 01-26-2025, 02:03 PM

If it were mine, I would grab a multimeter to start checking voltage in and out of the switch while duplicating the issue. Is there 12v coming to the switch on initial start up? If no, disconnect the switch and check again. The wire coming from the GM is Black/gray, the wire leaving to the components is red/gray. Brown will be your ground. Doing this should point you in the right direction.

**DeadPixel** · 02-25-2025, 01:47 AM

MWBURG’s son / E38 co-owner here - we finally figured this one out!

Long story short, it’s a known issue with the BlueBus module that is installed on our E38. BlueBus has a feature that enables you to have the parking lamps on as running lights (even when headlight switch is in “off” position). We weren’t even aware this option was active, but the previous owner must have turned it on. In any case, the BlueBus manual notes that this feature “may impair your ability to alter the in-vehicle dimmer while it's active”. Ah-ha!

Sure enough, once I disabled that option, dimmer works as normal! Glad I happened to notice this “fine print” while updating the BlueBus firmware. Guess it really does pay to read the manual sometimes. Thanks for all the advice either way!