1999 E38 728i - Weird Park Distance related issue

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  1. 1999 E38 728i - Weird Park Distance related issue 
    #1
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    Hello, this being the auto electric forum,I thought it prudent to post here. I have also posted on the 7 Series forum but I'm new here so didn't realise there was a specific forum for electric issues!!

    Sometimes, ONLY in the colder months of the year my park distance circuit picks up a current from somewhere independently of the ignition. With the fuse (#64) inserted it causes a feedback to the ignition when the car's switched off and it comes to life, dash lights, brake light, alarm tripping,etc. I have even had it where I've taken the key out and the engine's still running. As soon as I take the fuse out, it all goes dead again but the PDC circuit is still picking up some current and works until outside temp gets warmer, by even a degree or two, then it cuts out as well.

    I need to know where the circuit could be picking up this live feed from, and it's strange that it only occurs when it's really cold, and only sometimes. Appreciate any advice. Alternatively, if anyone knows any competent, honest and competitive auto electricians in the manchester/northwest area please let me know.

    I really need to sort this out as I'd rather not just use my PDC as normal with the fuse inserted and then wait for all the lights to come on, drain the battery or embarrasingly blare out the alarm. At the moment I've left the fuse out, but my car is not a small motor and the PDC comes in extremely handy. You know what I mean!! Thanks
     
     

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    Have you tried checking each sensor?
     
     

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    I have indeed. Each sensor, individually, front and back works normally with all the right "gongs" and "beeps". This seems to be a problem with the whole PDC function being powered from somewhere intermittently and without the ignition being turned on with my key.
     
     

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    That sounds a super strange one indeed.
    Maybe if you can get to grips with the WDS system you can find the right diagram for your problems.
    Thread and links here.
    (P.S. Adobe SVG plugin/viewer and J A V A is needed for the 2 WDS sites mentioned.)
    Last edited by compactM54; 02-01-2010 at 03:24. Reason: links
     
     

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    Thanks a lot for that link, the application certainly looks useful.I'll try and see what I can make of it. I'm no electrician though!!

    I'm also wondering whether when a sensor was replaced in its history (which I can see from previous invoices) whether it wasn't done so properly which is causing this. Is it possible for a worn out or stripped wire to pick up a current from an adjacent wire from a completely different function? But then why does it happen only in the cold??
     
     

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    Had a very quick search about.
    Fuse 64 does power the PDC control unit and only the PDC unit it seems.
    The PDC unit receive signals from the IKE module which I think is fancy name for instrument cluster.
    So if the PDC unit is bad/not well and as it is directly connected to the cluster, could explain all the dash lights maybe?
    Also, looks like the fuse box on a '99 7 is found RH of boot and the PDC unit seems to live directly below it, should be possible to trace the power wire from the PDC back and inspect for damage.
    Is the area free from moisture/leaks to?
    Could try swapping out the PDC unit with another.
    If you can find the p/n off the unit and checking here you can see if your unit was one of the ones also used on E39 5 series or if yours is a 7 only unit.
    Good luck.
     
     

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    Local BMW garage ordered a brand new PDC control unit last week and tried with that, the fault was still there,so unfortunately it's not the unit itself. I'm beginning to suspect it's the wires/connections, etc, some damaged wire or joint.

    Going to try and see if I can have a look at all the wiring around the pdc unit in the boot, see if everything looks "healthy". They looked at that area a few times, and doesn't look like any water's getting in or anything.
     
     

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    Ok, I got into the boot today, took the fuse box out, had a look at the PDC control unit and especially the wires coming out of it and following them. There doesn't look like any damaged wires or tears or quickly taped-over wires or anything like that. There's definitely not any moisture or water anywhere inside that area.

    Next I'm going to try and get into the rear sensor assembly by taking the trims off, I'll probably need help with this to be honest. I tried following some instructions elsewhere on the net and it's not straightforward (or maybe it's me!!). I keep coming back to this "rear parking sensor" that it says in the invoice history was replaced. I'm suspicious that there may be a stripped wire somewhere...
     
     

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    Is it possible to just disconnect the sensors from the PDC unit and then see if the fault still appears?

    Barney
     
     

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    There may be a way to do that, but I spoke to two different independent BMW mechanics over the phone from London today and with the symptoms I explained they both said it can't be something up with the sensors. He said to zero in on the power line to the park distance circuit.

    He also said that if this power line can be traced back through the car, most likely by visual inspection (sounds like a costly job due to the need to strip down the car) then most likely we'd be able to find some wire that's shorting, touching metal or being powered by another wire for something else.

    I'm weighing whether to book it back in for further investigation.
     
     

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