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Very odd RPM/OBC problem

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  • Very odd RPM/OBC problem

    Hey guys,
    A while back, my tachometer would stop along with OBC displaying blank only backlight, it would then spring into action later.
    After a while, it just stopped completely and didnt work again. So i checked the related fuses, all good.
    Then i removed the cluster, put new batteries, no dice.
    Then i put a different SI board, then another SI board, then another coding plug, then another cluster from a 325i, but no luck on the tachometer or OBC.
    I checked with a multimeter pin #7 on the blue connector, it showed a value that goes up with the rpm, so the signal is there and seems working.
    Any ideas what might be the problem? I'm going to open up the fusebox and see if anything is shorted there maybe
    Thanks
    e30 m3; euro S50B32 twin-turbo'd

  • #2
    Power to the tach comes in to the cluster via C1 (blue) pin 2, via fuse 12. For reference, here's a pic of C1 showing where pins 1 and 13 are. (the numbering pattern is the same on all 3 connectors)

    Power to the OBC is delivered from the back of the cluster out to the OBC via pin 2 of the C3 (yellow) connector. Check that you have 12v coming out of that pin on the back of the cluster.

    If you don't have power at that pin, check C2 (white) pin 6. That is a 12v input to the cluster via fuse 10. It powers the park brake light, low fuel light, and sends output power to C3 pin 2 for the OBC.

    The odd thing about your issue is that there doesn't seem to be a relationship between tach power and OBC power.


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    • #3
      Reflow your solder joints at the connectors on the cluster PCB.

      88 M3 - LACHSSILBER/M TECH
      89 M3 - ALPINEWEISS II/SCHWARZ
      85 323I S52 - ALPINEWEISS/SCHWARZ
      91 M TECHNIC TURBO - MACAOBLAU/M TECH


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      • #4
        I've tested pin#2 on C1(blue) with ignition on, no 12v.
        The fuse #12 has 12v with ignition on.

        I tested pin#6 on the white connector, there is 12v.

        Also it be should noted, i came across a thread yesterday mentioning someone who had the tach/mpg/obc and interior lights all dead. It turned out to be splice S319 under the driver's door sill was corroded. I tested my interior lights, no 12v there, also the trunk light has only 1-2v with the lid button loose and 0v with it depressed. All fuses seem intact.
        I've removed the doorsill and carpeting but there's just a big loom of wires, nothing seems out of the ordinary.
        Any suggestions regarding pin#2 on C1 and fuse#12 or the door sill?
        Thanks for the help
        e30 m3; euro S50B32 twin-turbo'd

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        • #5
          Bit of progress on the car and update,
          I removed the wires and found the splice, i believe it is a bunch of red and red/green wires, i removed the end and cut fresh ends and put the splice together, no dice.
          There's no 12v at the splice though with the ignition on, it's supposed to have 12v there for tach/obc/trunk/interior, correct?
          This is a picture by the person who had the same problem
          Click image for larger version

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          Next ill remove the speaker and follow the lines
          e30 m3; euro S50B32 twin-turbo'd

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          • #6
            The wire from Fuse 12 to C1 Pin 2 should be red with green stripe.

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            • #7
              Hey guys,
              Glad that the rpm and obc/trunk lights are working again!
              I checked for 12v at the fuse#12 , there's 12v, also the red/green wire below it has 12v going into the big loom of wires, it's somehow losing continuity before the splice
              I couldn't follow the wire through the stack of wires of the fusebox going down, so i just took constant 12v from another source and gave it to the splice, rpm and everything working again.
              Thanks for the help, cheers!
              e30 m3; euro S50B32 twin-turbo'd

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