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Thread: charge trek VIII -"Scottie, break out the ETM!"

  1. #1
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    Default charge trek VIII -"Scottie, break out the ETM!"

    Alrighht .... were you all hoping this would go away by itself? me too.


    For those of you who didn't make it through the first seven (!) episodes...
    ...the story so far...


    Basically, an inexplicably bad charging system on my '89 525i has led me to investigate the circuit that connects to the D+ terminal at the alternator, which reads 1.5V (should read +/- 12.5V) with the engine running. The meter shows that there are 48.4 ohms of unwanted resistance somewhere on this circuit that I have to find, preferably before my workspace turns into planet Hoth.

    Let's have a look at that circuit in the trusted Bentley that we all have a copy of, shall we?



    The link to the instrument cluster is for the charge light, which works as it should. Hmmmm... better have a look at that 'uncoupling diode' and the 'unloader relay'. As it turns out i have a couple - both of which seem to work, each with it's own clue.




    An unloader relay is designed to reduce draw on the battery from vehicle electrical components during vehicle starting so as not to overtax the battery, from what I can gather. I think it activates the diode which regulates voltage to certain circuits. (?.....help?)

    Time to break out the ETM...
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  2. #2
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    so here is a more detailed chart. the arrows point to the burn marks i found.


    here is the supply to power sources B, G, and H, all of which seem to operate through some kind of switch. Seems they are 'hot at all times'.
    But what does 'partial' mean?


    H leads to the ignition switch. From this chart we can see C is active in pos3 (start) and pos2(run), while A is active in key positions 1,2, and 3.


    and here is something related i think... the final stage blower control unit, which had a meltdown last spring (shortly before the first alternator died).


    So.... I think I'll sleep on it for a bit.... but there's something to this i think.

    Or maybe there is something wrong at that link... which is supposed to be in the front power distribution box.... but I'm not sure where.
    Underneath somewhere I'm guessing.

    Well... that's where I'm at - ideas are most welcome. Those are the clues I'm looking at. Something up with that B power feed... guesses?

    Any tests I should do.... something I can look for...
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    Last edited by tim eh?; 11-16-2008 at 12:27 AM.

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    here's maybe something i can look for...

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    ok, the diode in the relay is 'flyback' protection. when you shut the power off to a relay the collapsing field sends a voltage spike back to the switch. the diode provides a path to ground for the induced spike, and helps reduce arcing (and carbon build up) at the switch itself.
    it is NOT anything to be concerned with.

  5. #5
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    Quote Originally Posted by attack eagle View Post
    ok, the diode in the relay is 'flyback' protection. when you shut the power off to a relay the collapsing field sends a voltage spike back to the switch. the diode provides a path to ground for the induced spike, and helps reduce arcing (and carbon build up) at the switch itself.
    it is NOT anything to be concerned with.
    I understand most of that - I don't get "the diode in the relay" - what relay?

  6. #6
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    Tim... STOP... you have to think of KISS... Keep It Simple Stupid!

    First thing to do is the bulb... you said it work, but you never said if it is bright or dim... What would break first in the entire electrical system? Bulbs... so investigate the bulb... make sure you got 12V there.

    My next suspect is if you don't get 12V at the cluster, then it has to be the ignition switch... that part is high wear item...

  7. #7
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    Quote Originally Posted by Tiger View Post
    Tim... STOP... you have to think of KISS... Keep It Simple Stupid!

    First thing to do is the bulb... you said it work, but you never said if it is bright or dim... What would break first in the entire electrical system? Bulbs... so investigate the bulb... make sure you got 12V there.
    ok.... i stopped.... kind of...

    ...did you just call me stupid?

    ok, so investigated the bulb. i can't figure out how i would measure voltage in the clusterboard with the ignition on, so i figured the easiest way would be to swap in the spare cluster i have, which i did.

    in both clusters the bulb works as it is supposed to, and is bright, just as bright as all the others. it comes on before and during start then shuts off, also flashes (once) on shutdown.

    i went for a testdrive with the spare cluster in and had the same poor charging results.

    so can we rule out the cluster / bulb / resistorbehindthebulb ?

    Quote Originally Posted by Tiger View Post
    My next suspect is if you don't get 12V at the cluster, then it has to be the ignition switch... that part is high wear item...
    hmmm.... remember this?

    here's a better photo



    here's the etm again, with the PO's splices marked with caffeine-laden arrows.



    what am i doing still in front of the computer?
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  8. #8
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    Open up those ignition switch wires and re-solder them properly.


    BMW E34 1992 525i Touring

    RIP Touring...probably never going to drive you again...

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    comments?
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  10. #10
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    Quote Originally Posted by tim eh? View Post
    comments?
    Defiently the problem, or at least part of it, replace the wires from the first splice to the last and solder the connections, no crimp connectors for this.

    Same problem for a friend with a 2001 Elantra...Had an aftermarket alarm installed when he bought it and then after 4 months his starter would work whenever it felt like it, so I opened up the dash and took everything out, EVERYTHING, didn't leave a single wire, re-soldered the ignition wires and everything works again.

    After you fix them, check D+ again


    BMW E34 1992 525i Touring

    RIP Touring...probably never going to drive you again...

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