Headlights on, engine idles rough??? [Archive] - Bimmerfest - BMW Forums

: Headlights on, engine idles rough???

09-29-2009, 07:35 PM
Hey guys,

When I turn my headlights on while the car is in gear the engine vibrates more...it maybe drops off 50rpm or so but starts vibrating like crazy! If i turn the lights off or put it in park the idle smoothes out. has this happened to anyone else? It also happens if i turn the blower motor coincidentally.

Also, if the headlights are on does the "low beam" indicator in the information display always light up, or does this mean something is up with the LKM? both headlights, fog lights, turn signals, and marker lights work...any thoughts there?

Thanks guys!

09-29-2009, 09:09 PM
I would suspect your Battery is weak and unable to supply the power to the headlights, transmission and/or blower without loading up the Alternator. Loading the alternator puts a higher load on the engine and might at low rev's cause some vibration, especially if your drive belt is worn or slipping. Have your battery load tested and check the wiring between the battery and alternator. Also check the Fusible Link adjacent to the battery to see if it is cracked. It is also possible your alternator is not charging properly.

If your Low Beam indicator is on that usually means that a headlight is out or the resistance in the circuit has changed. I suppose if there was a wiring fault and some sort of short, then this could also overload the charging circuit causing the alternator to load up. You would expect a fuse to blow if that was the case though.

09-30-2009, 08:52 AM
Thanks for the info, i'll check out the battery first. Do you think it could also be motor or transmission mounts? it vibrates like a massage chair at the sharper image....

09-30-2009, 08:55 AM
Thanks for the info, i'll check out the battery first. Do you think it could also be motor or transmission mounts? it vibrates like a massage chair at the sharper image....

That is doubtful given your symptoms. Stick with the first suggestion. Thats more than likely your problem. But then... You sure you want to fix it, the vibrating seats can be a good thing too.

09-30-2009, 09:11 AM
agreed vibrating seats are fine, but anything to keep the fiance interesting in driving this jalopy is better!!

09-30-2009, 11:16 AM
For the lights I would try replacing the bulbs, Like Turburgring mentioned the LKM uses resistance to measure the circuit. You could have a bulb that is causing a higher/lower than normal resistance. Also check the connectors for corrosion. That plays havoc on resistance measuring.

09-30-2009, 08:44 PM
Thanks for the info. I did recently replace one of the headlight bulbs...perhaps if i got one to match on the other side it will solve my trouble. Just to be clear as well, if i have a marker light out the "parking lights" indicator will display correct?

10-01-2009, 03:39 PM
I know I have read to only use Sylvania bulbs or OEM BMW ones for the other lights. Because the restistance varies too much with the other bulbs. For headlights I use the Silverstars and have not had a problem........yet!

10-09-2009, 12:59 PM
Definately a low voltage problem. If not battery or charging system look to a poor ground between the engine and chassis.

10-11-2009, 08:52 PM
THanks for all the help friends. I have not gotten a chance to get down to the bottom of things. i bought two new headlight bulbs and that didn't help at all, the battery is in good condition... i'll check the ground next.