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Discussion Starter #1 (Edited)
Hi Guys,

A couple of days ago the backlight for my instrument cluster and all dash buttons started flickering then went out. I've been reading various posts on different forums regarding the same issue but very few people have posted possible fixes.

I've been round various earth/ground points with nothing found loose, i removed my LCM to check for loose wires, wiggling all off them with no difference to the lighting issue. I've checked all the wiring in the boot/trunk, again nothing obvious. I've removed my light and dimmer switch and had a wee play with them, again didn't make any difference

The only thing that seems to work briefly is removing one of the battery leads for a few minutes then reconnecting. This results in the backlight working again but only for a few minutes.

When it comes to the mechanical stuff i've got no issues but the electickery is a different ball game!!

Can anyone point in the right direction as this one is doing my head in now!!

Thanks and Regards
Gordon
 

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check your shifter knob the illumination wires could be shorting on the shifter
 

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real easy to check lift the shifter boot and unplug the wires going into the shifter knob illumination
 

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Discussion Starter #7
Hi Guys,

I've been out diagnosing with TIS all afternoon, unfortunately no further forward as yet. Using some of the test features in TIS i was able to illuminate the dash, all switching and gear knob lights at the same time, however when the lights are switched on from the normal light switch still no joy. I've tested the light switch, dimmer switch etc and all are operating as they should, according to TIS anyway.

've just noticed the replies regarding gear knob possible shorting issues, i'll check that out tomorrow.

It's strange how using TIS all lights can be illuminated but under normal circumstances they don't operate. I'm not sure if they're fed power from a different route under test conditions?

Does anyone know where all the earth/ground points are? I reckon thats the next road to go down.

Ive checked most of obvious ones in the engine bay and boot/trunk but all were tight and in good condition. There must be some hidden away under the carpets somewhere but could do with knowing exactly where before removing seats etc to access them?

As always guys any help much appreciated.
 

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Do a search using "gear shift short dash light" in the Google box at the top of this page. There's some good stuff, including m5 daemon's suggestion of a short in the wiring to the gear shift light.
 

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Discussion Starter #9
Wiring round the gear knob checked out and all appears to be intact with no frayed wires visible, i didn't remove the console between the two front seats but the wires that feed in to power the hazard warning lights etc were wiggled around for a few minutes but nothing seemed to make a difference.

Using TIS and a mulitmeter helped me find what wires power all dash and centre console buttons from the LCM. However when i turn the light switch on it doesn't seem to give the signal to the LCM to initiate illumination. I checked the signal wire from the light switch back to the LCM and at first thought i had found a dodgy connection as the dash lights flickered a bit when i was wiggling the wire, unfortunately i wasn't able to repeat this to prove the wire was at fault. Boo hoo!!!

After testing today i believe it's the same wires from the LCM that power the dash lights during TIS test procedures and in actual use , hopefully ruling out earths as they work fine under test conditions. Next mission is to figure out why when i turn the light switch on it's not signalling the LCM to power the illumination lights, i'm sure it will be something simple, but tracking it down is awfully time consuming. Good times!!!……..
 

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mine does the same thing it stopped for awhile but then i removed the nav screen and re installed and it got worse.
 

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When you say TIS above, do you mean DIS, WDS, or INPA maybe? Do you have WDS? It's pretty essential for this type of stuff.

The light switch inputs come into pins 1, 33, and 51 of the black 54-pin plug at the LCM, and the dimmer signal comes into pin 16 of the same connector. If you have confirmed these switches are functioning OK and their signals are getting to the LCM, using INPA or whatever, the problem must be in the LCM, or the "58G" circuit running between the LCM and the various lights. And there are a lot of light on that circuit. I think the LCM is protected from shorts in the circuit by its internal switching transistor for that circuit, so any of those loads shorting anywhere will shut down the supply to all the lights, as you are seeing. A break or unplugged connector will do the same, but the break or unplugged connector would have to be in the common single wire part of the circuit, since you are losing ALL the lights.That might be a good place to start. (You are losing every dash light and switch backlight, aren't you? It would be a good clue if some were still lit.)

From WDS, the LCM sends the power (modified by the dimmer dial setting) through circuit 58G to the cluster and the switch lights from pin 1 of the 15-pin white plug on the LCM. It's a heavy grey/red wire. It goes to a large connector under the carpet in the RH floor well. It looks like it's all soldered joints rather than pins. There it splits and goes to all the separate loads. Worth checking that connector and the wiring to it. And maybe swapping in a spare LCM. Because checking all the loads for shorts is quite daunting.
 

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Discussion Starter #12
Good day, thanks for the informative post, i must admit i'm not up to much diagnosing today, perhaps the bottle of Jim Beam last night might have something to do with it!!!!

It's TIS i've been using but think DIS is part of the same program, in fact now that i think of it, DIS must be diagnostic part with TIS the technical info. I did stumble across WDS which i'm still getting to grips with, very handy indeed!! The misses is complaining how much time i spent playing with cars so only getting short bursts at sussing this issue out.

