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Will driving w/o instrument cluster kill the battery?

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  • Will driving w/o instrument cluster kill the battery?

    Wdriving my car without the instrument cluster allow the battery to go dead?

    Here's the story -- I had to remove the cluster to send the speedo off to get fixed (odometer quit). Because it's such a PITA to R&R the cluster on air bagged E30 M3's, I've been driving the car without the cluster (yes, I know it's a bad idea -- there's no oil pressure light, temp gauge or gas gauge to watch) waiting for the speedo to come back.

    Well, wouldn't you know it, the battery went dead. It appears the car's been running off the battery all this time. I haven't been doing any night driving, so it went about a week before dying. I'm thinking something about the cluster is needed to keep the alternator charging the battery. Does anyone know for sure?
    The day ain't over yet....

  • #2
    don't know for sure, but I doubt that was the cause. It could be the cold (and not driving the car enough to charge the battery), it could be the old battery or the alternator. You did not say how you cured the problem?

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    • #3
      Haven't cured it yet, but the speedo is on its way back to me. Also, not cold down here in Florida. The coldest it's been lately (during the day) is about 60F. Also, in a year and a half I haven't had any charging or starting problems, so I doubt it has to do with the driving I'm doing.

      BTW: Love your 911. That car looks great in yellow.
      The day ain't over yet....

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      • #4
        just do a quick check to see if the battery has enough juice and if the alternator is putting enough charge, any shop would have the voltmeter to do this.

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        • #5
          Yes, well the cluster contains the charging indicator circuit and uses a bypass resistor to keep the circuit going if the indicator bulb should go bad (86-on). They did this because on early cars when the bulb went out the battery would not charge.

          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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          • #6
            Originally posted by Reelizmpro
            Yes, well the cluster contains the charging indicator circuit and uses a bypass resistor to keep the circuit going if the indicator bulb should go bad (86-on). They did this because on early cars when the bulb went out the battery would not charge.
            All true. You need the cluster in for the charging system to work.

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            • #7
              Hey Barry & Adam: It'd seem contrary that operating the car w/o the cluster wouldn't affect charging, but I was told by my local independent BMW shop that leaving out the cluster causes the alternator to quit charging. You've just confirmed what they said. So, not to worry. I'll just get the cluster back in and jumpstart the car. Thanks!

              Interesting that this car would operate for more than a week just off the battery! No nighttime driving, but even so, all the electronics, fuel pumps, windows, radio, etc. running off the battery. Wow.
              The day ain't over yet....

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              • #8
                Perhaps your car is different, but in the '88s I've examined this closely there's a secondary path for the alternator field wire (the blue one that goes through the cluster's alt warning light) through the ABS computer. The ABS computer gets a voltage reference from the alternator, and this usually also allows the alternator field it's initial excitement at starting, same as the path through the instrument cluster. Who knows if they installed a diode or something on this input in the ABS computer at some point...or randomly...

                When I first installed a Stack dash, and still had ABS, I had no problems with the battery charging. At that point, it was the same as simply removing the cluster...I didn't make any special effort to replace the 12V to the blue wire from the cluster...

                Andrew

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                • #9
                  Theoretically, you can bypass the cluster altogether to get the battery to charge while the cluster is out. In the blue connector, you have the green wire which is switch ignition power. The blue wire in the same harness is the alternator charge signal. Jumpering this two wires together will allow the alternator to begin to charge.

                  Note, I have not performed this bypass yet. But it should work.

                  Alternatively, you can connect the blue wire from the alternator to any switch ignition power source. The diagnostic plug also has both constant power and the blue alternator charge wire also. Google or search for the diagnostic plug pinout.

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