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Thread: 2002 BMW E39 540 electrical fault

  1. #1
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    2002 BMW E39 540 electrical fault

    Hello, I have some electrical fault I was hoping to get some tips on.
    The car has a uneven throttle if I hold it on the same position, Dome lights and lights where the odometer is dimming up and down all the time. Also the navigation can restart itself, or just shut off the sound for a few seconds and come back. Sometimes the whole speedometer can reset itself, and all the fault codes will come on, and sometimes the battery light will indicate.

    The car seems to have the same power as before, also i've never had any issues starting the car.

    Its an 5/5/2002 BMW E39 540, with the big navigation screen.

    I would be so happy if anyone knows what could be wrong, or knows how I can fault search to find the fault.

    Some things i've considered: Bad alternator (But I've never had issues with low battery for starting the car), bad main wires for power and earth, or a bad battery.

  2. #2
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    Could be bad ground.

  3. #3
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    1997 BMW 540i 6 speed
    How many kms on the alternator? Sounds like intermittent output due to worn out brushes.

    Have you verified clean, tight battery connections?


    Ed in San Jose '97 540i 6 speed aspensilber over aubergine leather. Build date 3/97. Golden Gate Chapter BMW CCA Nr 62319.

  4. #4
    JimLev's Avatar
    JimLev is offline Artifically Aspirated Moderator
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    No brushes in the water cooled alternator.

  5. #5
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    Loose electric/ground connections, or most likely - failing alternator, about to give up.

  6. #6
    JimLev's Avatar
    JimLev is offline Artifically Aspirated Moderator
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    Ever have the doors leak water on to the carpets? Might be a good idea to check the fuse block under the carpet.
    All power for the cabin goes thru those fuses.

  7. #7
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    The car has run 258k KM, and as far as I know, the alternator has not been changed.

    I haven't checked the battery connections, or acually checked the voltage on the battery.

    How likely is it that the alternator is starting to go bad? As said earlier I've never had any issues with starting the car because of low voltage.

    Which spots are common for bad ground connections? Also saw a video that the wiring harness beneath the seat could be fried.

    Have never had any issues with leaking water inside the cabin, but where is the fuse block under the carpet located?

  8. #8
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    Test do not guess
    All you need is a volt meter and access to the battery
    test at idle
    test at 1500 rpm
    test at 2500
    Hold at each RPM for 15 secs and see what the results show. NOTE: you are looking for variation not big numbers!

    The real test is with a 100 amp load, but you can approx it by turning everything on (don't forget the seat heater and rear window defroster)
    remove jumper cable at engine and clean and tighten the connection, also do the grounds in the trunk.

    The chances of a big time water leak causing weird fluctuations all over E39s electrical systems is well documented. Once flooded is it only gets WORSE.
    Last edited by StephenVA; 06-16-2019 at 05:55 PM.
    Current Garage Highlights
    2003 525iT TiSilver
    2002 M5 TiSilver
    1998 528i KASCHMIRBEIGE METALLIC (301) (Goldie)

    Former Garage Highlights
    2005 X5 4.8is
    2004 325iTs (2x)
    1973 Pantera L
    1971 Dodge Dart Swinger "Lite Package"
    1970 Dodge Challenger T/A 340 Six Pack Alpine White
    1970 Dodge Challenger T/A 340 Six Pack GoManGo Green
    1969 Road Runner 383
    1968 Barracuda Formula S 340 Sea Foam Green

  9. #9
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    Quote Originally Posted by StephenVA View Post
    Test do not guess
    All you need is a volt meter and access to the battery
    test at idle
    test at 1500 rpm
    test at 2500
    Hold at each RPM for 15 secs and see what the results show. NOTE: you are looking for variation not big numbers!

    The real test is with a 100 amp load, but you can approx it by turning everything on (don't forget the seat heater and rear window defroster)
    remove jumper cable at engine and clean and tighten the connection, also do the grounds in the trunk.

    The chances of a big time water leak causing weird fluctuations all over E39s electrical systems is well documented. Once flooded is it only gets WORSE.
    Sadly I haven't had much time to fault search, I wanted to check if others had experienced the same thing. Should I remove the floor carpet to make sure there are no water leakage? I have never experienced any wet spots in the car before.

  10. #10
    JimLev's Avatar
    JimLev is offline Artifically Aspirated Moderator
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    Might as well.
    Before you disconnect any connectors under the seat make sure the car goes to sleep (16 min) then disconnect the battery.

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