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Unread 03-05-2012, 11:24 AM   #1

Name: bemwa
Title: United Newb
Status: Offline
Join Date: Mar 2012
Location: Earth
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Your Ride: E39 520i
E39, 1997, 520i, LCM, K-BUS, Low instrument cluster, no lights and turn signal indica

Short description of my problem:

A while ago my turning lights indicators together with clicking sound, low and high beam and fog lights indicators in the dashboard stopped working. Also the reversing light quit. And the alarm for forgotten lights also went AWOL.
At the beginning the malfunction came and went. Sometimes it was OK after I started the car but quit after a few minutes. It got worse and worse and it stopped working without revival.
I have a 97 E39 520i, with low instrument cluster.
I have informed myself a bit and all this is related to K-BUS. All the signals for the nonfunctioning control lights should come from LCM over KBUS to the dash. The signal for reversing light should com from dash to LCM which in turn engages the light bulb in the rear.
I strongly suspect something is wrong with the bus. Either in LCM, dash or the wiring in between?
I have located a X10116 (k-bus) connector behind the glove box and I have selectively disconnected other wires trying to pinpoint the branch with fault if any. No success. I have only 5 wires on this connector: KOM cluster, LCM, G II and I think IHKA and radio.
Next thing I tried was to measure the voltage on the bus. With no traffic it sits on cca. 12V, which is ok.
Next thing I hooked up an oscilloscope to the LCM k-bus wire and engaged the hazard lights (all 4 turn indicators). On the scope I could see regular signals, but what puzzled me that the spike were pulled down by only 5.8V, which is, I think is not enough. According to the the low spikes should come to below 2V. In my case it comes down to just about 6V. I would like to confirm that this is indeed to little.
What could be the cause of that? Is the k-bus interface on the LCM faulty? Could it be some fault in the wiring? I have no spare LCM so I cannot experiment. Is there something I could check in the LCM board, like some buffer or optocoupler or something?
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