I put a multimeter across terminal 1 and 2 of the white LCM plug which is like you say the heavier gauge grey/red stripe feed wire and ground. Under test conditions i am seeing a healthy voltage with all back lighting illuminating however nothing when using the light switch. I did wonder where the grey/red wire terminates so thanks for that, i'll lift the carpet and check out the connections. I would have thought if there was a genuine short in the circuit somewhere that the lights would not illuminate under test conditions, what you reckon? Perhaps i'm wrong.

One thing i did notice a few days ago was a check control fault for the fog lights, however on closer inspection there was nothing wrong with the lights. Not sure if this is connected in any way but just a thought.

Hopefully when the hangover lifts i'll get outside to have another play.
 

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The DIS tests work through the various modules so from your testing, it is successfully telling the LCM to power the 58G circuit. Since you are getting power at the grey/red wire and all the lights are working, there can't be anything wrong with the LCM output transistor or the wiring downstream so don't bother checking that big connector under the carpet. It must be good.

That seems to narrow it down to the two switches, the wiring between the switches and the LCM, or the LCM internals related to reading the status of the switches. In WDS, do a search for A3, that's the LCM module. The light switch is identified as S8 and the dimmer as N3. The pins numbers are in my previous post.

The light switch contacts are all closed when selected OFF. One contact opens when parking lights or head lights is selected. You could check this is happening with an ohm meter at the pins at the LCM plug. But best technique is to back-probe the pins with the key on and the plug connected to the LCM. This also confirms the LCM is sending the monitoring signal of 5 or 12V to the switch.

The dimmer is a potentiometer. The LCM pin could be back-probed also as the pot regulates a 5V monitoring voltage from the LCM to 0.5V to 4.5V.

Operation of these switches can also be monitored in INPA, which is a handy app for this kind of problem. It gives a live reading of the inputs from the two switches. From your description, I'm not sure if TIS/DIS does that for you.
 

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Discussion Starter #14
Evening, not been near the car in a few days as finally our summer seems to be kicking in so out enjoying the sunshine!! Woo hoo.

Getting a bit stumped now on this one, all wiring from the LCM to the light and dimmer switch seems to be in good working order, the light switch and dimmer potentiometer were function checked using DIS test procedures with no issues found. It's strange that when activating the "58G" terminal using DIS all lights illuminate, however, although both light switch and dimmer potentiometer are functioning correctly they do not activate the "58G" terminal??

I was going to borrow an LCM from my mates 1998 540i but unfortunately there was a death in his family so don't want to bother him for a bit.

Any more thoughts what to try next guys? Do you think it sound like and LCM related issue?

Thanks.
 

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Discussion Starter #16 (Edited)
Hi,

I have version V47, not sure how that differs to the one you've got?

When I first scanned for codes i had an LCM check error and a error for a trailer module? It was when these faults were present I was able to access the LCM module and within it were four different functions, i think they were-
1 Diagnosis
2 Function check
3 Component activation
4 Expert mode

Within these modes there were many different tests you could carry out, can't remember which one now but the dimmer and light switch were on the same menu.

Since i've cleared the codes no more have being thrown by the LCM, which has resulted in being unable to access the menu where all the different tests were available. I'm no expert with this program as still getting used to it, so perhaps there is another way to navigate to these options. All my night lights are still not working so waiting to try a different LCM to see if that makes any difference before doing anything too intrusive.

Cheers
 

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Ah, I think that explains why I can't find the LCM tests you used. DIS only suggests tests for a module if you have a code for it, or you can force a test by picking a module from a list of user-pickable ones, but the LCM isn't in the list. I would have to force a code. What code did you have?
 

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Discussion Starter #18
I can't remember what the exact codes were now but one was related to a trailer module and the other for a check control fault. After clearing them they've not come back so not sure what they were related to? I've still not had time to investigate further, busy doing up an Audi S2 thats been sat in the garage for years so thats taking up ALL my spare time!! I really need to get this sorted though as i've not been doing any night driving since the fault appeared so not been too much of an issue.

I've checked the wiring between the LCM and light/dimmer switch several times thinking i've missed something but all appears normal, as mentioned before this was confirmed by DIS. If there was an earth/ground fault in this system do you think the light/dimmer switch would work as normal? I'm struggling to figure out where this fault lies if all lights are illuminated under test conditions? How common is it for LCM internal solder joints to fail? Perhaps clutching at straws with this train of thought but you never know i guess?…
 

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DIS does what it does by talking to the various modules. So it sounds like it tapped into the LCM and confirmed it was receiving good signals from the dimmer and light switch. And when it told the LCM to energize the 58g circuit, the lights lit. So if the switch input doesn't light the backlights, then it sure sounds like a problem in the LCM, corrupted code, an internal fault, whatever.

You need to swap in that other LCM. I've always found you have to keep moving forward on these problems. Try something, learn from it, etc., until you get to the bottom of it.
 

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Discussion Starter #20
Quick update for this thread guys.

After spending many hours chasing this problem down weeks ago, it appears, as if by magic to have sorted itself out???

Not the best outcome as i still don't know what caused it but after leaving the car parked up for two weeks whilst working offshore, on my return the lights were working perfectly. Several days later the lights flickered on and off momentarily but that was 3 weeks ago now and still working fine.

I've probably gone and tempted fate but so far so good.

Now the darker nights are starting to draw in slowly i bet they will go on the blink again!……If they do i'll keep you posted with any developments.

Cheers all
 
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